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+page.title=How Android Draws Views
+parent.title=User Interface
+parent.link=index.html
+@jd:body
+
+
+<p>When an Activity receives focus, it will be requested to draw its layout.
+The Android framework will handle the procedure for drawing, but the Activity must provide
+the root node of its layout hierarchy.</p>
+
+<p>Drawing begins with the root node of the layout. It is requested to measure and 
+draw the layout tree. Drawing is handled by walking the tree and rendering each View that
+   intersects the invalid region. In turn, each View group is responsible for requesting
+each of its children to be drawn (with the <code>{@link android.view.View#draw(Canvas) draw()}</code> method) 
+and each View is responsible for drawing itself.
+ Because the tree is traversed in-order,
+   this means that parents will be drawn before (i.e., behind) their children, with
+   siblings drawn in the order they appear in the tree.
+   </p>
+
+<div class="sidebox">
+  <p>The framework will not draw Views that are not in the invalid region, and also 
+   will take care of drawing the Views background for you.</p>
+   <p>You can force a View to draw, by calling <code>{@link android.view.View#invalidate()}</code>.
+   </p>
+</div>
+
+<p>
+   Drawing the layout is a two pass process: a measure pass and a layout pass. The measuring
+   pass is implemented in <code>{@link android.view.View#measure(int, int)}</code> and is a top-down traversal
+   of the View tree. Each View pushes dimension specifications down the tree
+   during the recursion. At the end of the measure pass, every View has stored
+   its measurements. The second pass happens in
+   <code>{@link android.view.View#layout(int,int,int,int)}</code> and is also top-down. During
+   this pass each parent is responsible for positioning all of its children
+   using the sizes computed in the measure pass.
+   </p>
+   
+   <p>
+   When a View's <code>measure()</code> method returns, its <code>{@link android.view.View#getMeasuredWidth()}</code> and
+   <code>{@link android.view.View#getMeasuredHeight()}</code> values must be set, along with those for all of
+   that View's descendants. A View's measured width and measured height values
+   must respect the constraints imposed by the View's parents. This guarantees
+   that at the end of the measure pass, all parents accept all of their
+   children's measurements. A parent View may call <code>measure()</code> more than once on
+   its children. For example, the parent may measure each child once with
+   unspecified dimensions to find out how big they want to be, then call
+   <code>measure()</code> on them again with actual numbers if the sum of all the children's
+   unconstrained sizes is too big or too small (i.e., if the children don't agree among themselves
+  as to how much space they each get, the parent will intervene and set the rules on the second pass).
+   </p>
+   
+   <div class="sidebox"><p>
+   To intiate a layout, call <code>{@link android.view.View#requestLayout}</code>. This method is typically
+   called by a View on itself when it believes that is can no longer fit within
+   its current bounds.</p>
+   </div>
+
+   <p>
+   The measure pass uses two classes to communicate dimensions. The
+   {@link android.view.View.MeasureSpec} class is used by Views to tell their parents how they
+   want to be measured and positioned. The base LayoutParams class just
+   describes how big the View wants to be for both width and height. For each
+   dimension, it can specify one of:</p>
+   <ul>
+    <li> an exact number
+    <li><var>FILL_PARENT</var>, which means the View wants to be as big as its parent
+    (minus padding)</li>
+    <li><var>WRAP_CONTENT</var>, which means that the View wants to be just big enough to
+    enclose its content (plus padding).</li>
+   </ul>
+  <p>There are subclasses of LayoutParams for different subclasses of ViewGroup.
+   For example, AbsoluteLayout has its own subclass of LayoutParams which adds
+   an X and Y value.
+   </p>
+   
+   <p>
+   MeasureSpecs are used to push requirements down the tree from parent to
+   child. A MeasureSpec can be in one of three modes:</p>
+   <ul>
+    <li><var>UNSPECIFIED</var>: This is used by a parent to determine the desired dimension
+    of a child View. For example, a LinearLayout may call <code>measure()</code> on its child
+    with the height set to <var>UNSPECIFIED</var> and a width of <var>EXACTLY</var> 240 to find out how
+    tall the child View wants to be given a width of 240 pixels.</li>
+    <li><var>EXACTLY</var>: This is used by the parent to impose an exact size on the
+    child. The child must use this size, and guarantee that all of its
+    descendants will fit within this size.</li>
+    <li><var>AT_MOST</var>: This is used by the parent to impose a maximum size on the
+    child. The child must gurantee that it and all of its descendants will fit
+    within this size.</li>
+   </ul>
+   
+
+