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diff --git a/docs/html/guide/topics/ui/drag-drop.jd b/docs/html/guide/topics/ui/drag-drop.jd new file mode 100644 index 0000000..588b05b --- /dev/null +++ b/docs/html/guide/topics/ui/drag-drop.jd @@ -0,0 +1,995 @@ +page.title=Dragging and Dropping +@jd:body +<div id="qv-wrapper"> + <div id="qv"> + <h2>Quickview</h2> + <ul> + <li> + Allow users to move data within your Activity layout using graphical gestures. + </li> + <li> + Supports operations besides data movement. + </li> + <li> + Only works within a single application. + </li> + <li> + Requires API 11. + </li> + </ul> + <h2>In this document</h2> + <ol> + <li> + <a href="#AboutDragging">Overview</a> + <ol> + <li> + <a href="#DragDropLifecycle">The drag/drop process</a> + </li> + <li> + <a href="#AboutDragListeners">The drag event listener and callback method</a> + </li> + <li> + <a href="#AboutDragEvent">Drag events</a> + </li> + <li> + <a href="#AboutDragShadowBuilder"> + The drag shadow</a> + </li> + </ol> + </li> + <li> + <a href="#DesignDragOperation">Designing a Drag and Drop Operation</a> + <ol> + <li> + <a href="#StartDrag">Starting a drag</a> + </li> + <li> + <a href="#HandleStart">Responding to a drag start</a> + </li> + <li> + <a href="#HandleDuring">Handling events during the drag</a> + </li> + <li> + <a href="#HandleDrop">Responding to a drop</a> + </li> + <li> + <a href="#HandleEnd">Responding to a drag end</a> + </li> + <li> + <a href="#RespondEventSample">Responding to drag events: an example</a> + </li> + </ol> + </li> + </ol> + <h2>Key classes</h2> + <ol> + <li> + {@link android.view.View View} + </li> + <li> + {@link android.view.View.OnLongClickListener OnLongClickListener} + </li> + <li> + {@link android.view.View.OnDragListener OnDragListener} + </li> + <li> + {@link android.view.DragEvent DragEvent} + </li> + <li> + {@link android.view.View.DragShadowBuilder DragShadowBuilder} + </li> + <li> + {@link android.content.ClipData ClipData} + </li> + <li> + {@link android.content.ClipDescription ClipDescription} + </li> + </ol> + <h2>Related Samples</h2> + <ol> + <li> + <a href="{@docRoot}resources/samples/Honeycomb-Gallery/index.html"> + Honeycomb-Gallery</a> sample application. + </li> + <li> + <a href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/view/DragAndDropDemo.html"> +DragAndDropDemo.java</a> and + <a href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/view/DraggableDot.html"> +DraggableDot.java</a> in <a href="{@docRoot}resources/samples/ApiDemos/index.html">Api Demos</a>. + </li> + </ol> + <h2>See also</h2> + <ol> + <li> + <a href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a> + </li> + <li> + <a href="{@docRoot}guide/topics/ui/ui-events.html">Handling UI Events</a> + </li> + </ol> + </div> +</div> +<p> + With the Android drag/drop framework, you can allow your users to move data + from one View to another View in the current layout using a graphical drag and drop gesture. + The framework includes a drag event class, drag listeners, and helper methods and classes. +</p> +<p> + Although the framework is primarily designed for data movement, you can use + it for other UI actions. For example, you could create an app that mixes colors when the user + drags a color icon over another icon. The rest of this topic, however, describes the + framework in terms of data movement. +</p> +<h2 id="AboutDragging">Overview</h2> +<p> + A drag and drop operation starts when the user makes some gesture that you recognize as a + signal to start dragging data. In response, your application tells the system that the drag is + starting. The system calls back to your application to get a representation of the data + being dragged. As the user's finger moves this representation (a "drag shadow") + over the current layout, the system sends drag events to the drag event listener objects and + drag event callback methods associated with the {@link android.view.View} objects in the layout. + Once the user releases the drag shadow, the system ends the drag operation. +</p> +<p> + You create a drag event listener object ("listeners") from a class that implements + {@link android.view.View.OnDragListener}. You set the drag event listener object for a View + with the View object's + {@link android.view.View#setOnDragListener(View.OnDragListener) setOnDragListener()} method. + Each View object also has a {@link android.view.View#onDragEvent(DragEvent) onDragEvent()} + callback method. Both of these are described in more detail in the section + <a href="#AboutDragListeners">The drag event listener and callback method</a>. +</p> +<p class="note"> + <strong>Note</strong>: For the sake of simplicity, the following sections refer to the routine + that receives drag events as the "drag event listener", even though it may actually + be a callback