Detecting Common Gestures

A "touch gesture" occurs when a user places one or more fingers on the touch screen, and your application interprets that pattern of touches as a particular gesture. There are correspondingly two phases to gesture detection:

1. Gathering data about touch events.
2. Interpreting the data to see if it meets the criteria for any of the gestures your app supports.

Support Library Classes

The examples in this lesson use the GestureDetectorCompat and MotionEventCompat classes. These classes are in the Support Library. You should use Support Library classes where possible to provide compatibility with devices running Android 1.6 and higher. Note that MotionEventCompat is not a replacement for the MotionEvent class. Rather, it provides static utility methods to which you pass your MotionEvent object in order to receive the desired action associated with that event.

Gather Data

When a user places one or more fingers on the screen, this triggers the callback onTouchEvent() on the View that received the touch events. For each sequence of touch events (position, pressure, size, addition of another finger, etc.) that is ultimately identified as a gesture, onTouchEvent() is fired several times.

The gesture starts when the user first touches the screen, continues as the system tracks the position of the user's finger(s), and ends by capturing the final event of the user's fingers leaving the screen. Throughout this interaction, the MotionEvent delivered to onTouchEvent() provides the details of every interaction. Your app can use the data provided by the MotionEvent to determine if a gesture it cares about happened.

Capturing touch events for an Activity or View

To intercept touch events in an Activity or View, override the onTouchEvent() callback.

The following snippet uses getActionMasked() to extract the action the user performed from the event parameter. This gives you the raw data you need to determine if a gesture you care about occurred:

public class MainActivity extends Activity {
...
// This example shows an Activity, but you would use the same approach if
// you were subclassing a View.
@Override
public boolean onTouchEvent(MotionEvent event){

    int action = MotionEventCompat.getActionMasked(event);

    switch(action) {
        case (MotionEvent.ACTION_DOWN) :
            Log.d(DEBUG_TAG,"Action was DOWN");
            return true;
        case (MotionEvent.ACTION_MOVE) :
            Log.d(DEBUG_TAG,"Action was MOVE");
            return true;
        case (MotionEvent.ACTION_UP) :
            Log.d(DEBUG_TAG,"Action was UP");
            return true;
        case (MotionEvent.ACTION_CANCEL) :
            Log.d(DEBUG_TAG,"Action was CANCEL");
            return true;
        case (MotionEvent.ACTION_OUTSIDE) :
            Log.d(DEBUG_TAG,"Movement occurred outside bounds " +
                    "of current screen element");
            return true;
        default :
            return super.onTouchEvent(event);
    }
}

You can then do your own processing on these events to determine if a gesture occurred. This is the kind of processing you would have to do for a custom gesture. However, if your app uses common gestures such as double tap, long press, fling, and so on, you can take advantage of the GestureDetector class. GestureDetector makes it easy for you to detect common gestures without processing the individual touch events yourself. This is discussed below in Detect Gestures.

Capturing touch events for a single view

As an alternative to onTouchEvent(), you can attach an View.OnTouchListener object to any View object using the setOnTouchListener() method. This makes it possible to to listen for touch events without subclassing an existing View. For example:

View myView = findViewById(R.id.my_view);
myView.setOnTouchListener(new OnTouchListener() {
    public boolean onTouch(View v, MotionEvent event) {
        // ... Respond to touch events
        return true;
    }
});

Beware of creating a listener that returns false for the ACTION_DOWN event. If you do this, the listener will not be called for the subsequent ACTION_MOVE and ACTION_UP string of events. This is because ACTION_DOWN is the starting point for all touch events.

If you are creating a custom View, you can override onTouchEvent(), as described above.

Detect Gestures

Android provides the GestureDetector class for detecting common gestures. Some of the gestures it supports include onDown(), onLongPress(), onFling(), and so on. You can use GestureDetector in conjunction with the onTouchEvent() method described above.

Detecting All Supported Gestures

When you instantiate a GestureDetectorCompat object, one of the parameters it takes is a class that implements the GestureDetector.OnGestureListener interface. GestureDetector.OnGestureListener notifies users when a particular touch event has occurred. To make it possible for your GestureDetector object to receive events, you override the View or Activity's onTouchEvent() method, and pass along all observed events to the detector instance.

