Gestures and Touch Events

Overview

Gesture recognition and handling touch events is an important part of developing user interactions. Handling standard events such as clicks, long clicks, key presses, etc are very basic and handled in other guides. This guide is focused on handling other more specialized gestures such as:

Swiping in a direction
Double tapping for zooming
Pinch to zoom in or out
Dragging and dropping

You can see a visual guide of common gestures on the gestures design patterns guide.

See the new Material Design information about gestures.

Usage

Handling Touches

At the heart of all gestures is the onTouchListener and the onTouch method which has access to MotionEvent data. Every view has an onTouchListener which can be specified:

myView.setOnTouchListener(new OnTouchListener() {
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        // Interpret MotionEvent data
        // Handle touch here
        return true;
    }
});

Each onTouch event has access to the MotionEvent which describe movements in terms of an action code and a set of axis values. The action code specifies the state change that occurred such as a pointer going down or up. The axis values describe the position and other movement properties:

getAction() - Returns an integer constant such as MotionEvent.ACTION_DOWN, MotionEvent.ACTION_MOVE, and MotionEvent.ACTION_UP
getX() - Returns the x coordinate of the touch event
getY() - Returns the y coordinate of the touch event

Handling Multi Touch Events

Note that getAction() normally includes both the action as well as the pointer index. In single-touch events, there is only one pointer (set to 0), so no bitmap mask is needed. In multiple touch events (i.e pinch open or pinch close), however, there are multiple fingers involved and therefore multiple pointer indexes included with the touch event. As a result, there are other methods that should be used:

getActionMasked() - extract the action event without the pointer index
getActionIndex() - extract the pointer index used

The events associated with other pointers usually start with MotionEvent.ACTION_POINTER_xxxx such as MotionEvent.ACTION_POINTER_DOWN and MotionEvent.ACTION_POINTER_UP. The getPointerCount() on the MotionEvent can be used to determine how many pointers are active in this touch sequence.

See the official Android documentation and this blog post for more details about how multi-touch events work.

Gesture Detectors

Within an onTouch event, we can then use a GestureDetector to understand gestures based on a series of motion events. Gestures are often used for user interactions within an app. Let's take a look at how to implement common gestures.

Double Tapping

You can enable double tap events for any view within your activity using the OnDoubleTapListener. First, copy the code for OnDoubleTapListener into your application and then you can apply the listener with:

myView.setOnTouchListener(new OnDoubleTapListener(this) {
  @Override
  public void onDoubleTap(MotionEvent e) {
    Toast.makeText(MainActivity.this, "Double Tap", Toast.LENGTH_SHORT).show();
  }
});

Now that view will be able to respond to a double tap event and you can handle the event accordingly.

Swipe Gesture Detection

Detecting finger swipes in a particular direction is best done using the built-in [onFling](http://developer.android.com/reference/android/view/GestureDetector.OnGestureListener.html#onFling\(android.view.MotionEvent, android.view.MotionEvent, float, float)) event in the GestureDetector.OnGestureListener.

A helper class that makes handling swipes as easy as possible can be found in the OnSwipeTouchListener class. Copy the OnSwipeTouchListener class to your own application and then you can use the listener to manage the swipe events with:

myView.setOnTouchListener(new OnSwipeTouchListener(this) {
  @Override
  public void onSwipeDown() {
    Toast.makeText(MainActivity.this, "Down", Toast.LENGTH_SHORT).show();
  }
  
  @Override
  public void onSwipeLeft() {
    Toast.makeText(MainActivity.this, "Left", Toast.LENGTH_SHORT).show();
  }
  
  @Override
  public void onSwipeUp() {
    Toast.makeText(MainActivity.this, "Up", Toast.LENGTH_SHORT).show();
  }
  
  @Override
  public void onSwipeRight() {
    Toast.makeText(MainActivity.this, "Right", Toast.LENGTH_SHORT).show();
  }
});

With that code in place, swipe gestures should be easily manageable.

ListView Swipe Detection

If you are interested in having a ListView that recognizes swipe gestures for each item, consider using the popular third-party library android-swipelistview which is a ListView replacement that supports swipe-eable items. Once setup, you can configure a layout that will appear when the item is swiped.

