Add @Nullable/@NonNull annotations to the Rule API

Change-Id: If7b6d7b505476676c5683ab4966fa5f587b15d1e

20 files changed, 366 insertions, 126 deletions

diff --git a/rule_api/src/com/android/ide/common/api/AbstractViewRule.java b/rule_api/src/com/android/ide/common/api/AbstractViewRule.java
index 86e7896..3bbf8e7 100644
--- a/rule_api/src/com/android/ide/common/api/AbstractViewRule.java
+++ b/rule_api/src/com/android/ide/common/api/AbstractViewRule.java
@@ -15,6 +15,7 @@
  */
 package com.android.ide.common.api;
 
+import com.android.annotations.Nullable;
 import com.google.common.annotations.Beta;
 
 import java.util.List;
@@ -39,6 +40,7 @@ public class AbstractViewRule implements IViewRule {
     }
 
     @Override
+    @Nullable
     public String getDisplayName() {
         // Default is to not override the selection display name.
         return null;
@@ -47,6 +49,7 @@ public class AbstractViewRule implements IViewRule {
     // ==== Selection ====
 
     @Override
+    @Nullable
     public List<String> getSelectionHint(INode parentNode, INode childNode) {
         return null;
     }
@@ -69,11 +72,13 @@ public class AbstractViewRule implements IViewRule {
 
     // By default Views do not accept drag'n'drop.
     @Override
+    @Nullable
     public DropFeedback onDropEnter(INode targetNode, Object targetView, IDragElement[] elements) {
         return null;
     }
 
     @Override
+    @Nullable
     public DropFeedback onDropMove(INode targetNode, IDragElement[] elements,
             DropFeedback feedback, Point p) {
         return null;
@@ -115,6 +120,7 @@ public class AbstractViewRule implements IViewRule {
     // ==== Resizing ====
 
     @Override
+    @Nullable
     public DropFeedback onResizeBegin(INode child, INode parent, SegmentType horizontalEdge,
             SegmentType verticalEdge, Object childView, Object parentView) {
         return null;
diff --git a/rule_api/src/com/android/ide/common/api/DropFeedback.java b/rule_api/src/com/android/ide/common/api/DropFeedback.java
index 4a526e5..3920958 100755..100644
--- a/rule_api/src/com/android/ide/common/api/DropFeedback.java
+++ b/rule_api/src/com/android/ide/common/api/DropFeedback.java
@@ -16,6 +16,7 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.Nullable;
 import com.google.common.annotations.Beta;
 
 /**
@@ -33,6 +34,7 @@ public class DropFeedback {
      * <p/>
      * Filled and owned by the view rule.
      */
+    @Nullable
     public Object userData;
 
     /**
@@ -67,6 +69,7 @@ public class DropFeedback {
      * Filled by the view rule, called by the engine.
      * <p/>
      */
+    @Nullable
     public IFeedbackPainter painter;
 
     /**
@@ -78,6 +81,7 @@ public class DropFeedback {
      * <p/>
      * Filled by the view rule, read by the engine.
      */
+    @Nullable
     public Rect captureArea;
 
     /**
@@ -98,6 +102,7 @@ public class DropFeedback {
      * NOTE: The coordinate units will be in layout/view coordinates. In other words, they
      * are unaffected by the canvas zoom.
      */
+    @Nullable
     public Rect dragBounds;
 
     /**
@@ -126,7 +131,7 @@ public class DropFeedback {
      * @param userData Data stored for later retrieval by the client
      * @param painter A callback invoked to paint the drop feedback
      */
-    public DropFeedback(Object userData, IFeedbackPainter painter) {
+    public DropFeedback(@Nullable Object userData, @Nullable IFeedbackPainter painter) {
         this.userData = userData;
         this.painter = painter;
         this.requestPaint = painter != null;
@@ -136,28 +141,33 @@ public class DropFeedback {
     /**
      * A message to be displayed to the user, if any. Should not contain line separators.
      */
+    @Nullable
     public String message;
 
     /**
      * An error message to be displayed to the user, if any. Should not contain line
      * separators.
      */
+    @Nullable
     public String errorMessage;
 
     /**
      * A message to be displayed in a tooltip to the user, which should be short, but
      * can be multiple lines (use embedded newlines)
      */
+    @Nullable
     public String tooltip;
 
     /**
      * Horizontal alignment for the tooltip, or null if no preference
      */
+    @Nullable
     public SegmentType tooltipX;
 
     /**
      * Vertical alignment for the tooltip, or null if no preference
      */
+    @Nullable
     public SegmentType tooltipY;
 
     /**
diff --git a/rule_api/src/com/android/ide/common/api/IAttributeInfo.java b/rule_api/src/com/android/ide/common/api/IAttributeInfo.java
index 3dcb241..e1bb067 100755..100644
--- a/rule_api/src/com/android/ide/common/api/IAttributeInfo.java
+++ b/rule_api/src/com/android/ide/common/api/IAttributeInfo.java
@@ -16,6 +16,8 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
 import com.google.common.annotations.Beta;
 
 /**
@@ -51,7 +53,7 @@ public interface IAttributeInfo {
          * @return True if and only if the given array (if any) contains this
          *         format.
          */
-        public boolean in(Format[] formats) {
+        public boolean in(@Nullable Format[] formats) {
             if (formats == null) {
                 return false;
             }
@@ -66,24 +68,31 @@ public interface IAttributeInfo {
     }
 
     /** Returns the XML Name of the attribute */
+    @NonNull
     public String getName();
 
     /** Returns the formats of the attribute. Cannot be null.
      *  Should have at least one format. */
+    @NonNull
     public Format[] getFormats();
 
     /** Returns the values for enums. null for other types. */
+    @Nullable
     public String[] getEnumValues();
 
     /** Returns the values for flags. null for other types. */
+    @Nullable
     public String[] getFlagValues();
 
     /** Returns a short javadoc, .i.e. the first sentence. */
+    @NonNull
     public String getJavaDoc();
 
     /** Returns the documentation for deprecated attributes. Null if not deprecated. */
+    @Nullable
     public String getDeprecatedDoc();
 
     /** Returns the fully qualified class name of the view defining this attribute */
+    @NonNull
     public String getDefinedBy();
 }
diff --git a/rule_api/src/com/android/ide/common/api/IClientRulesEngine.java b/rule_api/src/com/android/ide/common/api/IClientRulesEngine.java
index 490ef0c..ec28520 100755..100644
--- a/rule_api/src/com/android/ide/common/api/IClientRulesEngine.java
+++ b/rule_api/src/com/android/ide/common/api/IClientRulesEngine.java
@@ -17,6 +17,7 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
 import com.android.annotations.Nullable;
 import com.google.common.annotations.Beta;
 
@@ -39,6 +40,7 @@ public interface IClientRulesEngine {
      *
      * @return the fully qualified name of the rule
      */
+    @NonNull
     String getFqcn();
 
     /**
@@ -47,7 +49,7 @@ public interface IClientRulesEngine {
      * @param msg A String format message.
      * @param params Optional parameters for the message.
      */
-    void debugPrintf(String msg, Object...params);
+    void debugPrintf(@NonNull String msg, Object...params);
 
     /**
      * Loads and returns an {@link IViewRule} for the given FQCN.
@@ -57,22 +59,28 @@ public interface IClientRulesEngine {
      *   inheritance chain. Rules are cached and requesting the same FQCN twice
      *   is fast and will return the same rule instance.
      */
-    IViewRule loadRule(String fqcn);
+    @Nullable
+    IViewRule loadRule(@NonNull String fqcn);
 
     /**
-     * Returns the metadata associated with the given fully qualified class name.
+     * Returns the metadata associated with the given fully qualified class name. Note that
+     * this will always return an {@link IViewMetadata} instance, even when the class name
+     * is unknown to the layout editor, such as for custom views. In that case, some
+     * heuristics will be applied to return metadata information such as guesses for
+     * what the most common attribute is, and so on.
      *
      * @param fqcn a fully qualified class name for an Android view class
      * @return the metadata associated with the given fully qualified class name.
      */
-    IViewMetadata getMetadata(String fqcn);
+    @NonNull
+    IViewMetadata getMetadata(@NonNull String fqcn);
 