method. +</p> +<p> + When you start a drag, you include both the data you are moving and metadata describing this + data as part of the call to the system. During the drag, the system sends drag events to the + drag event listeners or callback methods of each View in the layout. The listeners or callback + methods can use the metadata to decide if they want to accept the data when it is dropped. + If the user drops the data over a View object, and that View object's listener or callback + method has previously told the system that it wants to accept the drop, then the system sends + the data to the listener or callback method in a drag event. +</p> +<p> + Your application tells the system to start a drag by calling the + {@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()} + method. This tells the system to start sending drag events. The method also sends the data that + you are dragging. +</p> +<p> + You can call + {@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()} + for any attached View in the current layout. The system only uses the View object to get access + to global settings in your layout. +</p> +<p> + Once your application calls + {@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()}, + the rest of the process uses events that the system sends to the View objects in your current + layout. +</p> +<h3 id="DragDropLifecycle">The drag/drop process</h3> +<p> + There are basically four steps or states in the drag and drop process: +</p> +<dl> + <dt> + <em>Started</em> + </dt> + <dd> + In response to the user's gesture to begin a drag, your application calls + {@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()} + to tell the system to start a drag. The arguments + {@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()} + provide the data to be dragged, metadata for this data, and a callback for drawing the + drag shadow. + <p> + The system first responds by calling back to your application to get a drag shadow. It + then displays the drag shadow on the device. + </p> + <p> + Next, the system sends a drag event with action type + {@link android.view.DragEvent#ACTION_DRAG_STARTED} to the drag event listeners for + all the View objects in the current layout. To continue to receive drag events, + including a possible drop event, a drag event listener must return <code>true</code>. + This registers the listener with the system. Only registered listeners continue to + receive drag events. At this point, listeners can also change the appearance of their + View object to show that the listener can accept a drop event. + </p> + <p> + If the drag event listener returns <code>false</code>, then it will not receive drag + events for the current operation until the system sends a drag event with action type + {@link android.view.DragEvent#ACTION_DRAG_ENDED}. By sending <code>false</code>, the + listener tells the system that it is not interested in the drag operation and + does not want to accept the dragged data. + </p> + </dd> + <dt> + <em>Continuing</em> + </dt> + <dd> + The user continues the drag. As the drag shadow intersects the bounding box of a View + object, the system sends one or more drag events to the View object's drag event + listener (if it is registered to receive events). The listener may choose to + alter its View object's appearance in response to the event. For example, if the event + indicates that the drag shadow has entered the bounding box of the View + (action type {@link android.view.DragEvent#ACTION_DRAG_ENTERED}), the listener + can react by highlighting its View. + </dd> + <dt> + <em>Dropped</em> + </dt> + <dd> + The user releases the drag shadow within the bounding box of a View that can accept the + data. The system sends the View object's listener a drag event with action type + {@link android.view.DragEvent#ACTION_DROP}. The drag event contains the data that was + passed to the system in the call to + {@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()} + that started the operation. The listener is expected to return boolean <code>true</code> to + the system if code for accepting the drop succeeds. + <p> + Note that this step only occurs if the user drops the drag shadow within the bounding + box of a View whose listener is registered to receive drag events. If the user releases + the drag shadow in any other situation, no {@link android.view.DragEvent#ACTION_DROP} + drag event is sent. + </p> + </dd> + <dt> + <em>Ended</em> + </dt> + <dd> + After the user releases the drag shadow, and after the system sends out (if necessary) + a drag event with action type {@link android.view.DragEvent#ACTION_DROP}, the system sends + out a drag event with action type {@link android.view.DragEvent#ACTION_DRAG_ENDED} to + indicate that the drag operation is over. This is done regardless of where the user released + the drag shadow. The event is sent to every listener that is registered to receive drag + events, even if the listener received the {@link android.view.DragEvent#ACTION_DROP} event. + </dd> +</dl> +<p> + Each of these four steps is described in more detail in the section + <a href="#DesignDragOperation">Designing a Drag and Drop Operation</a>. +</p> +<h3 id="AboutDragListeners">The drag event listener and callback method</h3> +<p> + A View receives drag events with either a drag event listener that implements + {@link android.view.View.OnDragListener} or with its + {@link android.view.View#onDragEvent(DragEvent)} callback method. + When the system calls the method or listener, it passes to them + a {@link android.view.DragEvent} object. +</p> +<p> + You will probably want to use the listener in most cases. When you design UIs, you usually + don't subclass View classes, but using the callback method forces you to do this in order to + override the method. In comparison, you can implement one listener class and then use it with + several different View objects. You can also implement it as an anonymous inline class. To + set the listener for a View object, call +{@link android.view.View#setOnDragListener(android.view.View.OnDragListener) setOnDragListener()}. +</p> +<p> + You can have both a listener and a callback method for View object. If this occurs, + the system first calls the listener. The system doesn't call the callback method unless the + listener returns <code>false</code>. +</p> +<p> + The combination of the {@link android.view.View#onDragEvent(DragEvent)} method and + {@link android.view.View.OnDragListener} is analogous to the combination + of the {@link android.view.View#onTouchEvent(MotionEvent) onTouchEvent()} and + {@link android.view.View.OnTouchListener} used with touch events. +</p> +<h3 id="AboutDragEvent">Drag events</h3> +<p> + The system sends out a drag event in the form of a {@link android.view.DragEvent} object. The + object contains an action type that tells the listener what is happening in the drag/drop + process. The object contains other data, depending on the action type. +</p> +<p> + To get the action type, a listener calls {@link android.view.DragEvent#getAction()}. There + are six possible values, defined by constants in the {@link android.view.DragEvent} class. These + are listed in <a href="table1">table 1</a>. +</p> +<p> + The {@link android.view.DragEvent} object also contains the data that your application provided + to the system in the call to + {@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()}. + Some of the data is valid only for certain action types. The data that is valid for each action + type is summarized in <a href="table2">table 2</a>. It is also described in detail with + the event for which it is valid in the section + <a href="#DesignDragOperation">Designing a Drag and Drop Operation</a>. +</p> +<p class="table-caption" id="table1"> + <strong>Table 1.</strong> DragEvent action types +</p> +<table> + <tr> + <th scope="col">getAction() value</th> + <th scope="col">Meaning</th> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_STARTED}</td> + <td> + A View object's drag event listener receives this event action type just after the + application calls +{@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()} and + gets a drag shadow. + </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_ENTERED}</td> + <td> + A View object's drag event listener receives this event action type when the drag shadow + has just entered the bounding box of the View. This is the first event action type the + listener receives when the drag shadow enters the bounding box. If the listener wants to + continue receiving drag events for this operation, it must return boolean + <code>true</code> to the system. + </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_LOCATION}</td> + <td> + A View object's drag event listener receives this event action type after it receives a + {@link android.view.DragEvent#ACTION_DRAG_ENTERED} event while the drag shadow is + still within the bounding box of the View. + </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_EXITED}</td> + <td> + A View object's drag event listener receives this event action type after it receives a + {@link android.view.DragEvent#ACTION_DRAG_ENTERED} and at least one + {@link android.view.DragEvent#ACTION_DRAG_LOCATION} event, and after the user has moved + the drag shadow outside the bounding box of the View. + </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DROP}</td> + <td> + A View object's drag event listener receives this event action type when the user + releases the drag shadow over the View object. This action type is only sent to a View + object's listener if the listener returned boolean <code>true</code> in response to the + {@link