cherrypick from jb-dev-docs: Gestures Class Change-Id: I9abebf58a9607c8f52f72ef2ce46308304386596 (518edbfa) · Commits · e / os / android_frameworks_base

docs/html/training/gestures/detector.jd

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			page.title=Detecting Common Gestures
			parent.title=Using Touch Gestures
			parent.link=index.html

			trainingnavtop=true
			next.title=Tracking Movement
			next.link=movement.html

			@jd:body

			<div id="tb-wrapper">
			<div id="tb">

			<!-- table of contents -->
			<h2>This lesson teaches you to</h2>
			<ol>
			<li><a href="#data">Gather Data</a></li>
			<li><a href="#detect">Detect Gestures</a></li>
			</ol>

			<!-- other docs (NOT javadocs) -->
			<h2>You should also read</h2>

			<ul>
			<li><a href="http://developer.android.com/guide/topics/ui/ui-events.html">Input Events</a> API Guide
			</li>
			<li><a href="{@docRoot}guide/topics/sensors/sensors_overview.html">Sensors Overview</a></li>
			<li><a href="http://android-developers.blogspot.com/2010/06/making-sense-of-multitouch.html">Making Sense of Multitouch</a> blog post</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>
			</ul>


			</div>
			</div>

			<p>A "touch gesture" occurs when a user places one or more fingers on the touch
			screen, and your application interprets
			that pattern of touches as a particular gesture. There are correspondingly two
			phases to gesture detection:</p>

			<ol>
			<li>Gathering data about touch events.</li>

			<li>Interpreting the data to see if it meets the criteria for any of the
			gestures your app supports. </li>

			</ol>

			<h4>Support Library Classes</h4>

			<p>The examples in this lesson use the {@link android.support.v4.view.GestureDetectorCompat}
			and {@link android.support.v4.view.MotionEventCompat} classes. These classes are in the
			<a href="{@docRoot}tools/extras/support-library.html">Support Library</a>. You should use
			Support Library classes where possible to provide compatibility with devices
			running Android 1.6 and higher. Note that {@link android.support.v4.view.MotionEventCompat} is <em>not</em> a
			replacement for the {@link android.view.MotionEvent} class. Rather, it provides static utility
			methods to which you pass your {@link android.view.MotionEvent} object in order to receive
			the desired action associated with that event.</p>

			<h2 id="data">Gather Data</h2>

			<p>When a user places one or more fingers on the screen, this triggers the
			callback {@link android.view.View#onTouchEvent onTouchEvent()}
			on the View that received the touch events.
			For each sequence of touch events (position, pressure, size, addition of another finger, etc.)
			that is ultimately identified as a gesture,
			{@link android.view.View#onTouchEvent onTouchEvent()} is fired several times.</p>

			<p>The gesture starts when the user first touches the screen, continues as the system tracks
			the position of the user's finger(s), and ends by capturing the final event of
			the user's fingers leaving the screen. Throughout this interaction,
			the {@link android.view.MotionEvent} delivered to {@link android.view.View#onTouchEvent onTouchEvent()}
			provides the details of every interaction. Your app can use the data provided by the {@link android.view.MotionEvent}
			to determine if a gesture it cares
			about happened.</p>

			<h3>Capturing touch events for an Activity or View</h3>

			<p><p>To intercept touch events in an Activity or View, override
			the {@link android.view.View#onTouchEvent onTouchEvent()} callback.</p>

			<p>The following snippet uses
			{@link android.support.v4.view.MotionEventCompat#getActionMasked getActionMasked()}
			to extract the action the user performed from the {@code event} parameter. This gives you the raw
			data you need to determine if a gesture you care about occurred:</p>

			<pre>
			public class MainActivity extends Activity {
			...
			// This example shows an Activity, but you would use the same approach if
			// you were subclassing a View.
			@Override
			public boolean onTouchEvent(MotionEvent event){

			int action = MotionEventCompat.getActionMasked(event);

			switch(action) {
			case (MotionEvent.ACTION_DOWN) :
			Log.d(DEBUG_TAG,"Action was DOWN");
			return true;
			case (MotionEvent.ACTION_MOVE) :
			Log.d(DEBUG_TAG,"Action was MOVE");
			return true;
			case (MotionEvent.ACTION_UP) :
			Log.d(DEBUG_TAG,"Action was UP");
			return true;
			case (MotionEvent.ACTION_CANCEL) :
			Log.d(DEBUG_TAG,"Action was CANCEL");
			return true;
			case (MotionEvent.ACTION_OUTSIDE) :
			Log.d(DEBUG_TAG,"Movement occurred outside bounds " +
			"of current screen element");
			return true;
			default :
			return super.onTouchEvent(event);
			}
			}</pre>