     /**
      * Displays the given message string in an alert dialog with an "OK" button.
      *
      * @param message the message to be shown
      */
-    void displayAlert(String message);
+    void displayAlert(@NonNull String message);
 
     /**
      * Displays a simple input alert dialog with an OK and Cancel buttons.
@@ -83,10 +91,10 @@ public interface IClientRulesEngine {
      *            a validator which always returns true) if you do not want
      *            input validation.
      * @return Null if canceled by the user. Otherwise the possibly-empty input string.
-     * @null Return value is null if dialog was canceled by the user.
      */
     @Nullable
-    String displayInput(String message, @Nullable String value, @Nullable IValidator filter);
+    String displayInput(@NonNull String message, @Nullable String value,
+            @Nullable IValidator filter);
 
     /**
      * Returns the minimum API level that the current Android project is targeting.
@@ -101,6 +109,7 @@ public interface IClientRulesEngine {
      * @return an {@link IValidator} for validating new resource name in the current
      *         project
      */
+    @NonNull
     IValidator getResourceValidator();
 
     /**
@@ -109,6 +118,7 @@ public interface IClientRulesEngine {
      * @param currentValue the current reference to select
      * @return the reference selected by the user, or null
      */
+    @Nullable
     String displayReferenceInput(String currentValue);
 
     /**
@@ -121,7 +131,8 @@ public interface IClientRulesEngine {
      * @return the margins selected by the user in the same order as the input arguments,
      *         or null
      */
-    String displayResourceInput(String resourceTypeName, String currentValue);
+    @Nullable
+    String displayResourceInput(@NonNull String resourceTypeName, @Nullable String currentValue);
 
     /**
      * Displays an input dialog tailored for editing margin properties.
@@ -133,9 +144,15 @@ public interface IClientRulesEngine {
      * @param top The current, initial value to display for the "top" margin
      * @param bottom The current, initial value to display for the "bottom" margin
      * @return an array of length 5 containing the user entered values for the all, left,
-     *         right, top and bottom margins respectively
+     *         right, top and bottom margins respectively, or null if canceled
      */
-    String[] displayMarginInput(String all, String left, String right, String top, String bottom);
+    @Nullable
+    String[] displayMarginInput(
+            @Nullable String all,
+            @Nullable String left,
+            @Nullable String right,
+            @Nullable String top,
+            @Nullable String bottom);
 
     /**
      * Displays an input dialog tailored for inputing the source of an {@code <include>}
@@ -143,16 +160,18 @@ public interface IClientRulesEngine {
      * "layout", but should also attempt to filter out layout resources that cannot be
      * included from the current context (because it would result in a cyclic dependency).
      *
-     * @return the layout resource to include
+     * @return the layout resource to include, or null if canceled
      */
+    @Nullable
     String displayIncludeSourceInput();
 
     /**
      * Displays an input dialog tailored for inputing the source of a {@code <fragment>}
      * layout tag.
      *
-     * @return the fully qualified class name of the fragment activity
+     * @return the fully qualified class name of the fragment activity, or null if canceled
      */
+    @Nullable
     String displayFragmentSourceInput();
 
     /**
@@ -160,7 +179,7 @@ public interface IClientRulesEngine {
      *
      * @param nodes the nodes to be selected, never null
      */
-    void select(Collection<INode> nodes);
+    void select(@NonNull Collection<INode> nodes);
 
     /**
      * Triggers a redraw
@@ -206,9 +225,10 @@ public interface IClientRulesEngine {
      * @param filter a filter to change attributes in the process of measuring, for
      *            example forcing the layout_width to wrap_content or the layout_weight to
      *            unset
-     * @return the corresponding bounds of the nodes
+     * @return the corresponding bounds of the nodes, or null if a rendering error occurs
      */
-    Map<INode, Rect> measureChildren(INode parent, AttributeFilter filter);
+    @Nullable
+    Map<INode, Rect> measureChildren(@NonNull INode parent, @Nullable AttributeFilter filter);
 
     /**
      * The {@link AttributeFilter} allows a client of
@@ -222,14 +242,18 @@ public interface IClientRulesEngine {
          * allows a client to adjust the attribute values that a node presents to the
          * layout library.
          * <p>
-         * Return "" to unset an attribute. Return null to return the unfiltered value.
+         * Returns "" to unset an attribute. Returns null to return the unfiltered value.
          *
          * @param node the node for which the attribute value should be returned
          * @param namespace the attribute namespace
          * @param localName the attribute local name
          * @return an override value, or null to return the unfiltered value
          */
-        String getAttribute(INode node, String namespace, String localName);
+        @Nullable
+        String getAttribute(
+                @NonNull INode node,
+                @Nullable String namespace,
+                @NonNull String localName);
     }
 
     /**
@@ -242,7 +266,8 @@ public interface IClientRulesEngine {
      * @return A suitable generated id in the attribute form needed by the XML id tag
      *         (e.g. "@+id/something")
      */
-    public String getUniqueId(String fqcn);
+    @NonNull
+    public String getUniqueId(@NonNull String fqcn);
 
     /**
      * Returns the namespace URI for attributes declared and used inside the
@@ -250,6 +275,7 @@ public interface IClientRulesEngine {
      *
      * @return the namespace URI
      */
+    @NonNull
     public String getAppNameSpace();
 }
 
diff --git a/rule_api/src/com/android/ide/common/api/IDragElement.java b/rule_api/src/com/android/ide/common/api/IDragElement.java
index e03b718..885ba35 100755..100644
--- a/rule_api/src/com/android/ide/common/api/IDragElement.java
+++ b/rule_api/src/com/android/ide/common/api/IDragElement.java
@@ -16,6 +16,8 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
 import com.google.common.annotations.Beta;
 
 /**
@@ -35,6 +37,7 @@ public interface IDragElement {
      * Returns the element name, which must match a fully qualified class name of
      * a View to inflate.
      */
+    @NonNull
     public abstract String getFqcn();
 
     /**
@@ -44,6 +47,7 @@ public interface IDragElement {
      *
      * The bounds are absolute for the canvas.
      */
+    @NonNull
     public abstract Rect getBounds();
 
     /**
@@ -51,6 +55,7 @@ public interface IDragElement {
      * from an existing canvas. Returns null if the element has no parent, such as a top
      * level element or an element originating from the object palette.
      */
+    @Nullable
     public abstract String getParentFqcn();
 
     /**
@@ -59,21 +64,25 @@ public interface IDragElement {
      *
      * The returned rectangle can be invalid. It is never null.
      */
+    @NonNull
     public abstract Rect getParentBounds();
 
     /**
      * Returns a list of attributes. The list can be empty but is never null.
      */
+    @NonNull
     public abstract IDragAttribute[] getAttributes();
 
     /**
      * Returns the requested attribute or null if not found.
      */
-    public abstract IDragAttribute getAttribute(String uri, String localName);
+    @Nullable
+    public abstract IDragAttribute getAttribute(@Nullable String uri, @NonNull String localName);
 
     /**
      * Returns a list of inner elements. The list can be empty but is never null.
      */
+    @NonNull
     public abstract IDragElement[] getInnerElements();
 
     /**
@@ -90,12 +99,15 @@ public interface IDragElement {
          * Returns the namespace URI of the attribute.
          * Can be empty for an attribute without a namespace but is never null.
          */
+        @NonNull
         public abstract String getUri();
 
         /** Returns the XML local name of the attribute. Cannot be null nor empty. */
+        @NonNull
         public abstract String getName();
 