android.view.DragEvent#ACTION_DRAG_STARTED} drag event. This action type is not + sent if the user releases the drag shadow on a View whose listener is not registered, + or if the user releases the drag shadow on anything that is not part of the current + layout. + <p> + The listener is expected to return boolean <code>true</code> if it successfully + processes the drop. Otherwise, it should return <code>false</code>. + </p> + </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_ENDED}</td> + <td> + A View object's drag event listener receives this event action type + when the system is ending the drag operation. This action type is not necessarily + preceded by an {@link android.view.DragEvent#ACTION_DROP} event. If the system sent + a {@link android.view.DragEvent#ACTION_DROP}, receiving the + {@link android.view.DragEvent#ACTION_DRAG_ENDED} action type does not imply that the + drop operation succeeded. The listener must call + {@link android.view.DragEvent#getResult()} to get the value that was + returned in response to {@link android.view.DragEvent#ACTION_DROP}. If an + {@link android.view.DragEvent#ACTION_DROP} event was not sent, then + {@link android.view.DragEvent#getResult()} returns <code>false</code>. + </td> + </tr> +</table> +<p class="table-caption" id="table2"> + <strong>Table 2.</strong> Valid DragEvent data by action type</p> +<table> + <tr> + <th scope="col">{@link android.view.DragEvent#getAction()} value</th> + <th scope="col">{@link android.view.DragEvent#getClipDescription()} value</th> + <th scope="col">{@link android.view.DragEvent#getLocalState()} value</th> + <th scope="col">{@link android.view.DragEvent#getX()} value</th> + <th scope="col">{@link android.view.DragEvent#getY()} value</th> + <th scope="col">{@link android.view.DragEvent#getClipData()} value</th> + <th scope="col">{@link android.view.DragEvent#getResult()} value</th> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_STARTED}</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;"> </td> + <td style="text-align: center;"> </td> + <td style="text-align: center;"> </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_ENTERED}</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;"> </td> + <td style="text-align: center;"> </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_LOCATION}</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;"> </td> + <td style="text-align: center;"> </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_EXITED}</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;"> </td> + <td style="text-align: center;"> </td> + <td style="text-align: center;"> </td> + <td style="text-align: center;"> </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DROP}</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;"> </td> + </tr> + <tr> + <td>{@link android.view.DragEvent#ACTION_DRAG_ENDED}</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;">X</td> + <td style="text-align: center;"> </td> + <td style="text-align: center;"> </td> + <td style="text-align: center;"> </td> + <td style="text-align: center;">X</td> + </tr> +</table> +<p> + The {@link android.view.DragEvent#getAction()}, + {@link android.view.DragEvent#describeContents()}, + {@link android.view.DragEvent#writeToParcel(Parcel,int) writeToParcel()}, and + {@link android.view.DragEvent#toString()} methods always return valid data. +</p> +<p> + If a method does not contain valid data for a particular action type, it returns either + <code>null</code> or 0, depending on its result type. +</p> +<h3 id="AboutDragShadowBuilder"> + The drag shadow +</h3> +<p> + During a drag and drop operation, the system displays a image that the user drags. + For data movement, this image represents the data being dragged. For other operations, the + image represents some aspect of the drag operation. +</p> +<p> + The image is called a drag shadow. You create it with methods you declare for a + {@link android.view.View.DragShadowBuilder} object, and then pass it to the system when you + start a drag using + {@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()}. + As part of its response to + {@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()}, + the system invokes the callback methods you've defined in + {@link android.view.View.DragShadowBuilder} to obtain a drag shadow. +</p> +<p> + The {@link android.view.View.DragShadowBuilder} class has two constructors: +</p> + <dl> + <dt>{@link android.view.View.DragShadowBuilder#View.DragShadowBuilder(View)}</dt> + <dd> + This constructor accepts any of your application's + {@link android.view.View} objects. The constructor stores the View object + in the {@link android.view.View.DragShadowBuilder} object, so during + the callback you can access it as you construct your drag shadow. + It doesn't have to be associated with the View (if