Check out the swipelistview project for more details but the general usage looks like:

 <com.fortysevendeg.swipelistview.SwipeListView
 xmlns:swipe="http://schemas.android.com/apk/res-auto"
 android:id="@+id/example_lv_list"
 android:listSelector="#00000000"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 swipe:swipeFrontView="@+id/front"
 swipe:swipeBackView="@+id/back"
 swipe:swipeActionLeft="reveal"
 swipe:swipeActionRight="dismiss"
 swipe:swipeMode="both"
 swipe:swipeCloseAllItemsWhenMoveList="true"
 />

and then define the individual list item layout with:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
             android:layout_width="match_parent"
             android:layout_height="match_parent">
    <LinearLayout
            android:id="@+id/back"
            android:tag="back"
            style="@style/ListBackContent">
        <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/example_row_b_action_1"
                style="@style/ListButtonAction"
                android:text="@string/open"/>
    </LinearLayout>
    <RelativeLayout
            android:orientation="vertical"
            android:id="@+id/front"
            android:tag="front"
            style="@style/ListFrontContent">
        <ImageView
                style="@style/ListImage"
                android:id="@+id/example_row_iv_image"/>
    </RelativeLayout>
</FrameLayout>

Now front will be displayed by default and if I swipe left on an item, then the back will be displayed for that item. This simplifies swipes for the common case of menus for a ListView.

Another more recent alternative is the AndroidSwipeLayout library which can be more flexible and is worth checking out as an alternative.

Pinch to Zoom

Supporting Pinch to Zoom in and out is fairly straightforward thanks to the ScaleGestureDetector class. Easiest way to manage pinch events is to subclass a view and manage the pinch event from within:

public class ScaleableTextView extends TextView 
        implements OnTouchListener, OnScaleGestureListener {

  ScaleGestureDetector mScaleDetector = 
      new ScaleGestureDetector(getContext(), this);

  public ScaleableTextView(Context context, AttributeSet attrs) {
    super(context, attrs);
  }

  @Override
  public boolean onScale(ScaleGestureDetector detector) {
    // Code for scale here
    return true;
  }

  @Override
  public boolean onScaleBegin(ScaleGestureDetector detector) {
    // Code for scale begin here
    return true;
  }

  @Override
  public void onScaleEnd(ScaleGestureDetector detector) {
    // Code for scale end here
  }

  @Override
  public boolean onTouch(View v, MotionEvent event) {
    if (mScaleDetector.onTouchEvent(event))
      return true;
    return super.onTouchEvent(event);
  }
}

Using the ScaleGestureDetector makes managing this fairly straightforward.

Zooming Image View

One of the most common use cases for a pinch or pannable view is for an ImageView that displays a Photo which can be zoomed or panned around on screen similar to the Facebook client. To achieve the zooming image view, rather than developing this yourself, be sure to check out the PhotoView third-party library. Using the PhotoView just requires the XML:

<uk.co.senab.photoview.PhotoView
    android:id="@+id/iv_photo"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

and then in the Java:

// Any implementation of ImageView can be used!
mImageView = (ImageView) findViewById(R.id.iv_photo);
// Set the image bitmap
mImageView.setImageDrawable(someBitmap);
// Setup view attacher
PhotoViewAttacher mAttacher = new PhotoViewAttacher(mImageView);

Check out the PhotoView readme and sample for more details. You can also check the TouchImageView library which is a nice alternative.

Dragging and Dropping

Dragging and dropping views is not particularly difficult to do thanks to the OnDragListener built in since API 11. Unfortunately, to support gingerbread managing drag and drop becomes much more manual as you have to implement it using the onTouch handlers. With API 11 and above, you can leverage the built in drag handling.