         /** Returns the value of the attribute. Cannot be null. Can be empty. */
+        @NonNull
         public abstract String getValue();
     }
 }
diff --git a/rule_api/src/com/android/ide/common/api/IFeedbackPainter.java b/rule_api/src/com/android/ide/common/api/IFeedbackPainter.java
index d6e256d..2b3d14d 100644
--- a/rule_api/src/com/android/ide/common/api/IFeedbackPainter.java
+++ b/rule_api/src/com/android/ide/common/api/IFeedbackPainter.java
@@ -16,6 +16,7 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
 import com.google.common.annotations.Beta;
 
 /**
@@ -34,5 +35,5 @@ public interface IFeedbackPainter {
      * @param targetNode The node being dragged
      * @param feedback The feedback data
      */
-    void paint(IGraphics gc, INode targetNode, DropFeedback feedback);
+    void paint(@NonNull IGraphics gc, @NonNull INode targetNode, @NonNull DropFeedback feedback);
 }
diff --git a/rule_api/src/com/android/ide/common/api/IGraphics.java b/rule_api/src/com/android/ide/common/api/IGraphics.java
index cb1cac5..1a7f64a 100755..100644
--- a/rule_api/src/com/android/ide/common/api/IGraphics.java
+++ b/rule_api/src/com/android/ide/common/api/IGraphics.java
@@ -16,6 +16,7 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
 import com.google.common.annotations.Beta;
 
 import java.util.List;
@@ -45,7 +46,7 @@ public interface IGraphics {
      * Draws a line between 2 points, using the current foreground color and
      * alpha.
      */
-    void drawLine(Point p1, Point p2);
+    void drawLine(@NonNull Point p1, @NonNull Point p2);
 
     /**
      * Draws an arrow from (x1, y1) to (x2, y2).
@@ -76,13 +77,13 @@ public interface IGraphics {
      * Draws a rectangle outline between 2 points, using the current foreground
      * color and alpha.
      */
-    void drawRect(Point p1, Point p2);
+    void drawRect(@NonNull Point p1, @NonNull Point p2);
 
     /**
      * Draws a rectangle outline between 2 points, using the current foreground
      * color and alpha.
      */
-    void drawRect(Rect r);
+    void drawRect(@NonNull Rect r);
 
     /**
      * Fills a rectangle outline between 2 points, using the current background
@@ -94,13 +95,13 @@ public interface IGraphics {
      * Fills a rectangle outline between 2 points, using the current background
      * color and alpha.
      */
-    void fillRect(Point p1, Point p2);
+    void fillRect(@NonNull Point p1, @NonNull Point p2);
 
     /**
      * Fills a rectangle outline between 2 points, using the current background
      * color and alpha.
      */
-    void fillRect(Rect r);
+    void fillRect(@NonNull Rect r);
 
     /**
      * Draws the given string, using the current foreground color. No tab
@@ -110,7 +111,7 @@ public interface IGraphics {
      * @param x the x coordinate of the top left corner of the text.
      * @param y the y coordinate of the top left corner of the text.
      */
-    void drawString(String string, int x, int y);
+    void drawString(@NonNull String string, int x, int y);
 
     /**
      * Draws the given string, using the current foreground color. No tab
@@ -119,7 +120,7 @@ public interface IGraphics {
      * @param string the string to be drawn.
      * @param topLeft the top left corner of the text.
      */
-    void drawString(String string, Point topLeft);
+    void drawString(@NonNull String string, @NonNull Point topLeft);
 
     /**
      * Draw the given strings, using the current stroke color and alpha for the
@@ -134,7 +135,7 @@ public interface IGraphics {
      *            The actual String used is the {@link Object#toString()} value
      *            of each list item.
      */
-    void drawBoxedStrings(int x, int y, List<?> strings);
+    void drawBoxedStrings(int x, int y, @NonNull List<?> strings);
 
     /**
      * Set up the graphics context to use the given style for subsequent drawing
@@ -142,7 +143,7 @@ public interface IGraphics {
      *
      * @param style The drawing style to be used. May not be null.
      */
-    void useStyle(DrawingStyle style);
+    void useStyle(@NonNull DrawingStyle style);
 
     /**
      * Registers a color using 0x00rrggbb where each component is 0..0xFF.
@@ -155,6 +156,7 @@ public interface IGraphics {
      * to ensure that your colors work properly across multiple current and
      * future themes.
      */
+    @NonNull
     IColor registerColor(int rgb);
 
     /**
@@ -166,25 +168,27 @@ public interface IGraphics {
      * Returns the current foreground color.
      * The foreground color is used for drawing operations including when text is drawn.
      */
+    @NonNull
     IColor getForeground();
 
     /**
      * Sets the foreground color. The foreground color is used for drawing
      * operations including when text is drawn.
      */
-    void setForeground(IColor color);
+    void setForeground(@NonNull IColor color);
 
     /**
      * Returns the current background color. The background color is used for
      * fill operations.
      */
+    @NonNull
     IColor getBackground();
 
     /**
      * Sets the background color. The background color is used for fill
      * operations.
      */
-    void setBackground(IColor color);
+    void setBackground(@NonNull IColor color);
 
     /**
      * Returns the current alpha value (varies between 0 for transparent and 255
@@ -226,7 +230,7 @@ public interface IGraphics {
     /**
      * Sets the current line style.
      */
-    void setLineStyle(LineStyle style);
+    void setLineStyle(@NonNull LineStyle style);
 
     /**
      * Sets the width that will be used when drawing lines.
diff --git a/rule_api/src/com/android/ide/common/api/IMenuCallback.java b/rule_api/src/com/android/ide/common/api/IMenuCallback.java
index 6c55222..fd100ee 100644
--- a/rule_api/src/com/android/ide/common/api/IMenuCallback.java
+++ b/rule_api/src/com/android/ide/common/api/IMenuCallback.java
@@ -16,6 +16,8 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
 import com.google.common.annotations.Beta;
 
 import java.util.List;
@@ -42,6 +44,9 @@ public interface IMenuCallback {
      *            useful; however for flags it allows one to add or remove items
      *            to the flag's choices.
      */
-    void action(RuleAction action, List<? extends INode> selectedNodes, String valueId,
-            Boolean newValue);
+    void action(
+            @NonNull RuleAction action,
+            @NonNull List<? extends INode> selectedNodes,
+            @Nullable String valueId,
+            @Nullable Boolean newValue);
 }
diff --git a/rule_api/src/com/android/ide/common/api/INode.java b/rule_api/src/com/android/ide/common/api/INode.java
index 7c60d82..d957419 100755..100644
--- a/rule_api/src/com/android/ide/common/api/INode.java
+++ b/rule_api/src/com/android/ide/common/api/INode.java
@@ -17,6 +17,8 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
 import com.android.ide.common.api.IDragElement.IDragAttribute;
 import com.google.common.annotations.Beta;
 
@@ -46,6 +48,7 @@ public interface INode {
     /**
      * Returns the FQCN of the view class represented by this node.
      */
+    @NonNull
     String getFqcn();
 
     /**
@@ -60,6 +63,7 @@ public interface INode {
      *
      * @return A non-null rectangle, in canvas coordinates.
      */
+    @NonNull
     Rect getBounds();
 
     /**
@@ -67,6 +71,7 @@ public interface INode {
      *
      * @return the margins for this node, never null
      */
+    @NonNull
     Margins getMargins();
 
     /**
@@ -86,6 +91,7 @@ public interface INode {
      * This may return null if the {@link INode} was not created using a correct UiNode,
      * which is unlikely.
      */
+    @Nullable
     INode getRoot();
 
     /**
@@ -93,11 +99,13 @@ public interface INode {
      * The returned parent can be null when the node is the root element, or when the node is
      * not yet or no longer attached to the hierarchy.
      */
+    @Nullable
     INode getParent();
 
     /**
      * Returns the list of valid children nodes. The list can be empty but not null.
      */
+    @NonNull
     INode[] getChildren();
 