any) that the user + selected to start the drag operation. + <p> + If you use this constructor, you don't have to extend + {@link android.view.View.DragShadowBuilder} or override its methods. By default, + you will get a drag shadow that has the same appearance as the View you pass as an + argument, centered under the location where the user is touching the screen. + </p> + </dd> + <dt>{@link android.view.View.DragShadowBuilder#View.DragShadowBuilder()}</dt> + <dd> + If you use this constructor, no View object is available in the + {@link android.view.View.DragShadowBuilder} object (the field is set to <code>null</code>). + If you use this constructor, and you don't extend + {@link android.view.View.DragShadowBuilder} or override its methods, + you will get an invisible drag shadow. + The system does <em>not</em> give an error. + </dd> +</dl> +<p> + The {@link android.view.View.DragShadowBuilder} class has two methods: +</p> +<dl> + <dt> +{@link android.view.View.DragShadowBuilder#onProvideShadowMetrics(Point,Point) onProvideShadowMetrics()} + </dt> + <dd> + The system calls this method immediately after you call +{@link android.view.View#startDrag(ClipData,View.DragShadowBuilder,Object,int) startDrag()}. Use it + to send to the system the dimensions and touch point of the drag shadow. The method has two + arguments: + <dl> + <dt><em>dimensions</em></dt> + <dd> + A {@link android.graphics.Point} object. The drag shadow width goes in + {@link android.graphics.Point#x} and its height goes in + {@link android.graphics.Point#y}. + </dd> + <dt><em>touch_point</em></dt> + <dd> + A {@link android.graphics.Point} object. The touch point is the location within the + drag shadow that should be under the user's finger during the drag. Its X + position goes in {@link android.graphics.Point#x} and its Y position goes in + {@link android.graphics.Point#y} + </dd> + </dl> + </dd> + <dt> + {@link android.view.View.DragShadowBuilder#onDrawShadow(Canvas) onDrawShadow()} + </dt> + <dd> + Immediately after the call to +{@link android.view.View.DragShadowBuilder#onProvideShadowMetrics(Point,Point) onProvideShadowMetrics()} + the system calls + {@link android.view.View.DragShadowBuilder#onDrawShadow(Canvas) onDrawShadow()} to get the + drag shadow itself. The method has a single argument, a {@link android.graphics.Canvas} + object that the system constructs from the parameters you provide in +{@link android.view.View.DragShadowBuilder#onProvideShadowMetrics(Point,Point) onProvideShadowMetrics()} + Use it to draw the drag shadow in the provided {@link android.graphics.Canvas} object. + </dd> +</dl> +<p> + To improve performance, you should keep the size of the drag shadow small. For a single item, + you may want to use a icon. For a multiple selection, you may want to use icons in a stack + rather than full images spread out over the screen. +</p> +<h2 id="DesignDragOperation">Designing a Drag and Drop Operation</h2> +<p> + This section shows step-by-step how to start a drag, how to respond to events during + the drag, how respond to a drop event, and how to end the drag and drop operation. +</p> +<h3 id="StartDrag">Starting a drag</h3> +<p> + The user starts a drag with a drag gesture, usually a long press, on a View object. + In response, you should do the following: +</p> +<ol> + <li> + As necessary, create a {@link android.content.ClipData} and + {@link android.content.ClipData.Item} for the data being moved. As part of the + ClipData object, supply metadata that is stored in a {@link android.content.ClipDescription} + object within the ClipData. For a drag and drop operation that does not represent data + movement, you may want to use <code>null</code> instead of an actual object. + <p> + For example, this code snippet shows how to respond to a long press on a ImageView + by creating a ClipData object that contains the tag or label of an + ImageView. Following this snippet, the next snippet shows how to override the methods in + {@link android.view.View.DragShadowBuilder}: + </p> +<pre> +// Create a string for the ImageView label +private static final String IMAGEVIEW_TAG = "icon bitmap" + +// Creates a new ImageView +ImageView imageView = new ImageView(this); + +// Sets the bitmap for the ImageView from an icon bit map (defined elsewhere) +imageView.setImageBitmap(mIconBitmap); + +// Sets the tag +imageView.setTag(IMAGEVIEW_TAG); + + ... + +// Sets a long click listener for the ImageView using an anonymous listener object that +// implements the OnLongClickListener interface +imageView.setOnLongClickListener(new View.OnLongClickListener() { + + // Defines the one method for the interface, which is called when the View is long-clicked + public boolean onLongClick(View v) { + + // Create a new ClipData. + // This is done in two steps to provide clarity. The convenience method + // ClipData.newPlainText() can create a plain text ClipData in one step. + + // Create a new ClipData.Item from the ImageView object's tag + ClipData.Item item = new ClipData.Item(v.getTag()); + + // Create a new ClipData using the tag as a label, the plain text MIME type, and + // the already-created item. This will create a new ClipDescription object within the + // ClipData, and set its MIME type entry to "text/plain" + ClipData dragData = new ClipData(v.getTag(),ClipData.MIMETYPE_TEXT_PLAIN,item); + + // Instantiates the drag shadow builder. + View.DrawShadowBuilder myShadow = new MyDragShadowBuilder(imageView); + + // Starts the drag + + v.startDrag(dragData, // the data to be dragged + myShadow, // the drag shadow builder + null, // no need to use local data + 0 // flags (not currently used, set to 0) + ); + + } +} +</pre> + </li> + <li> + The following code snippet defines {@code myDragShadowBuilder} + It creates a drag shadow for dragging a TextView as a small gray rectangle: +<pre> + private static class MyDragShadowBuilder extends View.DragShadowBuilder { + + // The drag shadow image, defined as a drawable thing + private static Drawable shadow; + + // Defines the constructor for myDragShadowBuilder + public MyDragShadowBuilder(View v) { + + // Stores the View parameter passed to myDragShadowBuilder. + super(v); + + // Creates a draggable image that will fill the Canvas provided by the system. + shadow = new ColorDrawable(Color.LTGRAY); + } + + // Defines a callback that sends the drag shadow dimensions and touch point back to the + // system. + @Override + public void onProvideShadowMetrics (Point size, Point touch) + // Defines local variables + private int width, height; + + // Sets the width of the shadow to half the width of the original View + width = getView().getWidth() / 2; + + // Sets the height of the shadow to half the height of the original View + height = getView().getHeight() / 2; + + // The drag shadow is a ColorDrawable. This sets its dimensions to be the same as the + // Canvas that the system will provide. As a result, the drag shadow will fill the + // Canvas. + shadow.setBounds(0, 0, width, height); + + // Sets the size parameter's width and height values. These get back to the system + // through the size parameter. + size.set(width, height); + + // Sets the touch point's position to be in the middle of the drag shadow + touch.set(width / 2, height / 2); + } + + // Defines a callback that draws the drag shadow in a Canvas that the system constructs + // from the dimensions passed in onProvideShadowMetrics(). + @Override + public void onDrawShadow(Canvas canvas) { + + // Draws the ColorDrawable in the Canvas passed in from the system. + shadow.draw(canvas); + } + } +</pre> + <p class="note"> + <strong>Note:</strong> Remember that you don't have to extend + {@link android.view.View.DragShadowBuilder}. The constructor + {@link android.view.View.DragShadowBuilder#View.DragShadowBuilder(View)} creates a + default drag shadow that's the same size as the View argument passed to it, with the + touch point centered in the drag shadow. + </p> + </li> +</ol> +<h3 id="HandleStart">Responding to a drag start</h3> +<p> + During the drag operation, the system dispatches drag events to the drag event listeners + of the View objects in the current layout. The listeners should react + by calling {@link android.view.DragEvent#getAction()} to get the action type. + At the start of a drag, this methods returns {@link android.view.DragEvent#ACTION_DRAG_STARTED}. +</p> +<p> + In response to an event with the action type {@link android.view.DragEvent#ACTION_DRAG_STARTED}, + a listener should do the following: +</p> +<ol> + <li> + Call {@link android.view.DragEvent#getClipDescription()} to get the + {@link android.content.ClipDescription}. Use the MIME type methods in + {@link android.content.ClipDescription} to see if the listener can accept the data being + dragged. + <p> + If the drag and drop operation does not represent data movement, this may not be + necessary. + </p> + </li> + <li> + If the listener can accept a drop, it should return <code>true</code>. This tells + the system to continue to send drag events to the listener. + If it can't accept a drop, it should return <code>false</code>, and the system + will stop sending drag events until it sends out + {@link android.view.DragEvent#ACTION_DRAG_ENDED}. + </li> +</ol> +<p> + Note that for an {@link android.view.DragEvent#ACTION_DRAG_STARTED} event, these + the following {@link android.view.DragEvent} methods are not valid: + {@link android.view.DragEvent#getClipData()}, {@link android.view.DragEvent#getX()}, + {@link android.view.DragEvent#getY()}, and {@link android.view.DragEvent#getResult()}. +</p> +<h3 id="HandleDuring">Handling events during the drag</h3> +<p> + During the drag, listeners