			<p>You can then do your own processing on these events to determine if a
			gesture occurred. This is the kind of processing you would have to do for a
			custom gesture. However, if your app uses
			common gestures such as double tap, long press, fling, and so on, you can
			take advantage of the {@link
			android.view.GestureDetector} class. {@link
			android.view.GestureDetector} makes it easy for you to detect common
			gestures without processing the individual touch events yourself. This is
			discussed below in <a href="#detect">Detect Gestures</a>.</p>

			<h3>Capturing touch events for a single view</h3>

			<p>As an alternative to {@link android.view.View#onTouchEvent onTouchEvent()},
			you can attach an {@link android.view.View.OnTouchListener} object to any {@link
			android.view.View} object using the {@link android.view.View#setOnTouchListener
			setOnTouchListener()} method. This makes it possible to to listen for touch
			events without subclassing an existing {@link android.view.View}. For
			example:</p>

			<pre>View myView = findViewById(R.id.my_view);
			myView.setOnTouchListener(new OnTouchListener() {
			public boolean onTouch(View v, MotionEvent event) {
			// ... Respond to touch events
			return true;
			}
			});</pre>

			<p>Beware of creating a listener that returns {@code false} for the
			{@link android.view.MotionEvent#ACTION_DOWN} event. If you do this, the listener will
			not be called for the subsequent {@link android.view.MotionEvent#ACTION_MOVE}
			and {@link android.view.MotionEvent#ACTION_UP} string of events. This is because
			{@link android.view.MotionEvent#ACTION_DOWN} is the starting point for all touch events.</p>

			<p>If you are creating a custom View, you can override
			{@link android.view.View#onTouchEvent onTouchEvent()},
			as described above.</p>

			<h2 id="detect">Detect Gestures</h2>

			<p>Android provides the {@link android.view.GestureDetector} class for detecting
			common gestures. Some of the gestures it supports include {@link
			android.view.GestureDetector.OnGestureListener#onDown onDown()}, {@link
			android.view.GestureDetector.OnGestureListener#onLongPress onLongPress()},
			{@link android.view.GestureDetector.OnGestureListener#onFling onFling()}, and so
			on. You can use {@link android.view.GestureDetector} in conjunction with the
			{@link android.view.View#onTouchEvent onTouchEvent()}
			method described above.</p>


			<h3>Detecting All Supported Gestures</h3>

			<p>When you instantiate a {@link android.support.v4.view.GestureDetectorCompat}
			object, one of the parameters it takes is a class that implements the
			{@link android.view.GestureDetector.OnGestureListener} interface.
			{@link android.view.GestureDetector.OnGestureListener} notifies users when
			a particular touch event has occurred. To make it possible for your
			{@link android.view.GestureDetector} object to receive events, you override
			the View or Activity's {@link android.view.View#onTouchEvent onTouchEvent()} method,
			and pass along all observed events to the detector instance.</p>


			<p>In the following snippet, a return value of {@code true} from the individual
			{@code on<em><TouchEvent></em>} methods indicates that you
			have handled the touch event. A return value of {@code false} passes events down
			through the view stack until the touch has been successfully handled.</p>

			<p>Run the following snippet to get a feel for how actions are triggered when
			you interact with the touch screen, and what the contents of the {@link
			android.view.MotionEvent} are for each touch event. You will realize how much
			data is being generated for even simple interactions.</p>

			<pre>public class MainActivity extends Activity implements
			GestureDetector.OnGestureListener,
			GestureDetector.OnDoubleTapListener{

			private static final String DEBUG_TAG = "Gestures";
			private GestureDetectorCompat mDetector;

			// Called when the activity is first created.
			@Override
			public void onCreate(Bundle savedInstanceState) {
			super.onCreate(savedInstanceState);
			setContentView(R.layout.activity_main);
			// Instantiate the gesture detector with the
			// application context and an implementation of
			// GestureDetector.OnGestureListener
			mDetector = new GestureDetectorCompat(this,this);
			// Set the gesture detector as the double tap
			// listener.
			mDetector.setOnDoubleTapListener(this);
			}

