Quickview
- Allow users to move data within your Activity layout using graphical gestures.
- Supports operations besides data movement.
- Only works within a single application.
- Requires API 11.
In this document
Key classes
Related Samples
See also
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.
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.
Overview
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 View
objects in the layout.
Once the user releases the drag shadow, the system ends the drag operation.
You create a drag event listener object ("listeners") from a class that implements
View.OnDragListener
. You set the drag event listener object for a View
with the View object's
setOnDragListener()
method.
Each View object also has a onDragEvent()
callback method. Both of these are described in more detail in the section
The drag event listener and callback method.
Note: 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.
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.
Your application tells the system to start a drag by calling the
startDrag()
method. This tells the system to start sending drag events. The method also sends the data that
you are dragging.
You can call
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.
Once your application calls
startDrag()
,
the rest of the process uses events that the system sends to the View objects in your current
layout.
The drag/drop process
There are basically four steps or states in the drag and drop process:
- Started
-
In response to the user's gesture to begin a drag, your application calls
startDrag()
to tell the system to start a drag. The argumentsstartDrag()
provide the data to be dragged, metadata for this data, and a callback for drawing the drag shadow.The system first responds by calling back to your application to get a drag shadow. It then displays the drag shadow on the device.
Next, the system sends a drag event with action type
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 returntrue
. 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.If the drag event listener returns
false
, then it will not receive drag events for the current operation until the system sends a drag event with action typeACTION_DRAG_ENDED
. By sendingfalse
, the listener tells the system that it is not interested in the drag operation and does not want to accept the dragged data. - Continuing
-
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
ACTION_DRAG_ENTERED
), the listener can react by highlighting its View. - Dropped
-
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
ACTION_DROP
. The drag event contains the data that was passed to the system in the call tostartDrag()
that started the operation. The listener is expected to return booleantrue
to the system if code for accepting the drop succeeds.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
ACTION_DROP
drag event is sent. - Ended
-
After the user releases the drag shadow, and after the system sends out (if necessary)
a drag event with action type
ACTION_DROP
, the system sends out a drag event with action typeACTION_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 theACTION_DROP
event.
Each of these four steps is described in more detail in the section Designing a Drag and Drop Operation.
The drag event listener and callback method
A View receives drag events with either a drag event listener that implements
View.OnDragListener
or with its
onDragEvent(DragEvent)
callback method.
When the system calls the method or listener, it passes to them
a DragEvent
object.
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
setOnDragListener()
.
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 false
.
The combination of the onDragEvent(DragEvent)
method and
View.OnDragListener
is analogous to the combination
of the onTouchEvent()
and
View.OnTouchListener
used with touch events.
Drag events
The system sends out a drag event in the form of a 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.
To get the action type, a listener calls getAction()
. There
are six possible values, defined by constants in the DragEvent
class. These
are listed in table 1.
The DragEvent
object also contains the data that your application provided
to the system in the call to
startDrag()
.
Some of the data is valid only for certain action types. The data that is valid for each action
type is summarized in table 2. It is also described in detail with
the event for which it is valid in the section
Designing a Drag and Drop Operation.
getAction() value | Meaning |
---|---|
ACTION_DRAG_STARTED |
A View object's drag event listener receives this event action type just after the
application calls
startDrag() and
gets a drag shadow.
|
ACTION_DRAG_ENTERED |
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
true to the system.
|
ACTION_DRAG_LOCATION |
A View object's drag event listener receives this event action type after it receives a
ACTION_DRAG_ENTERED event while the drag shadow is
still within the bounding box of the View.
|
ACTION_DRAG_EXITED |
A View object's drag event listener receives this event action type after it receives a
ACTION_DRAG_ENTERED and at least one
ACTION_DRAG_LOCATION event, and after the user has moved
the drag shadow outside the bounding box of the View.
|
ACTION_DROP |
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 true in response to the
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.
The listener is expected to return boolean |
ACTION_DRAG_ENDED |
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 ACTION_DROP event. If the system sent
a ACTION_DROP , receiving the
ACTION_DRAG_ENDED action type does not imply that the
drop operation succeeded. The listener must call
getResult() to get the value that was
returned in response to ACTION_DROP . If an
ACTION_DROP event was not sent, then
getResult() returns false .
|
getAction() value |
getClipDescription() value |
getLocalState() value |
getX() value |
getY() value |
getClipData() value |
getResult() value |
---|---|---|---|---|---|---|
ACTION_DRAG_STARTED |
X | X | X | |||
ACTION_DRAG_ENTERED |
X | X | X | X | ||
ACTION_DRAG_LOCATION |
X | X | X | X | ||
ACTION_DRAG_EXITED |
X | X | ||||
ACTION_DROP |
X | X | X | X | X | |
ACTION_DRAG_ENDED |
X | X | X |
The getAction()
,
describeContents()
,
writeToParcel()
, and
toString()
methods always return valid data.
If a method does not contain valid data for a particular action type, it returns either
null
or 0, depending on its result type.
The drag shadow
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.
The image is called a drag shadow. You create it with methods you declare for a
View.DragShadowBuilder
object, and then pass it to the system when you
start a drag using
startDrag()
.
As part of its response to
startDrag()
,
the system invokes the callback methods you've defined in
View.DragShadowBuilder
to obtain a drag shadow.