that returned <code>true</code> in response to + the {@link android.view.DragEvent#ACTION_DRAG_STARTED} drag event continue to receive drag + events. The types of drag events a listener receives during the drag depend on the location of + the drag shadow and the visibility of the listener's View. +</p> +<p> + During the drag, listeners primarily use drag events to decide if they should change the + appearance of their View. +</p> +<p> + During the drag, {@link android.view.DragEvent#getAction()} returns one of three + values: +</p> +<ul> + <li> + {@link android.view.DragEvent#ACTION_DRAG_ENTERED}: + The listener receives this when the touch point + (the point on the screen underneath the user's finger) has entered the bounding box of the + listener's View. + </li> + <li> + {@link android.view.DragEvent#ACTION_DRAG_LOCATION}: Once the listener receives an + {@link android.view.DragEvent#ACTION_DRAG_ENTERED} event, and before it receives an + A{@link android.view.DragEvent#ACTION_DRAG_EXITED} event, it receives a new + {@link android.view.DragEvent#ACTION_DRAG_LOCATION} event every time the touch point moves. + The {@link android.view.DragEvent#getX()} and {@link android.view.DragEvent#getY()} methods + return the the X and Y coordinates of the touch point. + </li> + <li> + {@link android.view.DragEvent#ACTION_DRAG_EXITED}: This event is sent to a listener that + previously received {@link android.view.DragEvent#ACTION_DRAG_ENTERED}, after + the drag shadow is no longer within the bounding box of the listener's View. + </li> +</ul> +<p> + The listener does not need to react to any of these action types. If the listener returns a + value to the system, it is ignored. Here are some guidelines for responding to each of + these action types: +</p> +<ul> + <li> + In response to {@link android.view.DragEvent#ACTION_DRAG_ENTERED} or + {@link android.view.DragEvent#ACTION_DRAG_LOCATION}, the listener can change the appearance + of the View to indicate that it is about to receive a drop. + </li> + <li> + An event with the action type {@link android.view.DragEvent#ACTION_DRAG_LOCATION} contains + valid data for {@link android.view.DragEvent#getX()} and + {@link android.view.DragEvent#getY()}, corresponding to the location of the touch point. + The listener may want to use this information to alter the appearance of that part of the + View that is at the touch point. The listener can also use this information + to determine the exact position where the user is going to drop the drag shadow. + </li> + <li> + In response to {@link android.view.DragEvent#ACTION_DRAG_EXITED}, the listener should reset + any appearance changes it applied in response to + {@link android.view.DragEvent#ACTION_DRAG_ENTERED} or + {@link android.view.DragEvent#ACTION_DRAG_LOCATION}. This indicates to the user that + the View is no longer an imminent drop target. + </li> +</ul> +<h3 id="HandleDrop">Responding to a drop</h3> +<p> + When the user releases the drag shadow on a View in the application, and that View previously + reported that it could accept the content being dragged, the system dispatches a drag event + to that View with the action type {@link android.view.DragEvent#ACTION_DROP}. The listener + should do the following: +</p> +<ol> + <li> + Call {@link android.view.DragEvent#getClipData()} to get the + {@link android.content.ClipData} object that was originally supplied in the call + to +{@link android.view.View#startDrag(ClipData, View.DragShadowBuilder, Object, int) startDrag()} + and store it. If the drag and drop operation does not represent data movement, + this may not be necessary. + </li> + <li> + Return boolean <code>true</code> to indicate that the drop was processed successfully, or + boolean <code>false</code> if it was not. The returned value becomes the value returned by + {@link android.view.DragEvent#getResult()} for an + {@link android.view.DragEvent#ACTION_DRAG_ENDED} event. + <p> + Note that if the system does not send out an {@link android.view.DragEvent#ACTION_DROP} + event, the value of {@link android.view.DragEvent#getResult()} for an + {@link android.view.DragEvent#ACTION_DRAG_ENDED} event is <code>false</code>. + </p> + </li> +</ol> +<p> + For an {@link android.view.DragEvent#ACTION_DROP} event, + {@link android.view.DragEvent#getX()} and {@link android.view.DragEvent#getY()} + return the X and Y position of the drag point at the moment of the drop, using the coordinate + system of the View that received the drop. +</p> +<p> + The system does allow the user to release the drag shadow on a View whose listener is not + receiving drag events. It will also allow the user to release the drag shadow + on empty regions of the application's UI, or on areas outside of your application. + In all of these cases, the system does not send an event with action type + {@link