How to use Multi-touch in Android 2: Part 5, Implementing the Drag Gesture

This will probably get lost in all of today's iPad coverage, but it's time for another entry in the Android multi-touch series. In this part we fill in the touch event code to let us drag an image around on the screen. Next week we'll tie everything together by implementing the multi-touch pinch zoom gesture. All source code can be downloaded from the web site for Hello, Android! (3rd edition).

Implementing the Drag Gesture

A drag gesture starts when the first finger is pressed to the screen (ACTION_DOWN) and ends when it is removed (ACTION_UP or ACTION_POINTER_UP).

From: Touchv1/src/org/example/touch/Touch.java:

switch (event.getAction() & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_DOWN:
savedMatrix.set(matrix);
start.set(event.getX(), event.getY());
Log.d(TAG, "mode=DRAG" );
mode = DRAG;
break;
case MotionEvent.ACTION_UP:
case MotionEvent.ACTION_POINTER_UP:
mode = NONE;
Log.d(TAG, "mode=NONE" );
break;
case MotionEvent.ACTION_MOVE:
if (mode == DRAG) {
matrix.set(savedMatrix);
matrix.postTranslate(event.getX() - start.x,
event.getY() - start.y);
}
break;
}

When the gesture starts we remember the current value of the transformation matrix and the starting position of the pointer. Every time the finger moves, we start the transformation matrix over at its original value and call the postTranslate( ) method to add a translation vector, the difference between the current and starting positions.

If you run the program now you should be able to drag the image around the screen using your finger. Neat, huh?

Joe Asks: Why not use the built-in gesture library?

The android.gesture package provides a way to create, recognize, load, and save gestures. Unfortunately it is not very useful in practice. Among other problems, it doesn’t come with a standard collection of built-in gestures (like tap and drag) and it doesn’t support multi-touch gestures such as pinch zoom. Perhaps a future version of Android will include a better gesture library so the code in this chapter could be simplified.

Continued in Part 6, Implementing the Pinch Zoom Gesture >

Copyright notice: This is an excerpt from Hello, Android 3rd edition, published by the Pragmatic Bookshelf. For more information or to purchase a paperback or PDF copy, please visit http://www.pragprog.com/titles/eband3.

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