 
@@ -111,7 +119,7 @@ public interface INode {
      * @param undoName The UI name that will be given to the undo action.
      * @param callback The code to execute.
      */
-    void editXml(String undoName, final INodeHandler callback);
+    void editXml(@NonNull String undoName, @NonNull INodeHandler callback);
 
     // TODO define an exception that methods below will throw if editXml() is not wrapping
     // these calls.
@@ -127,7 +135,8 @@ public interface INode {
      *  depend on whether this is an Android view or a custom project view.
      * @return The node for the newly created element. Can be null if we failed to create it.
      */
-    INode appendChild(String viewFqcn);
+    @NonNull
+    INode appendChild(@NonNull String viewFqcn);
 
     /**
      * Creates a new XML element as a child of this node's XML element and inserts
@@ -143,7 +152,8 @@ public interface INode {
      *  (less than zero or larger that current last child), appends at the end.
      * @return The node for the newly created element. Can be null if we failed to create it.
      */
-    INode insertChildAt(String viewFqcn, int index);
+    @NonNull
+    INode insertChildAt(@NonNull String viewFqcn, int index);
 
     /**
      * Removes the given XML element child from this node's list of children.
@@ -152,7 +162,7 @@ public interface INode {
      *
      * @param node The child to be deleted.
      */
-    void removeChild(INode node);
+    void removeChild(@NonNull INode node);
 
     /**
      * Sets an attribute for the underlying XML element.
@@ -170,7 +180,7 @@ public interface INode {
      * @param value It's value. Cannot be null. An empty value <em>removes</em> the attribute.
      * @return Whether the attribute was actually set or not.
      */
-    boolean setAttribute(String uri, String localName, String value);
+    boolean setAttribute(@Nullable String uri, @NonNull String localName, @NonNull String value);
 
     /**
      * Returns a given XML attribute.
@@ -184,7 +194,8 @@ public interface INode {
      * @param attrName The <em>local</em> name of the attribute.
      * @return the attribute as a {@link String}, if it exists, or <code>null</code>.
      */
-    String getStringAttr(String uri, String attrName);
+    @Nullable
+    String getStringAttr(@Nullable String uri, @NonNull String attrName);
 
     /**
      * Returns the {@link IAttributeInfo} for a given attribute.
@@ -204,7 +215,8 @@ public interface INode {
      * @param attrName The <em>local</em> name of the attribute.
      * @return the {@link IAttributeInfo} if the attribute is known, or <code>null</code>.
      */
-    public IAttributeInfo getAttributeInfo(String uri, String attrName);
+    @Nullable
+    public IAttributeInfo getAttributeInfo(@Nullable String uri, @NonNull String attrName);
 
     /**
      * Returns the list of all attributes declared by this node's descriptor.
@@ -223,6 +235,7 @@ public interface INode {
      *
      * @return A non-null possibly-empty list of {@link IAttributeInfo}.
      */
+    @NonNull
     public IAttributeInfo[] getDeclaredAttributes();
 
     /**
@@ -237,6 +250,7 @@ public interface INode {
      *         which is never null because at least android.view.View will
      *         contribute attributes.
      */
+    @NonNull
     public List<String> getAttributeSources();
 
     /**
@@ -252,6 +266,7 @@ public interface INode {
      *
      * @return A non-null possibly-empty list of {@link IAttribute}.
      */
+    @NonNull
     public IAttribute[] getLiveAttributes();
 
     // -----------
diff --git a/rule_api/src/com/android/ide/common/api/INodeHandler.java b/rule_api/src/com/android/ide/common/api/INodeHandler.java
index 8a9b504..e810487 100644
--- a/rule_api/src/com/android/ide/common/api/INodeHandler.java
+++ b/rule_api/src/com/android/ide/common/api/INodeHandler.java
@@ -16,6 +16,7 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
 import com.google.common.annotations.Beta;
 
 /**
@@ -34,5 +35,5 @@ public interface INodeHandler {
      *
      * @param node The node to be operated on
      */
-    void handle(INode node);
+    void handle(@NonNull INode node);
 }
diff --git a/rule_api/src/com/android/ide/common/api/IValidator.java b/rule_api/src/com/android/ide/common/api/IValidator.java
index 29f43d5..0b88a4b 100644
--- a/rule_api/src/com/android/ide/common/api/IValidator.java
+++ b/rule_api/src/com/android/ide/common/api/IValidator.java
@@ -16,6 +16,8 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
 import com.google.common.annotations.Beta;
 
 /**
@@ -37,5 +39,6 @@ public interface IValidator {
      * @return Null if the text is valid, and otherwise a description (possibly
      *         empty) for why the text is not valid.
      */
-    String validate(String text);
+    @Nullable
+    String validate(@NonNull String text);
 }
diff --git a/rule_api/src/com/android/ide/common/api/IViewMetadata.java b/rule_api/src/com/android/ide/common/api/IViewMetadata.java
index 190a497..ea79e29 100644
--- a/rule_api/src/com/android/ide/common/api/IViewMetadata.java
+++ b/rule_api/src/com/android/ide/common/api/IViewMetadata.java
@@ -16,6 +16,7 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
 import com.google.common.annotations.Beta;
 
 import java.util.List;
@@ -40,6 +41,7 @@ public interface IViewMetadata {
      *
      * @return the user visible name of views of this type (never null)
      */
+    @NonNull
     public String getDisplayName();
 
     /**
@@ -47,13 +49,16 @@ public interface IViewMetadata {
      *
      * @return the insets for this view
      */
+    @NonNull
     public Margins getInsets();
 
     /**
      * Returns the {@link FillPreference} of this view
      *
-     * @return the {@link FillPreference} of this view
+     * @return the {@link FillPreference} of this view, never null but may be
+     *     {@link FillPreference#NONE}
      */
+    @NonNull
     public FillPreference getFillPreference();
 
     /**
@@ -62,6 +67,7 @@ public interface IViewMetadata {
      * @return a list of attribute names (not including a namespace prefix) that
      *         are commonly set for this type of view, never null
      */
+    @NonNull
     public List<String> getTopAttributes();
 
     /**
@@ -114,5 +120,4 @@ public interface IViewMetadata {
                     (!verticalContext && (this == OPPOSITE || this == HEIGHT_IN_HORIZONTAL)));
         }
     }
-
 }
diff --git a/rule_api/src/com/android/ide/common/api/IViewRule.java b/rule_api/src/com/android/ide/common/api/IViewRule.java
index c11f500..bcf4e89 100755..100644
--- a/rule_api/src/com/android/ide/common/api/IViewRule.java
+++ b/rule_api/src/com/android/ide/common/api/IViewRule.java
@@ -16,6 +16,9 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
+
 import java.util.List;
 
 
@@ -54,7 +57,7 @@ public interface IViewRule {
      * @return True if this rule can handle the given FQCN. False if the rule can't handle the
      *   given FQCN, in which case the rule engine will find another rule matching a parent class.
      */
-    boolean onInitialize(String fqcn, IClientRulesEngine engine);
+    boolean onInitialize(@NonNull String fqcn, @NonNull IClientRulesEngine engine);
 
     /**
      * This method is called by the rules engine just before the script is unloaded.
@@ -62,15 +65,16 @@ public interface IViewRule {
     void onDispose();
 
     /**
-     * Returns the class name to display when an element is selected in the GLE.
+     * Returns the class name to display when an element is selected in the layout editor.
      * <p/>
-     * If null is returned, the GLE will automatically shorten the class name using its
+     * If null is returned, the layout editor will automatically shorten the class name using its
      * own heuristic, which is to keep the first 2 package components and the class name.
      * The class name is the <code>fqcn</code> argument that was given
      * to {@link #onInitialize(String,IClientRulesEngine)}.
      *
      * @return Null for the default behavior or a shortened string.
      */
+    @Nullable
     String getDisplayName();
 