In the following snippet, a return value of true from the individual on<TouchEvent> methods indicates that you have handled the touch event. A return value of false passes events down through the view stack until the touch has been successfully handled.

Run the following snippet to get a feel for how actions are triggered when you interact with the touch screen, and what the contents of the MotionEvent are for each touch event. You will realize how much data is being generated for even simple interactions.

public class MainActivity extends Activity implements
        GestureDetector.OnGestureListener,
        GestureDetector.OnDoubleTapListener{

    private static final String DEBUG_TAG = "Gestures";
    private GestureDetectorCompat mDetector;

    // Called when the activity is first created.
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // Instantiate the gesture detector with the
        // application context and an implementation of
        // GestureDetector.OnGestureListener
        mDetector = new GestureDetectorCompat(this,this);
        // Set the gesture detector as the double tap
        // listener.
        mDetector.setOnDoubleTapListener(this);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event){
        this.mDetector.onTouchEvent(event);
        // Be sure to call the superclass implementation
        return super.onTouchEvent(event);
    }

    @Override
    public boolean onDown(MotionEvent event) {
        Log.d(DEBUG_TAG,"onDown: " + event.toString());
        return true;
    }

    @Override
    public boolean onFling(MotionEvent event1, MotionEvent event2,
            float velocityX, float velocityY) {
        Log.d(DEBUG_TAG, "onFling: " + event1.toString()+event2.toString());
        return true;
    }

    @Override
    public void onLongPress(MotionEvent event) {
        Log.d(DEBUG_TAG, "onLongPress: " + event.toString());
    }

    @Override
    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX,
            float distanceY) {
        Log.d(DEBUG_TAG, "onScroll: " + e1.toString()+e2.toString());
        return true;
    }

    @Override
    public void onShowPress(MotionEvent event) {
        Log.d(DEBUG_TAG, "onShowPress: " + event.toString());
    }

    @Override
    public boolean onSingleTapUp(MotionEvent event) {
        Log.d(DEBUG_TAG, "onSingleTapUp: " + event.toString());
        return true;
    }

    @Override
    public boolean onDoubleTap(MotionEvent event) {
        Log.d(DEBUG_TAG, "onDoubleTap: " + event.toString());
        return true;
    }

    @Override
    public boolean onDoubleTapEvent(MotionEvent event) {
        Log.d(DEBUG_TAG, "onDoubleTapEvent: " + event.toString());
        return true;
    }

    @Override
    public boolean onSingleTapConfirmed(MotionEvent event) {
        Log.d(DEBUG_TAG, "onSingleTapConfirmed: " + event.toString());
        return true;
    }
}

Detecting a Subset of Supported Gestures

If you only want to process a few gestures, you can extend GestureDetector.SimpleOnGestureListener instead of implementing the GestureDetector.OnGestureListener interface.

GestureDetector.SimpleOnGestureListener provides an implementation for all of the on<TouchEvent> methods by returning false for all of them. Thus you can override only the methods you care about. For example, the snippet below creates a class that extends GestureDetector.SimpleOnGestureListener and overrides onFling() and onDown().

Whether or not you use GestureDetector.OnGestureListener, it's best practice to implement an onDown() method that returns true. This is because all gestures begin with an onDown() message. If you return false from onDown(), as GestureDetector.SimpleOnGestureListener does by default, the system assumes that you want to ignore the rest of the gesture, and the other methods of GestureDetector.OnGestureListener never get called. This has the potential to cause unexpected problems in your app. The only time you should return false from onDown() is if you truly want to ignore an entire gesture.

public class MainActivity extends Activity {

    private GestureDetectorCompat mDetector;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mDetector = new GestureDetectorCompat(this, new MyGestureListener());
    }

    @Override
    public boolean onTouchEvent(MotionEvent event){
        this.mDetector.onTouchEvent(event);
        return super.onTouchEvent(event);
    }

    class MyGestureListener extends GestureDetector.SimpleOnGestureListener {
        private static final String DEBUG_TAG = "Gestures";

        @Override
        public boolean onDown(MotionEvent event) {
            Log.d(DEBUG_TAG,"onDown: " + event.toString());
            return true;
        }

        @Override
        public boolean onFling(MotionEvent event1, MotionEvent event2,
                float velocityX, float velocityY) {
            Log.d(DEBUG_TAG, "onFling: " + event1.toString()+event2.toString());
            return true;
        }
    }
}