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:
- Gathering data about touch events.
- 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 andMotionEventCompat 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 eventparameter. 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. GestureDetectormakes
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 theonTouchEvent() 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 fromonDown(),
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 fromonDown() 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; } } }