     /**
@@ -95,7 +99,7 @@ public interface IViewRule {
      *    {@link RuleAction#getSortPriority()} later.
      * @param node the node to add actions for.
      */
-    void addContextMenuActions(List<RuleAction> actions, INode node);
+    void addContextMenuActions(@NonNull List<RuleAction> actions, @NonNull INode node);
 
     /**
      * Invoked by the Rules Engine to ask the parent layout for the set of layout actions
@@ -109,8 +113,10 @@ public interface IViewRule {
      * @param parentNode the parent of the selection, or the selection itself if the root
      * @param targets the targeted/selected nodes, if any
      */
-    void addLayoutActions(List<RuleAction> actions,
-            INode parentNode, List<? extends INode> targets);
+    void addLayoutActions(
+            @NonNull List<RuleAction> actions,
+            @NonNull INode parentNode,
+            @NonNull List<? extends INode> targets);
 
     // ==== Selection ====
 
@@ -126,7 +132,8 @@ public interface IViewRule {
      * @param childNode The child node that was selected. Never null.
      * @return a list of strings to be displayed, or null or empty to display nothing
      */
-    List<String> getSelectionHint(INode parentNode, INode childNode);
+    @Nullable
+    List<String> getSelectionHint(@NonNull INode parentNode, @NonNull INode childNode);
 
     /**
      * Paints any layout-specific selection feedback for the given parent layout.
@@ -136,8 +143,11 @@ public interface IViewRule {
      * @param childNodes the child nodes selected in the parent layout
      * @param view An instance of the view to be painted (may be null)
      */
-    void paintSelectionFeedback(IGraphics graphics, INode parentNode,
-            List<? extends INode> childNodes, Object view);
+    void paintSelectionFeedback(
+            @NonNull IGraphics graphics,
+            @NonNull INode parentNode,
+            @NonNull List<? extends INode> childNodes,
+            @Nullable Object view);
 
     // ==== Drag'n'drop support ====
 
@@ -156,7 +166,9 @@ public interface IViewRule {
      *         supplied to a follow-up {@link #onDropMove} call), or null if the
      *         drop should be ignored
      */
-    DropFeedback onDropEnter(INode targetNode, Object targetView, IDragElement[] elements);
+    @Nullable
+    DropFeedback onDropEnter(@NonNull INode targetNode, @Nullable Object targetView,
+            @Nullable IDragElement[] elements);
 
     /**
      * Called after onDropEnter. Returns a DropFeedback passed to
@@ -173,10 +185,12 @@ public interface IViewRule {
      * @return a {@link DropFeedback} (which is usually just the same one passed
      *         into this method)
      */
-    DropFeedback onDropMove(INode targetNode,
-            IDragElement[] elements,
-            DropFeedback feedback,
-            Point where);
+    @Nullable
+    DropFeedback onDropMove(
+            @NonNull INode targetNode,
+            @NonNull IDragElement[] elements,
+            @Nullable DropFeedback feedback,
+            @NonNull Point where);
 
     /**
      * Called when drop leaves the target without actually dropping.
@@ -201,9 +215,10 @@ public interface IViewRule {
      * @param feedback the {@link DropFeedback} object created by
      *            {@link #onDropEnter(INode, Object, IDragElement[])}
      */
-    void onDropLeave(INode targetNode,
-            IDragElement[] elements,
-            DropFeedback feedback);
+    void onDropLeave(
+            @NonNull INode targetNode,
+            @NonNull IDragElement[] elements,
+            @Nullable DropFeedback feedback);
 
     /**
      * Called when drop is released over the target to perform the actual drop.
@@ -220,10 +235,11 @@ public interface IViewRule {
      *            {@link #onDropEnter(INode, Object, IDragElement[])}
      * @param where the mouse drop position
      */
-    void onDropped(INode targetNode,
-            IDragElement[] elements,
-            DropFeedback feedback,
-            Point where);
+    void onDropped(
+            @NonNull INode targetNode,
+            @NonNull IDragElement[] elements,
+            @Nullable DropFeedback feedback,
+            @NonNull Point where);
 
     /**
      * Called when pasting elements in an existing document on the selected target.
@@ -233,7 +249,8 @@ public interface IViewRule {
      *            null if not known
      * @param pastedElements The elements being pasted.
      */
-    void onPaste(INode targetNode, Object targetView, IDragElement[] pastedElements);
+    void onPaste(@NonNull INode targetNode, @Nullable Object targetView,
+            @NonNull IDragElement[] pastedElements);
 
     // ==== XML Creation ====
 
@@ -254,7 +271,7 @@ public interface IViewRule {
      * @param insertType whether this node was created as part of a newly created view, or
      *            as a copy, or as a move, etc.
      */
-    void onCreate(INode node, INode parent, InsertType insertType);
+    void onCreate(@NonNull INode node, @NonNull INode parent, @NonNull InsertType insertType);
 
     /**
      * Called when a child for this view has been created and is being inserted into the
@@ -269,7 +286,8 @@ public interface IViewRule {
      * @param insertType whether this node was created as part of a newly created view, or
      *            as a copy, or as a move, etc.
      */
-    void onChildInserted(INode child, INode parent, InsertType insertType);
+    void onChildInserted(@NonNull INode child, @NonNull INode parent,
+            @NonNull InsertType insertType);
 
     /**
      * Called when one or more children are about to be deleted by the user. Note that
@@ -284,7 +302,7 @@ public interface IViewRule {
      * @param parent the parent of the deleted children (which still contains the children
      *            since this method is called before the deletion is performed)
      */
-    void onRemovingChildren(List<INode> deleted, INode parent);
+    void onRemovingChildren(@NonNull List<INode> deleted, @NonNull INode parent);
 
     /**
      * Called by the IDE on the parent layout when a child widget is being resized. This
@@ -300,10 +318,14 @@ public interface IViewRule {
      * @return a {@link DropFeedback} object which performs an update painter callback
      *         etc.
      */
+    @Nullable
     DropFeedback onResizeBegin(
-            INode child, INode parent,
-            SegmentType horizEdge, SegmentType verticalEdge,
-            Object childView, Object parentView);
+            @NonNull INode child,
+            @NonNull INode parent,
+            @Nullable SegmentType horizEdge,
+            @Nullable SegmentType verticalEdge,
+            @Nullable Object childView,
+            @Nullable Object parentView);
 
     /**
      * Called by the IDE on the parent layout when a child widget is being resized. This
@@ -320,7 +342,11 @@ public interface IViewRule {
      *    of the constants {@link DropFeedback#MODIFIER1}, {@link DropFeedback#MODIFIER2}
      *    and {@link DropFeedback#MODIFIER3}.
      */
-    void onResizeUpdate(DropFeedback feedback, INode child, INode parent, Rect newBounds,
+    void onResizeUpdate(
+            @Nullable DropFeedback feedback,
+            @NonNull INode child,
+            @NonNull INode parent,
+            @NonNull Rect newBounds,
             int modifierMask);
 
     /**
@@ -335,5 +361,9 @@ public interface IViewRule {
      * @param newBounds the new bounds the user has chosen to resize the widget to,
      *    in absolute coordinates
      */
-    void onResizeEnd(DropFeedback feedback, INode child, INode parent, Rect newBounds);
+    void onResizeEnd(
+            @Nullable DropFeedback feedback,
+            @NonNull INode child,
+            @NonNull INode parent,
+            @NonNull Rect newBounds);
 }
diff --git a/rule_api/src/com/android/ide/common/api/Margins.java b/rule_api/src/com/android/ide/common/api/Margins.java
index 969d068..f38066f 100644
--- a/rule_api/src/com/android/ide/common/api/Margins.java
+++ b/rule_api/src/com/android/ide/common/api/Margins.java
@@ -17,6 +17,7 @@
 package com.android.ide.common.api;
 
 import com.google.common.annotations.Beta;
+import com.android.annotations.NonNull;
 