			@Override
			public boolean onTouchEvent(MotionEvent event){
			this.mDetector.onTouchEvent(event);
			// Be sure to call the superclass implementation
			return super.onTouchEvent(event);
			}

			@Override
			public boolean onDown(MotionEvent event) {
			Log.d(DEBUG_TAG,"onDown: " + event.toString());
			return true;
			}

			@Override
			public boolean onFling(MotionEvent event1, MotionEvent event2,
			float velocityX, float velocityY) {
			Log.d(DEBUG_TAG, "onFling: " + event1.toString()+event2.toString());
			return true;
			}

			@Override
			public void onLongPress(MotionEvent event) {
			Log.d(DEBUG_TAG, "onLongPress: " + event.toString());
			}

			@Override
			public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX,
			float distanceY) {
			Log.d(DEBUG_TAG, "onScroll: " + e1.toString()+e2.toString());
			return true;
			}

			@Override
			public void onShowPress(MotionEvent event) {
			Log.d(DEBUG_TAG, "onShowPress: " + event.toString());
			}

			@Override
			public boolean onSingleTapUp(MotionEvent event) {
			Log.d(DEBUG_TAG, "onSingleTapUp: " + event.toString());
			return true;
			}

			@Override
			public boolean onDoubleTap(MotionEvent event) {
			Log.d(DEBUG_TAG, "onDoubleTap: " + event.toString());
			return true;
			}

			@Override
			public boolean onDoubleTapEvent(MotionEvent event) {
			Log.d(DEBUG_TAG, "onDoubleTapEvent: " + event.toString());
			return true;
			}

			@Override
			public boolean onSingleTapConfirmed(MotionEvent event) {
			Log.d(DEBUG_TAG, "onSingleTapConfirmed: " + event.toString());
			return true;
			}
			}</pre>

			<h3>Detecting a Subset of Supported Gestures</h3>

			<p>If you only want to process a few gestures, you can extend {@link
			android.view.GestureDetector.SimpleOnGestureListener} instead of implementing
			the {@link android.view.GestureDetector.OnGestureListener} interface. </p>
			<p>
			{@link
			android.view.GestureDetector.SimpleOnGestureListener} provides an implementation
			for all of the {@code on<em><TouchEvent></em>} methods by returning {@code false}
			for all of them. Thus you can override only the methods you care about.
			For
			example, the snippet below creates a class that extends {@link
			android.view.GestureDetector.SimpleOnGestureListener} and overrides {@link
			android.view.GestureDetector.OnGestureListener#onFling onFling()} and {@link
			android.view.GestureDetector.OnGestureListener#onDown onDown()}.</p>

			<p>Whether or not you use {@link android.view.GestureDetector.OnGestureListener},
			it's best practice to implement an
			{@link android.view.GestureDetector.OnGestureListener#onDown onDown()}
			method that returns {@code true}. This is 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 by default,
			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.
			This has the potential to cause unexpected problems in your app.
			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. </p>

			<pre>public class MainActivity extends Activity {

			private GestureDetectorCompat mDetector;

			@Override
			public void onCreate(Bundle savedInstanceState) {
			super.onCreate(savedInstanceState);
			setContentView(R.layout.activity_main);
			mDetector = new GestureDetectorCompat(this, new MyGestureListener());
			}

			@Override
			public boolean onTouchEvent(MotionEvent event){
			this.mDetector.onTouchEvent(event);
			return super.onTouchEvent(event);
			}

			class MyGestureListener extends GestureDetector.SimpleOnGestureListener {
			private static final String DEBUG_TAG = "Gestures";

			@Override
			public boolean onDown(MotionEvent event) {
			Log.d(DEBUG_TAG,"onDown: " + event.toString());
			return true;
			}

			@Override
			public boolean onFling(MotionEvent event1, MotionEvent event2,
			float velocityX, float velocityY) {
			Log.d(DEBUG_TAG, "onFling: " + event1.toString()+event2.toString());
			return true;
			}
			}
			}
			</pre>

docs/html/training/gestures/index.jd

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			page.title=Using Touch Gestures
			trainingnavtop=true
			startpage=true
			next.title=Detect Built-in Gestures
			next.link=detector.html


