Managing Touch Events in a ViewGroup

Handling touch events in a ViewGroup takes special care, because it's common for a ViewGroup to have children that are targets for different touch events than the ViewGroup itself. To make sure that each view correctly receives the touch events intended for it, override the onInterceptTouchEvent() method.

Intercept Touch Events in a ViewGroup

The onInterceptTouchEvent() method is called whenever a touch event is detected on the surface of a ViewGroup, including on the surface of its children. If onInterceptTouchEvent() returns true, the MotionEvent is intercepted, meaning it will be not be passed on to the child, but rather to the onTouchEvent() method of the parent.

The onInterceptTouchEvent() method gives a parent the chance to see any touch event before its children do. If you return true from onInterceptTouchEvent(), the child view that was previously handling touch events receives an ACTION_CANCEL, and the events from that point forward are sent to the parent's onTouchEvent() method for the usual handling. onInterceptTouchEvent() can also return false and simply spy on events as they travel down the view hierarchy to their usual targets, which will handle the events with their own onTouchEvent().

In the following snippet, the class MyViewGroup extends ViewGroup. MyViewGroup contains multiple child views. If you drag your finger across a child view horizontally, the child view should no longer get touch events, and MyViewGroup should handle touch events by scrolling its contents. However, if you press buttons in the child view, or scroll the child view vertically, the parent shouldn't intercept those touch events, because the child is the intended target. In those cases, onInterceptTouchEvent() should return false, and MyViewGroup's onTouchEvent() won't be called.

public class MyViewGroup extends ViewGroup {

    private int mTouchSlop;

    ...

    ViewConfiguration vc = ViewConfiguration.get(view.getContext());
    mTouchSlop = vc.getScaledTouchSlop();

    ...

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        /*
         * This method JUST determines whether we want to intercept the motion.
         * If we return true, onTouchEvent will be called and we do the actual
         * scrolling there.
         */


        final int action = MotionEventCompat.getActionMasked(ev);

        // Always handle the case of the touch gesture being complete.
        if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
            // Release the scroll.
            mIsScrolling = false;
            return false; // Do not intercept touch event, let the child handle it
        }

        switch (action) {
            case MotionEvent.ACTION_MOVE: {
                if (mIsScrolling) {
                    // We're currently scrolling, so yes, intercept the 
                    // touch event!
                    return true;
                }

                // If the user has dragged her finger horizontally more than 
                // the touch slop, start the scroll

                // left as an exercise for the reader
                final int xDiff = calculateDistanceX(ev); 

                // Touch slop should be calculated using ViewConfiguration 
                // constants.
                if (xDiff > mTouchSlop) { 
                    // Start scrolling!
                    mIsScrolling = true;
                    return true;
                }
                break;
            }
            ...
        }

        // In general, we don't want to intercept touch events. They should be 
        // handled by the child view.
        return false;
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        // Here we actually handle the touch event (e.g. if the action is ACTION_MOVE, 
        // scroll this container).
        // This method will only be called if the touch event was intercepted in 
        // onInterceptTouchEvent
        ...
    }
}

Note that ViewGroup also provides a requestDisallowInterceptTouchEvent() method. The ViewGroup calls this method when a child does not want the parent and its ancestors to intercept touch events with onInterceptTouchEvent().

Use ViewConfiguration Constants

The above snippet uses the current ViewConfiguration to initialize a variable called mTouchSlop. You can use the ViewConfiguration class to access common distances, speeds, and times used by the Android system.

"Touch slop" refers to the distance in pixels a user's touch can wander before the gesture is interpreted as scrolling. Touch slop is typically used to prevent accidental scrolling when the user is performing some other touch operation, such as touching on-screen elements.

Two other commonly used ViewConfiguration methods are getScaledMinimumFlingVelocity() and getScaledMaximumFlingVelocity(). These methods return the minimum and maximum velocity (respectively) to initiate a fling, as measured in pixels per second. For example:

ViewConfiguration vc = ViewConfiguration.get(view.getContext());
private int mSlop = vc.getScaledTouchSlop();
private int mMinFlingVelocity = vc.getScaledMinimumFlingVelocity();
private int mMaxFlingVelocity = vc.getScaledMaximumFlingVelocity();

...

case MotionEvent.ACTION_MOVE: {
    ...
    float deltaX = motionEvent.getRawX() - mDownX;
    if (Math.abs(deltaX) > mSlop) {
        // A swipe occurred, do something
    }

...

case MotionEvent.ACTION_UP: {
    ...
    } if (mMinFlingVelocity <= velocityX && velocityX <= mMaxFlingVelocity
            && velocityY < velocityX) {
        // The criteria have been satisfied, do something
    }
}

Extend a Child View's Touchable Area

Android provides the TouchDelegate class to make it possible for a parent to extend the touchable area of a child view beyond the child's bounds. This is useful when the child has to be small, but should have a larger touch region. You can also use this approach to shrink the child's touch region if need be.

In the following example, an ImageButton is the "delegate view" (that is, the child whose touch area the parent will extend). Here is the layout file:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/parent_layout"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     tools:context=".MainActivity" >
 
     <ImageButton android:id="@+id/button"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:background="@null"
          android:src="@drawable/icon" />
</RelativeLayout>

The snippet below does the following:

• Gets the parent view and posts a Runnable on the UI thread. This ensures that the parent lays out its children before calling the getHitRect() method. The getHitRect() method gets the child's hit rectangle (touchable area) in the parent's coordinates.
• Finds the ImageButton child view and calls getHitRect() to get the bounds of the child's touchable area.
• Extends the bounds of the ImageButton's hit rectangle.
• Instantiates a TouchDelegate, passing in the expanded hit rectangle and the ImageButton child view as parameters.
• Sets the TouchDelegate on the parent view, such that touches within the touch delegate bounds are routed to the child.

In its capacity as touch delegate for the ImageButton child view, the parent view will receive all touch events. If the touch event occurred within the child's hit rectangle, the parent will pass the touch event to the child for handling.

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // Get the parent view
        View parentView = findViewById(R.id.parent_layout);
        
        parentView.post(new Runnable() {
            // Post in the parent's message queue to make sure the parent
            // lays out its children before you call getHitRect()
            @Override
            public void run() {
                // The bounds for the delegate view (an ImageButton
                // in this example)
                Rect delegateArea = new Rect();
                ImageButton myButton = (ImageButton) findViewById(R.id.button);
                myButton.setEnabled(true);
                myButton.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Toast.makeText(MainActivity.this, 
                                "Touch occurred within ImageButton touch region.", 
                                Toast.LENGTH_SHORT).show();
                    }
                });
     
                // The hit rectangle for the ImageButton
                myButton.getHitRect(delegateArea);
            
                // Extend the touch area of the ImageButton beyond its bounds
                // on the right and bottom.
                delegateArea.right += 100;
                delegateArea.bottom += 100;
            
                // Instantiate a TouchDelegate.
                // "delegateArea" is the bounds in local coordinates of 
                // the containing view to be mapped to the delegate view.
                // "myButton" is the child view that should receive motion
                // events.
                TouchDelegate touchDelegate = new TouchDelegate(delegateArea, 
                        myButton);
     
                // Sets the TouchDelegate on the parent view, such that touches 
                // within the touch delegate bounds are routed to the child.
                if (View.class.isInstance(myButton.getParent())) {
                    ((View) myButton.getParent()).setTouchDelegate(touchDelegate);
                }
            }
        });
    }
}