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page.title=Making the View Interactive
parent.title=Creating Custom Views
previous.title=Custom Drawing
next.title=Optmizing the View
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<h2>This lesson teaches you to</h2>
<li><a href="#inputgesture">Handle Input Gestures</a></li>
<li><a href="#motion">Create Physically Plausible Motion</a></li>
<li><a href="#makesmooth">Make Your Transitions Smooth</a></li>
<h2>You should also read</h2>
<li><a href="{@docRoot}guide/topics/ui/ui-events.html">Input Events</a></li>
<li><a href="{@docRoot}guide/topics/graphics/prop-animation.html">Property Animation</a>
<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>Drawing a UI is only one part of creating a custom view. You also need to make your view respond
to user input in a
way that closely resembles the real-world action you're mimicking. Objects should always act in the
same way that real
objects do. For example, images should not immediately pop out of existence and reappear somewhere
else, because objects
in the real world don't do that. Instead, images should move from one place to another.</p>
<p>Users also sense subtle behavior or feel in an interface, and react best to subtleties that
mimic the real world.
For example, when users fling a UI object, they should sense friction at the beginning that delays
the motion, and then
at the end sense momentum that carries the motion beyond the fling.</p>
<p>This lesson demonstrates how to use features of the Android framework to add these real-world
behaviors to your
custom view.
<h2 id="inputgesture">Handle Input Gestures</h2>
<p>Like many other UI frameworks, Android supports an input event model. User actions are turned
into events that
trigger callbacks, and you can override the callbacks to customize how your application responds
to the user. The
most common input event in the Android system is <em>touch</em>, which triggers {@link
android.view.View#onTouchEvent(android.view.MotionEvent)}. Override this method to handle the
public boolean onTouchEvent(MotionEvent event) {
return super.onTouchEvent(event);
<p>Touch events by themselves are not particularly useful. Modern touch UIs define interactions in
terms of gestures
such as tapping, pulling, pushing, flinging, and zooming. To convert raw touch events into
gestures, Android
provides {@link android.view.GestureDetector}.</p>
<p>Construct a {@link android.view.GestureDetector} by passing in an instance of a class that
implements {@link
android.view.GestureDetector.OnGestureListener}. If you only want to process a few gestures, you
can extend {@link
android.view.GestureDetector.SimpleOnGestureListener} instead of implementing the {@link
interface. For instance, this code creates a class that extends {@link
android.view.GestureDetector.SimpleOnGestureListener} and overrides {@link
class mListener extends GestureDetector.SimpleOnGestureListener {
public boolean onDown(MotionEvent e) {
return true;
mDetector = new GestureDetector(PieChart.this.getContext(), new mListener());
<p>Whether or not you use {@link
android.view.GestureDetector.SimpleOnGestureListener}, you must always implement an
{@link android.view.GestureDetector.OnGestureListener#onDown onDown()} method that
returns {@code true}. This step is necessary because all gestures begin with an
{@link android.view.GestureDetector.OnGestureListener#onDown onDown()} message. If
you return {@code
false} from {@link android.view.GestureDetector.OnGestureListener#onDown onDown()}, as
{@link android.view.GestureDetector.SimpleOnGestureListener} does, the system assumes that
you want to ignore the
rest of the gesture, and the other methods of
{@link android.view.GestureDetector.OnGestureListener} never get called. The
only time you should
return {@code false} from {@link android.view.GestureDetector.OnGestureListener#onDown onDown()}
is if you truly want to ignore an entire gesture.