 /**
  * Set of margins - distances to outer left, top, right and bottom edges. These objects
@@ -56,6 +57,7 @@ public class Margins {
         this.bottom = bottom;
     }
 
+    @NonNull
     @Override
     public String toString() {
         return "Margins [left=" + left + ", right=" + right + ", top=" + top + ", bottom=" + bottom
diff --git a/rule_api/src/com/android/ide/common/api/Point.java b/rule_api/src/com/android/ide/common/api/Point.java
index 199701a..b13f12a 100755..100644
--- a/rule_api/src/com/android/ide/common/api/Point.java
+++ b/rule_api/src/com/android/ide/common/api/Point.java
@@ -17,6 +17,7 @@
 package com.android.ide.common.api;
 
 import com.google.common.annotations.Beta;
+import com.android.annotations.NonNull;
 
 
 /**
@@ -35,7 +36,7 @@ public class Point {
         this.y = y;
     }
 
-    public Point(Point p) {
+    public Point(@NonNull Point p) {
         x = p.x;
         y = p.y;
     }
@@ -47,6 +48,7 @@ public class Point {
     }
 
     /** Returns a new instance of a point with the same values. */
+    @NonNull
     public Point copy() {
         return new Point(x, y);
     }
@@ -55,6 +57,7 @@ public class Point {
      * Offsets this point by adding the given x,y deltas to the x,y coordinates.
      * @return Returns self, for chaining.
      */
+    @NonNull
     public Point offsetBy(int x, int y) {
         this.x += x;
         this.y += y;
diff --git a/rule_api/src/com/android/ide/common/api/Rect.java b/rule_api/src/com/android/ide/common/api/Rect.java
index 0ae7a74..0fb791b 100755..100644
--- a/rule_api/src/com/android/ide/common/api/Rect.java
+++ b/rule_api/src/com/android/ide/common/api/Rect.java
@@ -17,7 +17,8 @@
 package com.android.ide.common.api;
 
 import com.google.common.annotations.Beta;
-
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
 
 
 /**
@@ -46,11 +47,12 @@ public class Rect {
     }
 
     /** Initialize rectangle to the given values. They can be invalid. */
-    public Rect(Rect r) {
+    public Rect(@NonNull Rect r) {
         set(r);
     }
 
     /** Initialize rectangle to the given values. They can be invalid. */
+    @NonNull
     public Rect set(int x, int y, int w, int h) {
         this.x = x;
         this.y = y;
@@ -60,12 +62,14 @@ public class Rect {
     }
 
     /** Initialize rectangle to match the given one. */
-    public Rect set(Rect r) {
+    @NonNull
+    public Rect set(@NonNull Rect r) {
         set(r.x, r.y, r.w, r.h);
         return this;
     }
 
     /** Returns a new instance of a rectangle with the same values. */
+    @NonNull
     public Rect copy() {
         return new Rect(x, y, w, h);
     }
@@ -85,7 +89,7 @@ public class Rect {
     }
 
     /** Returns true if the rectangle contains the x,y coordinates, borders included. */
-    public boolean contains(Point p) {
+    public boolean contains(@Nullable Point p) {
         return p != null && contains(p.x, p.y);
     }
 
@@ -93,6 +97,7 @@ public class Rect {
      * Moves this rectangle by setting it's x,y coordinates to the new values.
      * @return Returns self, for chaining.
      */
+    @NonNull
     public Rect moveTo(int x, int y) {
         this.x = x;
         this.y = y;
@@ -103,31 +108,37 @@ public class Rect {
      * Offsets this rectangle by adding the given x,y deltas to the x,y coordinates.
      * @return Returns self, for chaining.
      */
+    @NonNull
     public Rect offsetBy(int x, int y) {
         this.x += x;
         this.y += y;
         return this;
     }
 
+    @NonNull
     public Point getCenter() {
         return new Point(x + (w > 0 ? w / 2 : 0),
                          y + (h > 0 ? h / 2 : 0));
     }
 
+    @NonNull
     public Point getTopLeft() {
         return new Point(x, y);
     }
 
+    @NonNull
     public Point getBottomLeft() {
         return new Point(x,
                          y + (h > 0 ? h : 0));
     }
 
+    @NonNull
     public Point getTopRight() {
         return new Point(x + (w > 0 ? w : 0),
                          y);
     }
 
+    @NonNull
     public Point getBottomRight() {
         return new Point(x + (w > 0 ? w : 0),
                          y + (h > 0 ? h : 0));
@@ -207,6 +218,7 @@ public class Rect {
      *
      * @return the center point in the rectangle
      */
+    @NonNull
     public Point center() {
         return new Point(x + w / 2, y + h / 2);
     }
diff --git a/rule_api/src/com/android/ide/common/api/ResizePolicy.java b/rule_api/src/com/android/ide/common/api/ResizePolicy.java
index 3383cd0..f48095a 100644
--- a/rule_api/src/com/android/ide/common/api/ResizePolicy.java
+++ b/rule_api/src/com/android/ide/common/api/ResizePolicy.java
@@ -15,6 +15,7 @@
  */
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
 import com.google.common.annotations.Beta;
 
 /**
@@ -40,7 +41,7 @@ public class ResizePolicy {
 
     // Shared objects for common policies
 
-    public static final ResizePolicy sAny = new ResizePolicy(ANY);
+    private static final ResizePolicy sAny = new ResizePolicy(ANY);
     private static final ResizePolicy sNone = new ResizePolicy(NONE);
     private static final ResizePolicy sHorizontal = new ResizePolicy(HORIZONTAL);
     private static final ResizePolicy sVertical = new ResizePolicy(VERTICAL);
@@ -113,6 +114,7 @@ public class ResizePolicy {
      *
      * @return a resize policy allowing resizing in any direction
      */
+    @NonNull
     public static ResizePolicy full() {
         return sAny;
     }
@@ -122,6 +124,7 @@ public class ResizePolicy {
      *
      * @return a policy which does not allow any resizing
      */
+    @NonNull
     public static ResizePolicy none() {
         return sNone;
     }
@@ -131,6 +134,7 @@ public class ResizePolicy {
      *
      * @return a policy which allows horizontal resizing only
      */
+    @NonNull
     public static ResizePolicy horizontal() {
         return sHorizontal;
     }
@@ -140,6 +144,7 @@ public class ResizePolicy {
      *
      * @return a policy which allows vertical resizing only
      */
+    @NonNull
     public static ResizePolicy vertical() {
         return sVertical;
     }
@@ -149,6 +154,7 @@ public class ResizePolicy {
      *
      * @return a resize policy allowing scaled / aspect-ratio preserving resizing only
      */
+    @NonNull
     public static ResizePolicy scaled() {
         return sScaled;
     }
@@ -163,6 +169,7 @@ public class ResizePolicy {
      * @param preserve whether the policy requires the aspect ratio to be preserved
      * @return a resize policy recording the constraints required by the parameters
      */
+    @NonNull
     public static ResizePolicy create(boolean top, boolean right, boolean bottom, boolean left,
             boolean preserve) {
         int mask = NONE;
diff --git a/rule_api/src/com/android/ide/common/api/RuleAction.java b/rule_api/src/com/android/ide/common/api/RuleAction.java
index 5b6dc7f..f6c7e8c 100755..100644
--- a/rule_api/src/com/android/ide/common/api/RuleAction.java
+++ b/rule_api/src/com/android/ide/common/api/RuleAction.java
@@ -16,6 +16,8 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
 import com.android.util.Pair;
 import com.google.common.annotations.Beta;
 
