Drag and scale

This lesson describes how to use touch gestures to drag and scale on-screen objects, using onTouchEvent() to intercept touch events.

Refer to the following related resources:

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 compatibility 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);

        // 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;
}

Drag to pan

The previous section showed an example of dragging an object around the screen. Another common scenario is panning, which is when a user's dragging motion causes scrolling in both the x and y axes. The above snippet directly intercepted the MotionEvent actions to implement dragging. The snippet in this section takes advantage of the platform's built-in support for common gestures. It overrides onScroll() in GestureDetector.SimpleOnGestureListener.

To provide a little more context, onScroll() is called when a user is dragging his finger to pan the content. onScroll() is only called when a finger is down; as soon as the finger is lifted from the screen, the gesture either ends, or a fling gesture is started (if the finger was moving with some speed just before it was lifted). For more discussion of scrolling vs. flinging, see Animating a Scroll Gesture.

Here is the snippet for onScroll(): 

// The current viewport. This rectangle represents the currently visible
// chart domain and range.
private RectF mCurrentViewport =
        new RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX);

// The current destination rectangle (in pixel coordinates) into which the
// chart data should be drawn.
private Rect mContentRect;

private final GestureDetector.SimpleOnGestureListener mGestureListener
            = new GestureDetector.SimpleOnGestureListener() {
...

@Override
public boolean onScroll(MotionEvent e1, MotionEvent e2,
            float distanceX, float distanceY) {
    // Scrolling uses math based on the viewport (as opposed to math using pixels).

    // Pixel offset is the offset in screen pixels, while viewport offset is the
    // offset within the current viewport.
    float viewportOffsetX = distanceX * mCurrentViewport.width()
            / mContentRect.width();
    float viewportOffsetY = -distanceY * mCurrentViewport.height()
            / mContentRect.height();
    ...
    // Updates the viewport, refreshes the display.
    setViewportBottomLeft(
            mCurrentViewport.left + viewportOffsetX,
            mCurrentViewport.bottom + viewportOffsetY);
    ...
    return true;
}

The implementation of onScroll() scrolls the viewport in response to the touch gesture:

/**
 * Sets the current viewport (defined by mCurrentViewport) to the given
 * X and Y positions. Note that the Y value represents the topmost pixel position,
 * and thus the bottom of the mCurrentViewport rectangle.
 */
private void setViewportBottomLeft(float x, float y) {
    /*
     * Constrains within the scroll range. The scroll range is simply the viewport
     * extremes (AXIS_X_MAX, etc.) minus the viewport size. For example, if the
     * extremes were 0 and 10, and the viewport size was 2, the scroll range would
     * be 0 to 8.
     */

    float curWidth = mCurrentViewport.width();
    float curHeight = mCurrentViewport.height();
    x = Math.max(AXIS_X_MIN, Math.min(x, AXIS_X_MAX - curWidth));
    y = Math.max(AXIS_Y_MIN + curHeight, Math.min(y, AXIS_Y_MAX));

    mCurrentViewport.set(x, y - curHeight, x + curWidth, y);

    // Invalidates the View to update the display.
    ViewCompat.postInvalidateOnAnimation(this);
}

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.

Basic scaling example

Here is a snippet that illustrates the basic ingredients involved in scaling.

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;
    }
}

More complex scaling example

Here is a more complex example from the InteractiveChart sample provided with this class. The InteractiveChart sample supports both scrolling (panning) and scaling with multiple fingers, using the ScaleGestureDetector "span" (getCurrentSpanX/Y) and "focus" (getFocusX/Y) features:

@Override
private RectF mCurrentViewport =
        new RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX);
private Rect mContentRect;
private ScaleGestureDetector mScaleGestureDetector;
...
public boolean onTouchEvent(MotionEvent event) {
    boolean retVal = mScaleGestureDetector.onTouchEvent(event);
    retVal = mGestureDetector.onTouchEvent(event) || retVal;
    return retVal || super.onTouchEvent(event);
}

/**
 * The scale listener, used for handling multi-finger scale gestures.
 */
private final ScaleGestureDetector.OnScaleGestureListener mScaleGestureListener
        = new ScaleGestureDetector.SimpleOnScaleGestureListener() {
    /**
     * This is the active focal point in terms of the viewport. Could be a local
     * variable but kept here to minimize per-frame allocations.
     */
    private PointF viewportFocus = new PointF();
    private float lastSpanX;
    private float lastSpanY;

    // Detects that new pointers are going down.
    @Override
    public boolean onScaleBegin(ScaleGestureDetector scaleGestureDetector) {
        lastSpanX = ScaleGestureDetectorCompat.
                getCurrentSpanX(scaleGestureDetector);
        lastSpanY = ScaleGestureDetectorCompat.
                getCurrentSpanY(scaleGestureDetector);
        return true;
    }

    @Override
    public boolean onScale(ScaleGestureDetector scaleGestureDetector) {

        float spanX = ScaleGestureDetectorCompat.
                getCurrentSpanX(scaleGestureDetector);
        float spanY = ScaleGestureDetectorCompat.
                getCurrentSpanY(scaleGestureDetector);

        float newWidth = lastSpanX / spanX * mCurrentViewport.width();
        float newHeight = lastSpanY / spanY * mCurrentViewport.height();

        float focusX = scaleGestureDetector.getFocusX();
        float focusY = scaleGestureDetector.getFocusY();
        // Makes sure that the chart point is within the chart region.
        // See the sample for the implementation of hitTest().
        hitTest(scaleGestureDetector.getFocusX(),
                scaleGestureDetector.getFocusY(),
                viewportFocus);

        mCurrentViewport.set(
                viewportFocus.x
                        - newWidth * (focusX - mContentRect.left)
                        / mContentRect.width(),
                viewportFocus.y
                        - newHeight * (mContentRect.bottom - focusY)
                        / mContentRect.height(),
                0,
                0);
        mCurrentViewport.right = mCurrentViewport.left + newWidth;
        mCurrentViewport.bottom = mCurrentViewport.top + newHeight;
        ...
        // Invalidates the View to update the display.
        ViewCompat.postInvalidateOnAnimation(InteractiveLineGraphView.this);

        lastSpanX = spanX;
        lastSpanY = spanY;
        return true;
    }
};
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