			@jd:body
			<div id="tb-wrapper">
			<div id="tb">

			<!-- Required platform, tools, add-ons, devices, knowledge, etc. -->
			<h2>Dependencies and prerequisites</h2>

			<ul>
			<li>Android 1.6 (API Level 4) or higher</li>
			</ul>
			<h2>You should also read</h2>
			<ul>
			<li><a href="http://developer.android.com/guide/topics/ui/ui-events.html">Input Events</a> API Guide
			</li>
			<li><a href="{@docRoot}guide/topics/sensors/sensors_overview.html">Sensors Overview</a></li>
			<li><a href="http://android-developers.blogspot.com/2010/06/making-sense-of-multitouch.html">Making Sense of Multitouch</a> blog post</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>
			</ul>


			</div>
			</div>

			<p> This class describes how to write apps that allow users to interact with an
			app via touch gestures. Android provides a variety of APIs to
			help you create and detect gestures.</p>

			<p>Although your app should not depend on touch gestures for basic behaviors (since the gestures
			may not be available to all users in all contexts), adding touch-based
			interaction to your app can greatly increase its usefulness and appeal.</p>

			<p>To
			provide users with a consistent, intuitive experience, your app should follow
			the accepted Android conventions for touch gestures. The <a
			href="http://developer.android.com/design/patterns/gestures.html">Gestures
			design guide</a><a href="{@docRoot}design/patterns/notifications.html"></a>
			shows you how to use common gestures in Android apps. Also see the Design Guide
			for <a href="{@docRoot}design/style/touch-feedback.html">Touch Feedback</a>. </p>


			<h2>Lessons</h2>

			<dl>
			<dt>
			<strong><a href="detector.html">Detecting Common Gestures</a></strong>
			</dt>
			<dd>
			Learn how to detect basic touch gestures such as scrolling, flinging, and double-tapping, using
			{@link android.view.GestureDetector}.
			</dd>

			<dt>
			<strong><a href="movement.html">Tracking Movement</a></strong>
			</dt>
			<dd>
			Learn how to track movement.
			</dd>

			<dt>
			<strong><a href="scroll.html">Animating a Scroll Gesture</a></strong>
			</dt>
			<dd>
			Learn how to use scrollers ({@link android.widget.Scroller} or {@link
			android.widget.OverScroller}) to produce a scrolling animation in response to a
			touch event. </dd>

			<dt>
			<strong><a href="multi.html">Handling Multi-Touch Gestures</a></strong>
			</dt>
			<dd>
			Learn how to detect multi-pointer (finger) gestures.
			</dd>
			<dt>
			<strong><a href="scale.html">Dragging and Scaling</a></strong>
			</dt>
			<dd>
			Learn how to implement touch-based dragging and scaling.
			</dd>


			<dt><strong><a href="viewgroup.html">Managing Touch Events in a ViewGroup</a></strong></dt>

			<dd>Learn how to manage touch events in a {@link android.view.ViewGroup} to
			ensure that touch events are correctly dispatched to their target views.</dd>
			</dl>

docs/html/training/gestures/movement.jd

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			page.title=Tracking Movement
			parent.title=Using Touch Gestures
			parent.link=index.html

			trainingnavtop=true
			next.title=Animating a Scroll Gesture
			next.link=scroll.html

			@jd:body

			<div id="tb-wrapper">
			<div id="tb">

			<!-- table of contents -->
			<h2>This lesson teaches you to</h2>
			<ol>
			<li><a href="#velocity">Track Velocity</a></li>
			</ol>

			<!-- other docs (NOT javadocs) -->
			<h2>You should also read</h2>

			<ul>
			<li><a href="http://developer.android.com/guide/topics/ui/ui-events.html">Input Events</a> API Guide
			</li>
			<li><a href="{@docRoot}guide/topics/sensors/sensors_overview.html">Sensors Overview</a></li>
			<li><a href="http://android-developers.blogspot.com/2010/06/making-sense-of-multitouch.html">Making Sense of Multitouch</a> blog post</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>
			</ul>


			</div>
			</div>

			<p>This lesson describes how to track movement in touch events.</p>

			<p>A new {@link
			android.view.View#onTouchEvent onTouchEvent()} is triggered with an {@link
			android.view.MotionEvent#ACTION_MOVE} event whenever the current touch contact
			position, pressure, or size changes. As described in <a
			href="detector.html">Detecting Common Gestures</a>, all of these events are
			recorded in the {@link android.view.MotionEvent} parameter of {@link
			android.view.View#onTouchEvent onTouchEvent()}.</p>