@@ -99,6 +101,7 @@ public class RuleAction implements Comparable<RuleAction> {
      * @param sortPriority a priority used for sorting this action
      * @return a new separator
      */
+    @NonNull
     public static Separator createSeparator(int sortPriority) {
         return new Separator(sortPriority, true /* supportsMultipleNodes*/);
     }
@@ -116,8 +119,14 @@ public class RuleAction implements Comparable<RuleAction> {
      *            see {@link #supportsMultipleNodes()} for details
      * @return the new {@link RuleAction}
      */
-    public static RuleAction createAction(String id, String title,
-            IMenuCallback callback, URL iconUrl, int sortPriority, boolean supportsMultipleNodes) {
+    @NonNull
+    public static RuleAction createAction(
+            @NonNull String id,
+            @NonNull String title,
+            @NonNull IMenuCallback callback,
+            @Nullable URL iconUrl,
+            int sortPriority,
+            boolean supportsMultipleNodes) {
         RuleAction action = new RuleAction(id, title, callback, sortPriority,
                 supportsMultipleNodes);
         action.setIconUrl(iconUrl);
@@ -139,8 +148,14 @@ public class RuleAction implements Comparable<RuleAction> {
      *            see {@link #supportsMultipleNodes()} for details
      * @return the new {@link Toggle}
      */
-    public static Toggle createToggle(String id, String title, boolean isChecked,
-            IMenuCallback callback, URL iconUrl, int sortPriority,
+    @NonNull
+    public static Toggle createToggle(
+            @NonNull String id,
+            @NonNull String title,
+            boolean isChecked,
+            @NonNull IMenuCallback callback,
+            @Nullable URL iconUrl,
+            int sortPriority,
             boolean supportsMultipleNodes) {
         Toggle toggle = new Toggle(id, title, isChecked, callback, sortPriority,
                 supportsMultipleNodes);
@@ -165,9 +180,15 @@ public class RuleAction implements Comparable<RuleAction> {
      *            see {@link #supportsMultipleNodes()} for details
      * @return the new {@link NestedAction}
      */
-    public static NestedAction createChoices(String id, String title,
-            IMenuCallback callback, URL iconUrl,
-            int sortPriority, boolean supportsMultipleNodes, ActionProvider provider) {
+    @NonNull
+    public static NestedAction createChoices(
+            @NonNull String id,
+            @NonNull String title,
+            @NonNull IMenuCallback callback,
+            @Nullable URL iconUrl,
+            int sortPriority,
+            boolean supportsMultipleNodes,
+            @NonNull ActionProvider provider) {
         NestedAction choices = new NestedAction(id, title, provider, callback,
                 sortPriority, supportsMultipleNodes);
         choices.setIconUrl(iconUrl);
@@ -196,9 +217,18 @@ public class RuleAction implements Comparable<RuleAction> {
      *            see {@link #supportsMultipleNodes()} for details
      * @return the new {@link Choices}
      */
-    public static Choices createChoices(String id, String title,
-            IMenuCallback callback, List<String> titles, List<URL> iconUrls, List<String> ids,
-            String current, URL iconUrl, int sortPriority, boolean supportsMultipleNodes) {
+    @NonNull
+    public static Choices createChoices(
+            @NonNull String id,
+            @NonNull String title,
+            @NonNull IMenuCallback callback,
+            @NonNull List<String> titles,
+            @Nullable List<URL> iconUrls,
+            @NonNull List<String> ids,
+            @Nullable String current,
+            @Nullable URL iconUrl,
+            int sortPriority,
+            boolean supportsMultipleNodes) {
         Choices choices = new Choices(id, title, callback, titles, iconUrls,
                 ids, current, sortPriority, supportsMultipleNodes);
         choices.setIconUrl(iconUrl);
@@ -228,10 +258,17 @@ public class RuleAction implements Comparable<RuleAction> {
      *            menu items
      * @return the new {@link Choices}
      */
-    public static Choices createChoices(String id, String title,
-            IMenuCallback callback, List<URL> iconUrls,
-            String current, URL iconUrl, int sortPriority,
-            boolean supportsMultipleNodes, List<Pair<String, String>> idsAndTitles) {
+    @NonNull
+    public static Choices createChoices(
+            @NonNull String id,
+            @NonNull String title,
+            @NonNull IMenuCallback callback,
+            @Nullable List<URL> iconUrls,
+            @Nullable String current,
+            @Nullable URL iconUrl,
+            int sortPriority,
+            boolean supportsMultipleNodes,
+            @NonNull List<Pair<String, String>> idsAndTitles) {
         int itemCount = idsAndTitles.size();
         List<String> titles = new ArrayList<String>(itemCount);
         List<String> ids = new ArrayList<String>(itemCount);
@@ -264,9 +301,16 @@ public class RuleAction implements Comparable<RuleAction> {
      *            see {@link #supportsMultipleNodes()} for details
      * @return the new {@link Choices}
      */
-    public static Choices createChoices(String id, String title,
-            IMenuCallback callback, ChoiceProvider provider,
-            String current, URL iconUrl, int sortPriority, boolean supportsMultipleNodes) {
+    @NonNull
+    public static Choices createChoices(
+            @NonNull String id,
+            @NonNull String title,
+            IMenuCallback callback,
+            @NonNull ChoiceProvider provider,
+            @Nullable String current,
+            @Nullable URL iconUrl,
+            int sortPriority,
+            boolean supportsMultipleNodes) {
         Choices choices = new DelayedChoices(id, title, callback,
                 current, provider, sortPriority, supportsMultipleNodes);
         choices.setIconUrl(iconUrl);
@@ -286,7 +330,11 @@ public class RuleAction implements Comparable<RuleAction> {
      * @param supportsMultipleNodes the new return value for
      *            {@link #supportsMultipleNodes()}
      */
-    private RuleAction(String id, String title, IMenuCallback callback, int sortPriority,
+    private RuleAction(
+            @NonNull String id,
+            @NonNull String title,
+            @NonNull IMenuCallback callback,
+            int sortPriority,
             boolean supportsMultipleNodes) {
         mId = id;
         mTitle = title;
@@ -302,6 +350,7 @@ public class RuleAction implements Comparable<RuleAction> {
      *
      * @return the unique id of the action, never null
      */
+    @NonNull
     public String getId() {
         return mId;
     }
@@ -312,6 +361,7 @@ public class RuleAction implements Comparable<RuleAction> {
      *
      * @return the user name of the action, never null
      */
+    @NonNull
     public String getTitle() {
         return mTitle;
     }
@@ -371,6 +421,7 @@ public class RuleAction implements Comparable<RuleAction> {
      * @param iconUrl a URL pointing to an icon to use for this action, or null
      * @return this action, to allow setter chaining
      */
+    @NonNull
     public RuleAction setIconUrl(URL iconUrl) {
         mIconUrl = iconUrl;
 
@@ -392,13 +443,14 @@ public class RuleAction implements Comparable<RuleAction> {
      *
      * @return the callback, never null
      */
+    @NonNull
     public IMenuCallback getCallback() {
         return mCallback;
     }
 
     // Implements Comparable<MenuAciton>
     @Override
-    public int compareTo(RuleAction other) {
+    public int compareTo(@NonNull RuleAction other) {
         if (mSortPriority != other.mSortPriority) {
             return mSortPriority - other.mSortPriority;
         }
@@ -406,6 +458,7 @@ public class RuleAction implements Comparable<RuleAction> {
         return mTitle.compareTo(other.mTitle);
     }
 
+    @NonNull
     @Override
     public String toString() {
         return "RuleAction [id=" + mId + ", title=" + mTitle + ", priority=" + mSortPriority + "]";
@@ -442,8 +495,13 @@ public class RuleAction implements Comparable<RuleAction> {
          * @param callback A callback to execute when the context menu item is
          *            selected.
          */
-        private Toggle(String id, String title, boolean isChecked,
-                IMenuCallback callback, int sortPriority, boolean supportsMultipleNodes) {
+        private Toggle(
+                @NonNull String id,
+                @NonNull String title,
+                boolean isChecked,
+                @NonNull IMenuCallback callback,
+                int sortPriority,
+                boolean supportsMultipleNodes) {
             super(id, title, callback, sortPriority, supportsMultipleNodes);
             mIsChecked = isChecked;
         }
@@ -491,9 +549,16 @@ public class RuleAction implements Comparable<RuleAction> {
          */
         protected final String mCurrent;
 