First, we want to attach an onTouch handler on the views that are draggable which will start the drag by creating a DragShadow with the DragShadowBuilder which is then dragged around the Activity once [startDrag](http://developer.android.com/reference/android/view/View.html#startDrag\(android.content.ClipData, android.view.View.DragShadowBuilder, java.lang.Object, int)) is invoked on the view:

// This listener is attached to the view that should be draggable
draggableView.setOnTouchListener(new OnTouchListener() {
    public boolean onTouch(View view, MotionEvent motionEvent) {
        if (motionEvent.getAction() == MotionEvent.ACTION_DOWN) {
            // Construct draggable shadow for view
	    DragShadowBuilder shadowBuilder = new View.DragShadowBuilder(view);
            // Start the drag of the shadow
	    view.startDrag(null, shadowBuilder, view, 0);
            // Hide the actual view as shadow is being dragged
	    view.setVisibility(View.INVISIBLE);
	    return true;
	} else {
	    return false;
	}
   }
});

If we want to add "drag" or "drop" events, we should create a DragListener that is attached to a drop zone for the draggable object. We hook up the listener and manage the different dragging and dropping events for the zone:

// This listener is attached to the view that should be a drop target
viewDropZone.setOnDragListener(new OnDragListener() {
  // Drawable for when the draggable enters the drop target
  Drawable enteredZoneBackground = getResources().getDrawable(R.drawable.shape_border_green);
  // Drawable for the default background of the drop target
  Drawable defaultBackground = getResources().getDrawable(R.drawable.shape_border_red);

  @Override
  public boolean onDrag(View v, DragEvent event) {
      // Get the dragged view being dropped over a target view
      final View draggedView = (View) event.getLocalState();
      switch (event.getAction()) {
      case DragEvent.ACTION_DRAG_STARTED:
          // Signals the start of a drag and drop operation.
          // Code for that event here
          break;
      case DragEvent.ACTION_DRAG_ENTERED:
          // Signals to a View that the drag point has 
          // entered the bounding box of the View. 
          v.setBackground(enteredZoneBackground);
          break;
      case DragEvent.ACTION_DRAG_EXITED:
          // Signals that the user has moved the drag shadow 
          // outside the bounding box of the View. 
          v.setBackground(defaultBackground);
          break;
      case DragEvent.ACTION_DROP:
          // Signals to a View that the user has released the drag shadow, 
          // and the drag point is within the bounding box of the View. 
          // Get View dragged item is being dropped on
          View dropTarget = v;
          // Make desired changes to the drop target below
          dropTarget.setTag("dropped");
          // Get owner of the dragged view and remove the view (if needed)
          ViewGroup owner = (ViewGroup) draggedView.getParent();
          owner.removeView(draggedView);
          break;
      case DragEvent.ACTION_DRAG_ENDED:
          // Signals to a View that the drag and drop operation has concluded.
          // If event result is set, this means the dragged view was dropped in target
          if (event.getResult()) { // drop succeeded
              v.setBackground(enteredZoneBackground);
          } else { // drop did not occur
              // restore the view as visible
              draggedView.post(new Runnable() {
                  @Override
                  public void run() {
                      draggedView.setVisibility(View.VISIBLE);
                  }
              });
              // restore drop zone default background
              v.setBackground(defaultBackground);
          }
      default:
          break;
      }
      return true;
  }
});

Check out the vogella dragging tutorial or the javapapers dragging tutorial for a detailed look at handling dragging and dropping. Read the official drag and drop guide for a more detail overview.

Shake Detection

Detecting when the device is shaked requires using the sensor data to determine movement. We can whip up a special listener which manages this shake recognition for us. First, copy the ShakeListener into your project. Now, we can implement ShakeListener.Callback in any activity:

public class MainActivity extends Activity 
    implements ShakeListener.Callback {

  @Override
  public void shakingStarted() {
    // Code on started here
  }
  
  @Override
  public void shakingStopped() {
    // Code on stopped here
  }
}

Now we just have to implement the expected behavior for the shaking event in the two methods from the callback.

MultiTouch Events

Additional multi-touch events such as "rotation" of fingers, finger movement events, etc you can check out the multitouch-gesture-detectors third-party library. Read the documentation for more details about how to handle multi-touch gestures. Also, for a more generic approach, read the official multitouch guide.

Libraries

AndroidSwipeLayout
android-swipelistview - A List View with swipeable cells
gesticulate - Android swipe detection made simple
android-gesture-detectors - small framework for gesture detection
PhotoView - ImageView for Android that supports zooming, by various touch gestures.
DraggablePanel - Cool draggable panels like YouTube

References

Created by CodePath with much help from the community. Contributed content licensed under cc-wiki with attribution required. You are free to remix and reuse, as long as you attribute and use a similar license.

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