Once you've implemented {@link android.view.GestureDetector.OnGestureListener}
and created an instance of {@link android.view.GestureDetector}, you can use
your {@link android.view.GestureDetector} to interpret the touch events you receive in {@link
android.view.GestureDetector#onTouchEvent onTouchEvent()}.</p>
public boolean onTouchEvent(MotionEvent event) {
boolean result = mDetector.onTouchEvent(event);
if (!result) {
if (event.getAction() == MotionEvent.ACTION_UP) {
result = true;
return result;
<p>When you pass {@link android.view.GestureDetector#onTouchEvent onTouchEvent()} a touch event that
it doesn't
recognize as part of a gesture, it returns {@code false}. You can then run your own custom
<h2 id="motion">Create Physically Plausible Motion</h2>
<p>Gestures are a powerful way to control touchscreen devices, but they can be counterintuitive and
difficult to
remember unless they produce physically plausible results. A good example of this is the <em>fling</em>
gesture, where the
user quickly moves a finger across the screen and then lifts it. This gesture makes sense if the UI
responds by moving
quickly in the direction of the fling, then slowing down, as if the user had pushed on a
flywheel and set it
<p>However, simulating the feel of a flywheel isn't trivial. A lot of physics and math are required
to get a flywheel
model working correctly. Fortunately, Android provides helper classes to simulate this and other
behaviors. The
{@link android.widget.Scroller} class is the basis for handling flywheel-style <em>fling</em>
<p>To start a fling, call {@link android.widget.Scroller#fling fling()} with the starting velocity
and the minimum and
maximum x and y values of the fling. For the velocity value, you can use the value computed for
you by {@link android.view.GestureDetector}.</p>
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
mScroller.fling(currentX, currentY, velocityX / SCALE, velocityY / SCALE, minX, minY, maxX, maxY);
<p class="note"><strong>Note:</strong> Although the velocity calculated by
{@link android.view.GestureDetector} is physically accurate,
many developers feel
that using this value makes the fling animation too fast. It's common to divide the x and y
velocity by a factor of
4 to 8.</p>
<p>The call to {@link android.widget.Scroller#fling fling()} sets up the physics model for the fling
Afterwards, you need to update the {@link android.widget.Scroller Scroller} by calling {@link
android.widget.Scroller#computeScrollOffset Scroller.computeScrollOffset()} at regular
intervals. {@link
android.widget.Scroller#computeScrollOffset computeScrollOffset()} updates the {@link
Scroller} object's internal state by reading the current time and using the physics model to calculate
the x and y position
at that time. Call {@link android.widget.Scroller#getCurrX} and {@link
android.widget.Scroller#getCurrY} to
retrieve these values.</p>
<p>Most views pass the {@link android.widget.Scroller Scroller} object's x and y position directly to
android.view.View#scrollTo scrollTo()}. The PieChart example is a little different: it
uses the current scroll
y position to set the rotational angle of the chart.</p>
if (!mScroller.isFinished()) {
<p>The {@link android.widget.Scroller Scroller} class computes scroll positions for you, but it does
not automatically
apply those positions to your view. It's your responsibility to make sure you get and apply new
coordinates often
enough to make the scrolling animation look smooth. There are two ways to do this:</p>
<li>Call {@link android.view.View#postInvalidate() postInvalidate()} after calling
{@link android.widget.Scroller#fling(int, int, int, int, int, int, int, int) fling()},
in order to
force a redraw. This
technique requires that you compute scroll offsets in {@link android.view.View#onDraw onDraw()}
and call {@link android.view.View#postInvalidate() postInvalidate()} every
time the scroll offset changes.
<li>Set up a {@link android.animation.ValueAnimator} to animate for the duration of the fling,
and add a listener to process animation updates
by calling {@link android.animation.ValueAnimator#addUpdateListener addUpdateListener()}.
<p>The PieChart example uses the second approach. This technique is slightly more complex to set up, but
it works more
closely with the animation system and doesn't require potentially unnecessary view
invalidation. The drawback is that {@link android.animation.ValueAnimator}
is not available prior to API level 11, so this technique cannot be used
on devices running Android versions lower than 3.0.</p>
<p class="note"><strong>Note:</strong> {@link android.animation.ValueAnimator} isn't available
prior to API level 11, but you can still use it in applications that
target lower API levels. You just need to make sure to check the current API level
at runtime, and omit the calls to the view animation system if the current level is less than 11.</p>
mScroller = new Scroller(getContext(), null, true);
mScrollAnimator = ValueAnimator.ofFloat(0,1);
mScrollAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
public void onAnimationUpdate(ValueAnimator valueAnimator) {
if (!mScroller.isFinished()) {
} else {
<h2 id="makesmooth">Make Your Transitions Smooth</h2>
<p>Users expect a modern UI to transition smoothly between states. UI elements fade in and out
instead of appearing and
disappearing. Motions begin and end smoothly instead of starting and stopping abruptly. The
Android <a
href="{@docRoot}guide/topics/graphics/prop-animation.html">property animation
framework</a>, introduced in
Android 3.0, makes smooth transitions easy.</p>
<p>To use the animation system, whenever a property changes that will affect your view's appearance,
do not change the
property directly. Instead, use {@link android.animation.ValueAnimator} to make the change. In
the following
example, modifying the
currently selected pie slice in PieChart causes the entire chart to rotate so that the selection
pointer is centered
in the selected slice. {@link android.animation.ValueAnimator} changes the rotation over a
period of several
hundred milliseconds,
rather than immediately setting the new rotation value.</p>
mAutoCenterAnimator = ObjectAnimator.ofInt(PieChart.this, "PieRotation", 0);
<p>If the value you want to change is one of the base {@link android.view.View} properties, doing
the animation
is even easier,
because Views have a built-in {@link android.view.ViewPropertyAnimator} that is optimized for
simultaneous animation
of multiple properties. For example:</p>