Dragging and Scaling

This lesson describes how to use touch gestures to drag and scale on-screen objects, using onTouchEvent() to intercept touch events. Here is the original source code for the examples used in this lesson.

Drag an Object

If you are targeting Android 3.0 or higher, you can use the built-in drag-and-drop event listeners with View.OnDragListener, as described in Drag and Drop.

A common operation for a touch gesture is to use it to drag an object across the screen. The following snippet lets the user drag an on-screen image. Note the following:

• In a drag (or scroll) operation, the app has to keep track of the original pointer (finger), even if additional fingers get placed on the screen. For example, imagine that while dragging the image around, the user places a second finger on the touch screen and lifts the first finger. If your app is just tracking individual pointers, it will regard the second pointer as the default and move the image to that location.
• To prevent this from happening, your app needs to distinguish between the original pointer and any follow-on pointers. To do this, it tracks the ACTION_POINTER_DOWN and ACTION_POINTER_UP events described in Handling Multi-Touch Gestures. ACTION_POINTER_DOWN and ACTION_POINTER_UP are passed to the onTouchEvent() callback whenever a secondary pointer goes down or up.
• In the ACTION_POINTER_UP case, the example extracts this index and ensures that the active pointer ID is not referring to a pointer that is no longer touching the screen. If it is, the app selects a different pointer to be active and saves its current X and Y position. Since this saved position is used in the ACTION_MOVE case to calculate the distance to move the onscreen object, the app will always calculate the distance to move using data from the correct pointer.

The following snippet enables a user to drag an object around on the screen. It records the initial position of the active pointer, calculates the distance the pointer traveled, and moves the object to the new position. It correctly manages the possibility of additional pointers, as described above.

Notice that the snippet uses the getActionMasked() method. You should always use this method (or better yet, the compatability version MotionEventCompat.getActionMasked()) to retrieve the action of a MotionEvent. Unlike the older getAction() method, getActionMasked() is designed to work with multiple pointers. It returns the masked action being performed, without including the pointer index bits.

// The ‘active pointer’ is the one currently moving our object.
private int mActivePointerId = INVALID_POINTER_ID;

@Override
public boolean onTouchEvent(MotionEvent ev) {
    // Let the ScaleGestureDetector inspect all events.
    mScaleDetector.onTouchEvent(ev);
             
    final int action = MotionEventCompat.getActionMasked(ev); 
        
    switch (action) { 
    case MotionEvent.ACTION_DOWN: {
        final int pointerIndex = MotionEventCompat.getActionIndex(ev); 
        final float x = MotionEventCompat.getX(ev, pointerIndex); 
        final float y = MotionEventCompat.getY(ev, pointerIndex); 
            
        // Remember where we started (for dragging)
        mLastTouchX = x;
        mLastTouchY = y;
        // Save the ID of this pointer (for dragging)
        mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
        break;
    }
            
    case MotionEvent.ACTION_MOVE: {
        // Find the index of the active pointer and fetch its position
        final int pointerIndex = 
                MotionEventCompat.findPointerIndex(ev, mActivePointerId);  
            
        final float x = MotionEventCompat.getX(ev, pointerIndex);
        final float y = MotionEventCompat.getY(ev, pointerIndex);
            
        // Only move if the ScaleGestureDetector isn't processing a gesture.
        if (!mScaleDetector.isInProgress()) {
            // Calculate the distance moved
            final float dx = x - mLastTouchX;
            final float dy = y - mLastTouchY;

            mPosX += dx;
            mPosY += dy;

            invalidate();
        }
        // Remember this touch position for the next move event
        mLastTouchX = x;
        mLastTouchY = y;

        break;
    }
            
    case MotionEvent.ACTION_UP: {
        mActivePointerId = INVALID_POINTER_ID;
        break;
    }
            
    case MotionEvent.ACTION_CANCEL: {
        mActivePointerId = INVALID_POINTER_ID;
        break;
    }
        
    case MotionEvent.ACTION_POINTER_UP: {
            
        final int pointerIndex = MotionEventCompat.getActionIndex(ev); 
        final int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex); 

        if (pointerId == mActivePointerId) {
            // This was our active pointer going up. Choose a new
            // active pointer and adjust accordingly.
            final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
            mLastTouchX = MotionEventCompat.getX(ev, newPointerIndex); 
            mLastTouchY = MotionEventCompat.getY(ev, newPointerIndex); 
            mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex);
        }
        break;
    }
    }       
    return true;
}

Use Touch to Perform Scaling

As discussed in Detecting Common Gestures, GestureDetector helps you detect common gestures used by Android such as scrolling, flinging, and long press. For scaling, Android provides ScaleGestureDetector. GestureDetector and ScaleGestureDetector can be used together when you want a view to recognize additional gestures.

To report detected gesture events, gesture detectors use listener objects passed to their constructors. ScaleGestureDetector uses ScaleGestureDetector.OnScaleGestureListener. Android provides ScaleGestureDetector.SimpleOnScaleGestureListener as a helper class that you can extend if you don’t care about all of the reported events.

Here is a snippet that gives you the basic idea of how to perform scaling. Here is the original source code for the examples.

private ScaleGestureDetector mScaleDetector;
private float mScaleFactor = 1.f;

public MyCustomView(Context mContext){
    ...
    // View code goes here
    ...
    mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
}

@Override
public boolean onTouchEvent(MotionEvent ev) {
    // Let the ScaleGestureDetector inspect all events.
    mScaleDetector.onTouchEvent(ev);
    return true;
}

@Override
public void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    canvas.save();
    canvas.scale(mScaleFactor, mScaleFactor);
    ...
    // onDraw() code goes here
    ...
    canvas.restore();
}

private class ScaleListener 
        extends ScaleGestureDetector.SimpleOnScaleGestureListener {
    @Override
    public boolean onScale(ScaleGestureDetector detector) {
        mScaleFactor *= detector.getScaleFactor();

        // Don't let the object get too small or too large.
        mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));

        invalidate();
        return true;
    }
}