			<p>Because finger-based touch isn't always the most precise form of interaction,
			detecting touch events is often based more on movement than on simple contact.
			To help apps distinguish between movement-based gestures (such as a swipe) and
			non-movement gestures (such as a single tap), Android includes the notion of
			"touch slop." Touch slop refers to the distance in pixels a user's touch can wander
			before the gesture is interpreted as a movement-based gesture. For more discussion of this
			topic, see <a href="viewgroup.html#vc">Managing Touch Events in a ViewGroup</a>.</p>



			<p>There are several different ways to track movement in a gesture, depending on
			the needs of your application. For example:</p>

			<ul>

			<li>The starting and ending position of a pointer (for example, move an
			on-screen object from point A to point B).</li>

			<li>The direction the pointer is traveling in, as determined by the x and y coordinates.</li>

			<li>History. You can find the size of a gesture's history by calling the {@link
			android.view.MotionEvent} method {@link android.view.MotionEvent#getHistorySize
			getHistorySize()}. You can then obtain the positions, sizes, time, and pressures
			of each of the historical events by using the motion event's {@code
			getHistorical<em><Value></em>} methods. History is useful when rendering a trail of the user's finger,
			such as for touch drawing. See the {@link android.view.MotionEvent} reference for
			details.</li>

			<li>The velocity of the pointer as it moves across the touch screen.</li>

			</ul>



			<h2 id="velocity">Track Velocity</h2>

			<p> You could have a movement-based gesture that is simply based on the distance and/or direction the pointer traveled. But velocity often is a
			determining factor in tracking a gesture's characteristics or even deciding
			whether the gesture occurred. To make velocity calculation easier, Android
			provides the {@link android.view.VelocityTracker} class and the
			{@link android.support.v4.view.VelocityTrackerCompat} class in the
			<a href="{@docRoot}tools/extras/support-library.html">Support Library</a>.
			{@link
			android.view.VelocityTracker} helps you track the velocity of touch events. This
			is useful for gestures in which velocity is part of the criteria for the
			gesture, such as a fling.</p>


			<p>Here is a simple example that illustrates the purpose of the methods in the
			{@link android.view.VelocityTracker} API:</p>

			<pre>public class MainActivity extends Activity {
			private static final String DEBUG_TAG = "Velocity";
			...
			private VelocityTracker mVelocityTracker = null;
			@Override
			public boolean onTouchEvent(MotionEvent event) {
			int index = event.getActionIndex();
			int action = event.getActionMasked();
			int pointerId = event.getPointerId(index);

			switch(action) {
			case MotionEvent.ACTION_DOWN:
			if(mVelocityTracker == null) {
			// Retrieve a new VelocityTracker object to watch the velocity of a motion.
			mVelocityTracker = VelocityTracker.obtain();
			}
			else {
			// Reset the velocity tracker back to its initial state.
			mVelocityTracker.clear();
			}
			// Add a user's movement to the tracker.
			mVelocityTracker.addMovement(event);
			break;
			case MotionEvent.ACTION_MOVE:
			mVelocityTracker.addMovement(event);
			// When you want to determine the velocity, call
			// computeCurrentVelocity(). Then call getXVelocity()
			// and getYVelocity() to retrieve the velocity for each pointer ID.
			mVelocityTracker.computeCurrentVelocity(1000);
			// Log velocity of pixels per second
			// Best practice to use VelocityTrackerCompat where possible.
			Log.d("", "X velocity: " +
			VelocityTrackerCompat.getXVelocity(mVelocityTracker,
			pointerId));
			Log.d("", "Y velocity: " +
			VelocityTrackerCompat.getYVelocity(mVelocityTracker,
			pointerId));
			break;
			case MotionEvent.ACTION_UP:
			case MotionEvent.ACTION_CANCEL:
			// Return a VelocityTracker object back to be re-used by others.
			mVelocityTracker.recycle();
			break;
			}
			return true;
			}
			}
			</pre>

			<p class="note"><strong>Note:</strong> Note that you should calculate velocity after an
			{@link android.view.MotionEvent#ACTION_MOVE} event,
			not after {@link android.view.MotionEvent#ACTION_UP}. After an {@link android.view.MotionEvent#ACTION_UP},
			the X and Y velocities will be 0.
			</p>

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