android.view.DragEvent#ACTION_DROP}, although it does send out an + {@link android.view.DragEvent#ACTION_DRAG_ENDED} event. +</p> +<h3 id="HandleEnd">Responding to a drag end</h3> +<p> + Immediately after the user releases the drag shadow, the system sends a + drag event to all of the drag event listeners in your application, with an action type of + {@link android.view.DragEvent#ACTION_DRAG_ENDED}. This indicates that the drag operation is + over. +</p> +<p> + Each listener should do the following: +</p> +<ol> + <li> + If listener changed its View object's appearance during the operation, it should reset the + View to its default appearance. This is a visual indication to the user that the operation + is over. + </li> + <li> + The listener can optionally call {@link android.view.DragEvent#getResult()} to find out more + about the operation. If a listener returned <code>true</code> in response to an event of + action type {@link android.view.DragEvent#ACTION_DROP}, then + {@link android.view.DragEvent#getResult()} will return boolean <code>true</code>. In all + other cases, {@link android.view.DragEvent#getResult()} returns boolean <code>false</code>, + including any case in which the system did not send out a + {@link android.view.DragEvent#ACTION_DROP} event. + </li> + <li> + The listener should return boolean <code>true</code> to the system. + </li> +</ol> +<p> +</p> +<h3 id="RespondEventSample">Responding to drag events: an example</h3> +<p> + All drag events are initially received by your drag event method or listener. The following + code snippet is a simple example of reacting to drag events in a listener: +</p> +<pre> +// Creates a new drag event listener +mDragListen = new myDragEventListener(); + +View imageView = new ImageView(this); + +// Sets the drag event listener for the View +imageView.setOnDragListener(mDragListen); + +... + +protected class myDragEventListener implements View.OnDragEventListener { + + // This is the method that the system calls when it dispatches a drag event to the + // listener. + public boolean onDrag(View v, DragEvent event) { + + // Defines a variable to store the action type for the incoming event + final int action = event.getAction(); + + // Handles each of the expected events + switch(action) { + + case DragEvent.ACTION_DRAG_STARTED: + + // Determines if this View can accept the dragged data + if (event.getClipDescription().hasMimeType(ClipDescription.MIMETYPE_TEXT_PLAIN)) { + + // As an example of what your application might do, + // applies a blue color tint to the View to indicate that it can accept + // data. + v.setColorFilter(Color.BLUE); + + // Invalidate the view to force a redraw in the new tint + v.invalidate(); + + // returns true to indicate that the View can accept the dragged data. + return(true); + + } else { + + // Returns false. During the current drag and drop operation, this View will + // not receive events again until ACTION_DRAG_ENDED is sent. + return(false); + + } + break; + + case DragEvent.ACTION_DRAG_ENTERED: { + + // Applies a green tint to the View. Return true; the return value is ignored. + + v.setColorFilter(Color.GREEN); + + // Invalidate the view to force a redraw in the new tint + v.invalidate(); + + return(true); + + break; + + case DragEvent.ACTION_DRAG_LOCATION: + + // Ignore the event + return(true); + + break; + + case DragEvent.ACTION_DRAG_EXITED: + + // Re-sets the color tint to blue. Returns true; the return value is ignored. + v.setColorFilter(Color.BLUE); + + // Invalidate the view to force a redraw in the new tint + v.invalidate(); + + return(true); + + break; + + case DragEvent.ACTION_DROP: + + // Gets the item containing the dragged data + ClipData.Item item = event.getClipData().getItemAt(0); + + // Gets the text data from the item. + dragData = item.getText(); + + // Displays a message containing the dragged data. + Toast.makeText(this, "Dragged data is " + dragData, Toast.LENGTH_LONG); + + // Turns off any color tints + v.clearColorFilter(); + + // Invalidates the view to force a redraw + v.invalidate(); + + // Returns true. DragEvent.getResult() will return true. + return(true); + + break; + + case DragEvent.ACTION_DRAG_ENDED: + + // Turns off any color tinting + v.clearColorFilter(); + + // Invalidates the view to force a redraw + v.invalidate(); + + // Does a getResult(), and displays what happened. + if (event.getResult()) { + Toast.makeText(this, "The drop was handled.", Toast.LENGTH_LONG); + + } else { + Toast.makeText(this, "The drop didn't work.", Toast.LENGTH_LONG); + + }; + + // returns true; the value is ignored. + return(true); + + break; + + // An unknown action type was received. + default: + Log.e("DragDrop Example","Unknown action type received by OnDragListener."); + + break; + }; + }; +}; +</pre>
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