android - Drag and Drop

> 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.

A View receives drag events with either a drag event listener that implements View.OnDragListener or with itsonDragEvent(DragEvent) callback method. When the system calls the method or listener, it passes to them aDragEvent object.

>To get the action type, a listener calls getAction(). There are six possible values, defined by constants in theDragEvent class. These are listed in table 1.

ACTION_DRAG_STARTED ACTION_DRAG_ENTERED ACTION_DRAG_LOCATION ACTION_DRAG_EXITED ACTION_DROP ACTION_DRAG_ENDED 

> 

// Create a string for the ImageView labelprivate static final String IMAGEVIEW_TAG = "icon bitmap"// Creates a new ImageViewImageView imageView = new ImageView(this);// Sets the bitmap for the ImageView from an icon bit map (defined elsewhere)imageView.setImageBitmap(mIconBitmap);// Sets the tagimageView.setTag(IMAGEVIEW_TAG);    ...// Sets a long click listener for the ImageView using an anonymous listener object that// implements the OnLongClickListener interfaceimageView.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);        }    }

> 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 an ACTION_DRAG_ENTERED event, and before it receives an AACTION_DRAG_EXITED event, it receives a new ACTION_DRAG_LOCATION event every time the touch point moves. The getX() and getY() methods return the X and Y coordinates of the touch point.
• ACTION_DRAG_EXITED: This event is sent to a listener that previously received ACTION_DRAG_ENTERED, after the drag shadow is no longer within the bounding box of the listener's View.

> 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 listenermDragListen = new myDragEventListener();View imageView = new ImageView(this);// Sets the drag event listener for the ViewimageView.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;    }};