The View.DragShadowBuilder
class has two constructors:
View.DragShadowBuilder(View)
-
This constructor accepts any of your application's
View
objects. The constructor stores the View object in theView.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.If you use this constructor, you don't have to extend
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. View.DragShadowBuilder()
-
If you use this constructor, no View object is available in the
View.DragShadowBuilder
object (the field is set tonull
). If you use this constructor, and you don't extendView.DragShadowBuilder
or override its methods, you will get an invisible drag shadow. The system does not give an error.
The View.DragShadowBuilder
class has two methods:
-
onProvideShadowMetrics()
-
The system calls this method immediately after you call
startDrag()
. Use it to send to the system the dimensions and touch point of the drag shadow. The method has two arguments: -
onDrawShadow()
-
Immediately after the call to
onProvideShadowMetrics()
the system callsonDrawShadow()
to get the drag shadow itself. The method has a single argument, aCanvas
object that the system constructs from the parameters you provide inonProvideShadowMetrics()
Use it to draw the drag shadow in the providedCanvas
object.
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.
Designing a Drag and Drop Operation
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.
Starting a drag
The user starts a drag with a drag gesture, usually a long press, on a View object. In response, you should do the following:
-
As necessary, create a
ClipData
andClipData.Item
for the data being moved. As part of the ClipData object, supply metadata that is stored in aClipDescription
object within the ClipData. For a drag and drop operation that does not represent data movement, you may want to usenull
instead of an actual object.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
View.DragShadowBuilder
:// 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.DragShadowBuilder 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) ); } }
-
The following code snippet defines
myDragShadowBuilder
It creates a drag shadow for dragging a TextView as a small gray rectangle: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); } }
Note: Remember that you don't have to extend
View.DragShadowBuilder
. The constructorView.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.
Responding to a drag start
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 getAction()
to get the action type.
At the start of a drag, this methods returns ACTION_DRAG_STARTED
.
In response to an event with the action type ACTION_DRAG_STARTED
,
a listener should do the following:
-
Call
getClipDescription()
to get theClipDescription
. Use the MIME type methods inClipDescription
to see if the listener can accept the data being dragged.If the drag and drop operation does not represent data movement, this may not be necessary.
-
If the listener can accept a drop, it should return
true
. This tells the system to continue to send drag events to the listener. If it can't accept a drop, it should returnfalse
, and the system will stop sending drag events until it sends outACTION_DRAG_ENDED
.
Note that for an ACTION_DRAG_STARTED
event, these
the following DragEvent
methods are not valid:
getClipData()
, getX()
,
getY()
, and getResult()
.
Handling events during the drag
During the drag, listeners that returned true
in response to
the 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.
During the drag, listeners primarily use drag events to decide if they should change the appearance of their View.
During the drag, getAction()
returns one of three
values:
-
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. -
ACTION_DRAG_LOCATION
: Once the listener receives anACTION_DRAG_ENTERED
event, and before it receives an AACTION_DRAG_EXITED
event, it receives a newACTION_DRAG_LOCATION
event every time the touch point moves. ThegetX()
andgetY()
methods return the X and Y coordinates of the touch point. -
ACTION_DRAG_EXITED
: This event is sent to a listener that previously receivedACTION_DRAG_ENTERED
, after the drag shadow is no longer within the bounding box of the listener's View.
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:
-
In response to
ACTION_DRAG_ENTERED
orACTION_DRAG_LOCATION
, the listener can change the appearance of the View to indicate that it is about to receive a drop. -
An event with the action type
ACTION_DRAG_LOCATION
contains valid data forgetX()
andgetY()
, 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. -
In response to
ACTION_DRAG_EXITED
, the listener should reset any appearance changes it applied in response toACTION_DRAG_ENTERED
orACTION_DRAG_LOCATION
. This indicates to the user that the View is no longer an imminent drop target.
Responding to a drop
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 ACTION_DROP
. The listener
should do the following:
-
Call
getClipData()
to get theClipData
object that was originally supplied in the call tostartDrag()
and store it. If the drag and drop operation does not represent data movement, this may not be necessary. -
Return boolean
true
to indicate that the drop was processed successfully, or booleanfalse
if it was not. The returned value becomes the value returned bygetResult()
for anACTION_DRAG_ENDED
event.Note that if the system does not send out an
ACTION_DROP
event, the value ofgetResult()
for anACTION_DRAG_ENDED
event isfalse
.
For an ACTION_DROP
event,
getX()
and 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.
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
ACTION_DROP
, although it does send out an
ACTION_DRAG_ENDED
event.
Responding to a drag end
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
ACTION_DRAG_ENDED
. This indicates that the drag operation is
over.
Each listener should do the following:
- 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.
-
The listener can optionally call
getResult()
to find out more about the operation. If a listener returnedtrue
in response to an event of action typeACTION_DROP
, thengetResult()
will return booleantrue
. In all other cases,getResult()
returns booleanfalse
, including any case in which the system did not send out aACTION_DROP
event. -
The listener should return boolean
true
to the system.
Responding to drag events: an example
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:
// 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.OnDragListener { // 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; } // Returns false. During the current drag and drop operation, this View will // not receive events again until ACTION_DRAG_ENDED is sent. return false; 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; case DragEvent.ACTION_DRAG_LOCATION: // Ignore the event return true; 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; 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; 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; // An unknown action type was received. default: Log.e("DragDrop Example","Unknown action type received by OnDragListener."); break; } return false; } };