blob: f2e4eb82d12285e6c4263c947586dc2c4d8594a6 [file] [log] [blame]
page.title=Dragging and Scaling
parent.title=Using Touch Gestures
next.title=Managing Touch Events in a ViewGroup
<div id="tb-wrapper">
<div id="tb">
<!-- table of contents -->
<h2>This lesson teaches you to</h2>
<li><a href="#drag">Drag an Object</a></li>
<li><a href="#pan">Drag to Pan</a></li>
<li><a href="#scale">Use Touch to Perform Scaling</a></li>
<!-- other docs (NOT javadocs) -->
<h2>You should also read</h2>
<li><a href="">Input Events</a> API Guide
<li><a href="{@docRoot}guide/topics/sensors/sensors_overview.html">Sensors Overview</a></li>
<li><a href="{@docRoot}training/custom-views/making-interactive.html">Making the View Interactive</a> </li>
<li>Design Guide for <a href="{@docRoot}design/patterns/gestures.html">Gestures</a></li>
<li>Design Guide for <a href="{@docRoot}design/style/touch-feedback.html">Touch Feedback</a></li>
<h2>Try it out</h2>
<div class="download-box">
<a href="{@docRoot}shareables/training/"
class="button">Download the sample</a>
<p class="filename"></p>
<p>This lesson describes how to use touch gestures to drag and scale on-screen
objects, using {@link android.view.View#onTouchEvent onTouchEvent()} to intercept
touch events.
<h2 id="drag">Drag an Object</h2>
<p class="note">If you are targeting Android 3.0 or higher, you can use the built-in drag-and-drop event
listeners with {@link android.view.View.OnDragListener}, as described in
<a href="{@docRoot}guide/topics/ui/drag-drop.html">Drag and Drop</a>.
<p>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:</p>
<li>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.</li>
<li>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
{@link android.view.MotionEvent#ACTION_POINTER_DOWN} and
{@link android.view.MotionEvent#ACTION_POINTER_UP} events described in
<a href="multi.html">Handling Multi-Touch Gestures</a>.
{@link android.view.MotionEvent#ACTION_POINTER_DOWN} and
{@link android.view.MotionEvent#ACTION_POINTER_UP} are
passed to the {@link android.view.View#onTouchEvent onTouchEvent()} callback
whenever a secondary pointer goes down or up. </li>
<li>In the {@link android.view.MotionEvent#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 {@link android.view.MotionEvent#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.</li>
<p>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
<p>Notice that the snippet uses the {@link android.view.MotionEvent#getActionMasked getActionMasked()} method.
You should always use this method (or better yet, the compatability version
{@link MotionEventCompat.getActionMasked()})
to retrieve the action of a
{@link android.view.MotionEvent}. Unlike the older
{@link android.view.MotionEvent#getAction getAction()}
method, {@link getActionMasked()}
is designed to work with multiple pointers. It returns the masked action
being performed, without including the pointer index bits.</p>
<pre>// The ‘active pointer’ is the one currently moving our object.
private int mActivePointerId = INVALID_POINTER_ID;
public boolean onTouchEvent(MotionEvent ev) {
// Let the ScaleGestureDetector inspect all events.
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);
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;
// Remember this touch position for the next move event
mLastTouchX = x;
mLastTouchY = y;
case MotionEvent.ACTION_UP: {
mActivePointerId = INVALID_POINTER_ID;
case MotionEvent.ACTION_CANCEL: {
mActivePointerId = INVALID_POINTER_ID;
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);
return true;
<h2 id="pan">Drag to Pan</h2>
<p>The previous section showed an example of dragging an object around the screen. Another
common scenario is <em>panning</em>, which is when a user's dragging motion causes scrolling
in both the x and y axes. The above snippet directly intercepted the {@link android.view.MotionEvent}
actions to implement dragging. The snippet in this section takes advantage of the platform's
built-in support for common gestures. It overrides
{@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()} in
{@link android.view.GestureDetector.SimpleOnGestureListener}.</p>
<p>To provide a little more context, {@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()}
is called when a user is dragging his finger to pan the content.
{@link android.view.GestureDetector.OnGestureListener#onScroll 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 <a href="scroll.html">Animating a Scroll Gesture</a>.</p>
<p>Here is the snippet for {@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()}:
<pre>// The current viewport. This rectangle represents the currently visible
// chart domain and range.
private RectF mCurrentViewport =
// 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() {
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.
mCurrentViewport.left + viewportOffsetX,
mCurrentViewport.bottom + viewportOffsetY);
return true;
<p>The implementation of {@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()}
scrolls the viewport in response to the touch gesture:</p>
* 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.
<h2 id="scale">Use Touch to Perform Scaling</h2>
<p>As discussed in <a href="detector.html">Detecting Common Gestures</a>,
{@link android.view.GestureDetector} helps you detect common gestures used by
Android such as scrolling, flinging, and long press. For scaling, Android
provides {@link android.view.ScaleGestureDetector}. {@link
android.view.GestureDetector} and {@link android.view.ScaleGestureDetector} can
be used together when you want a view to recognize additional gestures.</p>
<p>To report detected gesture events, gesture detectors use listener objects
passed to their constructors. {@link android.view.ScaleGestureDetector} uses
{@link android.view.ScaleGestureDetector.OnScaleGestureListener}.
Android provides
{@link android.view.ScaleGestureDetector.SimpleOnScaleGestureListener}
as a helper class that you can extend if you dont care about all of the reported events.</p>
<h3>Basic scaling example</h3>
<p>Here is a snippet that illustrates the basic ingredients involved in scaling.</p>
<pre>private ScaleGestureDetector mScaleDetector;
private float mScaleFactor = 1.f;
public MyCustomView(Context mContext){
// View code goes here
mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
public boolean onTouchEvent(MotionEvent ev) {
// Let the ScaleGestureDetector inspect all events.
return true;
public void onDraw(Canvas canvas) {
canvas.scale(mScaleFactor, mScaleFactor);
// onDraw() code goes here
private class ScaleListener
extends ScaleGestureDetector.SimpleOnScaleGestureListener {
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));
return true;
<h3>More complex scaling example</h3>
<p>Here is a more complex example from the {@code InteractiveChart} sample provided with this class.
The {@code InteractiveChart} sample supports both scrolling (panning) and scaling with multiple fingers,
using the {@link android.view.ScaleGestureDetector} "span"
({@link android.view.ScaleGestureDetector#getCurrentSpanX getCurrentSpanX/Y}) and
"focus" ({@link android.view.ScaleGestureDetector#getFocusX getFocusX/Y}) features:</p>
private RectF mCurrentViewport =
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.
public boolean onScaleBegin(ScaleGestureDetector scaleGestureDetector) {
lastSpanX = ScaleGestureDetectorCompat.
lastSpanY = ScaleGestureDetectorCompat.
return true;
public boolean onScale(ScaleGestureDetector scaleGestureDetector) {
float spanX = ScaleGestureDetectorCompat.
float spanY = ScaleGestureDetectorCompat.
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().
- newWidth * (focusX - mContentRect.left)
/ mContentRect.width(),
- newHeight * (mContentRect.bottom - focusY)
/ mContentRect.height(),
mCurrentViewport.right = mCurrentViewport.left + newWidth;
mCurrentViewport.bottom = + newHeight;
// Invalidates the View to update the display.
lastSpanX = spanX;
lastSpanY = spanY;
return true;