고 투 더 멘토

  - If you are intercepting the BACK key in a view, you should track the key event on key-down (through the new startTracking() method), then invoke the action at key up. Here's a pattern you can use:

    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK
                && event.getRepeatCount() == 0) {
            event.startTracking();
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }

    public boolean onKeyUp(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK && event.isTracking()
                && !event.isCanceled()) {
            // *** DO ACTION HERE ***
            return true;
        }
        return super.onKeyUp(keyCode, event);
    }

If you want to update a legacy application so that its handling of the BACK key works properly for both Android 2.0 and older platform versions, you can use an approach similar to that shown above. Your code can catch the target button event on key-down, set a flag to track the key event, and then also catch the event on key-up, executing the desired action if the tracking flag is set. You'll also want to watch for focus changes and clear the tracking flag when gaining/losing focus.