-        private Choices(String id, String title, IMenuCallback callback,
-                List<String> titles, List<URL> iconUrls, List<String> ids, String current,
-                int sortPriority, boolean supportsMultipleNodes) {
+        private Choices(
+                @NonNull String id,
+                @NonNull String title,
+                @NonNull IMenuCallback callback,
+                @NonNull List<String> titles,
+                @Nullable List<URL> iconUrls,
+                @NonNull List<String> ids,
+                @Nullable String current,
+                int sortPriority,
+                boolean supportsMultipleNodes) {
             super(id, title, callback, sortPriority, supportsMultipleNodes);
             mTitles = titles;
             mIconUrls = iconUrls;
@@ -506,6 +571,7 @@ public class RuleAction implements Comparable<RuleAction> {
          *
          * @return the list of urls to icons to display for each choice, or null
          */
+        @Nullable
         public List<URL> getIconUrls() {
             return mIconUrls;
         }
@@ -515,6 +581,7 @@ public class RuleAction implements Comparable<RuleAction> {
          *
          * @return the list of ids for the menu choices, never null
          */
+        @NonNull
         public List<String> getIds() {
             return mIds;
         }
@@ -524,6 +591,7 @@ public class RuleAction implements Comparable<RuleAction> {
          *
          * @return the titles to be displayed for the menu choices, never null
          */
+        @NonNull
         public List<String> getTitles() {
             return mTitles;
         }
@@ -533,6 +601,7 @@ public class RuleAction implements Comparable<RuleAction> {
          *
          * @return the current value of the choice, possibly null
          */
+        @Nullable
         public String getCurrent() {
             return mCurrent;
         }
@@ -566,8 +635,12 @@ public class RuleAction implements Comparable<RuleAction> {
         /** The provider to produce the list of nested actions when needed */
         private final ActionProvider mProvider;
 
-        private NestedAction(String id, String title, ActionProvider provider,
-                IMenuCallback callback, int sortPriority,
+        private NestedAction(
+                @NonNull String id,
+                @NonNull String title,
+                @NonNull ActionProvider provider,
+                @NonNull IMenuCallback callback,
+                int sortPriority,
                 boolean supportsMultipleNodes) {
             super(id, title, callback, sortPriority, supportsMultipleNodes);
             mProvider = provider;
@@ -579,7 +652,8 @@ public class RuleAction implements Comparable<RuleAction> {
          * @param node the node to look up nested actions for
          * @return a list of nested actions
          */
-        public List<RuleAction> getNestedActions(INode node) {
+        @NonNull
+        public List<RuleAction> getNestedActions(@NonNull INode node) {
             return mProvider.getNestedActions(node);
         }
     }
@@ -587,21 +661,23 @@ public class RuleAction implements Comparable<RuleAction> {
     /** Like {@link Choices}, but the set of choices is computed lazily */
     private static class DelayedChoices extends Choices {
         private final ChoiceProvider mProvider;
-
-        private DelayedChoices(String id, String title,
-                IMenuCallback callback, String current, ChoiceProvider provider,
+        private boolean mInitialized;
+
+        private DelayedChoices(
+                @NonNull String id,
+                @NonNull String title,
+                @NonNull IMenuCallback callback,
+                @Nullable String current,
+                @NonNull ChoiceProvider provider,
                 int sortPriority, boolean supportsMultipleNodes) {
-            super(id, title, callback, null, null, null, current, sortPriority,
-                    supportsMultipleNodes);
+            super(id, title, callback, new ArrayList<String>(), new ArrayList<URL>(),
+                    new ArrayList<String>(), current, sortPriority, supportsMultipleNodes);
             mProvider = provider;
         }
 
         private void ensureInitialized() {
-            if (mTitles == null) {
-                mTitles = new ArrayList<String>();
-                mIconUrls = new ArrayList<URL>();
-                mIds = new ArrayList<String>();
-
+            if (!mInitialized) {
+                mInitialized = true;
                 mProvider.addChoices(mTitles, mIconUrls, mIds);
             }
         }
@@ -637,7 +713,8 @@ public class RuleAction implements Comparable<RuleAction> {
          * @param node the node to look up nested actions for
          * @return a list of nested actions
          */
-        public List<RuleAction> getNestedActions(INode node);
+        @NonNull
+        public List<RuleAction> getNestedActions(@NonNull INode node);
     }
 
     /**
@@ -654,6 +731,9 @@ public class RuleAction implements Comparable<RuleAction> {
          * @param iconUrls a list of icon URLs that the provider should append to
          * @param ids a list of ids that the provider should append to
          */
-        public void addChoices(List<String> titles, List<URL> iconUrls, List<String> ids);
+        public void addChoices(
+                @NonNull List<String> titles,
+                @NonNull List<URL> iconUrls,
+                @NonNull List<String> ids);
     }
 }
diff --git a/rule_api/src/com/android/ide/common/api/Segment.java b/rule_api/src/com/android/ide/common/api/Segment.java
index 681fd14..9f03286 100644
--- a/rule_api/src/com/android/ide/common/api/Segment.java
+++ b/rule_api/src/com/android/ide/common/api/Segment.java
@@ -16,6 +16,8 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
 import com.google.common.annotations.Beta;
 
 /**
@@ -38,25 +40,29 @@ public class Segment {
     public final int to;
 
     /** Whether the edge is a top edge, a baseline edge, a left edge, etc */
+    @NonNull
     public final SegmentType edgeType;
 
     /**
      * Whether the edge is offset from the node by a margin or not, or whether it has no
      * margin
      */
+    @NonNull
     public final MarginType marginType;
 
     /** The node that contains this edge */
+    @NonNull
     public final INode node;
 
     /**
      * The id of the node. May be null (in which case id should be generated when
      * move/resize is completed
      */
+    @Nullable
     public final String id;
 
-    public Segment(int at, int from, int to, INode node, String id, SegmentType edgeType,
-            MarginType marginType) {
+    public Segment(int at, int from, int to, @NonNull INode node, @Nullable String id,
+            @NonNull SegmentType edgeType, @NonNull MarginType marginType) {
         this.at = at;
         this.from = from;
         this.to = to;
@@ -66,6 +72,7 @@ public class Segment {
         this.marginType = marginType;
     }
 
+    @NonNull
     @Override
     public String toString() {
         String nodeStr = node == null ? "null" : node.getFqcn().substring(
diff --git a/rule_api/src/com/android/ide/common/api/SegmentType.java b/rule_api/src/com/android/ide/common/api/SegmentType.java
index 191d753..25635c7 100644
--- a/rule_api/src/com/android/ide/common/api/SegmentType.java
+++ b/rule_api/src/com/android/ide/common/api/SegmentType.java
@@ -16,6 +16,8 @@
 
 package com.android.ide.common.api;
 
+import com.android.annotations.NonNull;
+import com.android.annotations.Nullable;
 import com.google.common.annotations.Beta;
 
 /** A segment type describes the different roles or positions a segment can have in a node
@@ -38,7 +40,7 @@ public enum SegmentType {
      * @param bounds the bounds of the node
      * @return the X coordinate for an edge of this type given its bounds
      */
-    public int getX(INode node, Rect bounds) {
+    public int getX(@Nullable INode node, @NonNull Rect bounds) {
         // We pass in the bounds rather than look it up via node.getBounds() because
         // during a resize or move operation, we call this method to look up proposed
         // bounds rather than actual bounds
@@ -66,7 +68,7 @@ public enum SegmentType {
      * @param bounds the bounds of the node
      * @return the Y coordinate for an edge of this type given its bounds
      */
-    public int getY(INode node, Rect bounds) {
+    public int getY(@Nullable INode node, @NonNull Rect bounds) {
         switch (this) {
             case TOP:
                 return bounds.y;