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core/java/android/view/DragEvent.aidl
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core/java/android/view/DragEvent.java
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Drag/drop APIs and infrastructure
A View initiates a drag-and-drop operation (hereafter just called a "drag") by calling its startDrag(ClipData) method. Within the processing of that call, two callbacks are made into the originating View. The first is to onMeasureDragThumbnail(). Similarly to the core onMeasure() method, this callback must respond by calling setDragThumbnailDimension(width, height) to declare the size of the drag thumbnail image that should be used. Following this, the View's onDrawDragThumbnail(canvas) method will be invoked to actually produce the bits of the thumbnail image. If all goes well, startDrag() will return 'true', and the drag is off and running. (The other arguments to startDrag() provide reconciliation between the current finger position and where the thumbnail should be placed on the screen relative to it.) Potential receipients of the ClipData behind the drag are notified by a new dispatch mechanism, roughly parallel to motion event dispatch. The core routine is the View's onDragEvent(event) callback, with the mechanics of dispatch itself being routed through dispatchDragEvent(event) -- as in the case of motion events, the dispatch logic is in ViewGroup, with leaf View objects not needing to consider the dispatch flow. Several different event 'actions' are delivered through this dispatch mechanism: ACTION_DRAG_STARTED: this event is propagated to every View in every window (including windows created during the course of a drag). It serves as a global notification that a drag has started with a payload whose matching ClipDescription is supplied with the event. A View that is prepared to consume the data described in this event should return 'true' from their onDragEvent() method, and ideally will also make some visible on-screen indication that they are a potential target of the drop. ACTION_DRAG_ENTERED: this event is sent once when the drag point enters the View's bounds. It is an opportunity for the View to set up feedback that they are the one who will see the drop if the finger goes up now. ACTION_DRAG_LOCATION: when the drag point is over a given View, that View will receive a stream of DRAG_LOCATION events, providing an opportunity for the View to show visual feedback tied to the drag point. ACTION_DRAG_EXITED: like DRAG_ENTERED, but called when the drag point leaves the View's bounds. The View should undo any visuals meant to emphasize their being the hovered-over target. ACTION_DROP: when the drag ends at a given point, the View under that point is sent this event, with the full ClipData of the payload. ACTION_DRAG_ENDED: paralleling the DRAG_STARTED action, this is the global broadcast that the drag has ended and all Views should return to their normal visual state. This happens after the DROP event. Change-Id: Ia8d0fb1516bce8c735d87ffd101af0976d7e84b6