docs/html/training/gestures/multi.jd

page.title=Handling Multi-Touch Gestures
parent.title=Using Touch Gestures
parent.link=index.html

trainingnavtop=true
next.title=Dragging and Scaling
next.link=scale.html

@jd:body

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<!-- table of contents -->
<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#track">Track Multiple Pointers</a></li>
  <li><a href="#action">Get a MotionEvent's Action</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="{@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>

<h2>Try it out</h2>

<div class="download-box">
  <a href="{@docRoot}shareables/training/InteractiveChart.zip"
class="button">Download the sample</a>
 <p class="filename">InteractiveChart.zip</p>
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</div>
</div>

<p>A multi-touch gesture is when multiple pointers (fingers) touch the screen
at the same time. This lesson describes how to detect gestures that involve
multiple pointers.</p>

<h2 id="track">Track Multiple Pointers</h2>

<p>When multiple pointers touch the screen at the same time, the system generates the 
following touch events:</p>

<ul>
  <li>{@link android.view.MotionEvent#ACTION_DOWN}&mdash;For the first pointer that 
touches the screen. This starts the gesture. The pointer data for this pointer is 
always at index 0 in the {@link android.view.MotionEvent}.</li>
  <li>{@link android.support.v4.view.MotionEventCompat#ACTION_POINTER_DOWN}&mdash;For 
extra pointers that enter the screen beyond the first. The pointer data for this 
pointer is at the index returned by {@link android.support.v4.view.MotionEventCompat#getActionIndex getActionIndex()}.</li>
  <li>{@link android.view.MotionEvent#ACTION_MOVE}&mdash;A change has happened during a press gesture.</li>
  <li>{@link android.support.v4.view.MotionEventCompat#ACTION_POINTER_UP}&mdash;Sent when a non-primary pointer goes up.</li>
  <li>{@link android.view.MotionEvent#ACTION_UP}&mdash;Sent when the last pointer leaves the screen.</li>
</ul>

<p>You keep track of individual pointers within a {@link
android.view.MotionEvent} via each pointer's index and ID:</p>

<ul>
<li><strong>Index</strong>: A {@link android.view.MotionEvent} effectively 
stores information about each pointer in an array. The index of a pointer is its position 
within this array. Most of the {@link
android.view.MotionEvent} methods you use to interact with pointers take the
pointer index as a parameter, not the pointer ID. </li>
  
  
  <li><strong>ID</strong>: Each pointer also has an ID mapping that stays
persistent across touch events to allow tracking an individual pointer across 
the entire gesture.</li>
  
</ul>

<p>The  order in which individual pointers appear within a motion event is 
undefined. Thus the index of a pointer can change from one event to the
next, but the pointer ID of a pointer is guaranteed to remain  constant as long
as the pointer remains active. Use the  {@link
android.view.MotionEvent#getPointerId getPointerId()} method to obtain a
pointer's ID to track the pointer across all subsequent motion events in a
gesture. Then for successive  motion events, use the {@link
android.view.MotionEvent#findPointerIndex findPointerIndex()} method to obtain
the pointer index for a given pointer ID in that motion event. For example:</p>


<pre>private int mActivePointerId;
 
public boolean onTouchEvent(MotionEvent event) {
    ....
    // Get the pointer ID
    mActivePointerId = event.getPointerId(0);

    // ... Many touch events later...

    // Use the pointer ID to find the index of the active pointer 
    // and fetch its position
    int pointerIndex = event.findPointerIndex(mActivePointerId);
    // Get the pointer's current position
    float x = event.getX(pointerIndex);
    float y = event.getY(pointerIndex);
}</pre>

<h2 id="action">Get a MotionEvent's Action</h2>

<p>You should always use the method  
{@link android.view.MotionEvent#getActionMasked getActionMasked()} (or better yet, the compatability version 
{@link android.support.v4.view.MotionEventCompat#getActionMasked MotionEventCompat.getActionMasked()}) to retrieve 
the action of a
{@link android.view.MotionEvent}. Unlike the older {@link android.view.MotionEvent#getAction getAction()} 
method, {@link android.support.v4.view.MotionEventCompat#getActionMasked getActionMasked()} is designed to work with 
multiple pointers. It returns the masked action 
being performed, without including the pointer index bits. You can then use 
{@link android.support.v4.view.MotionEventCompat#getActionIndex getActionIndex()} to return the index of 
the pointer associated with the action. This is illustrated in the snippet below.</p>

<p class="note"><strong>Note:</strong> This example uses the 
{@link android.support.v4.view.MotionEventCompat}
class. This class is in the 
<a href="{@docRoot}tools/extras/support-library.html">Support Library</a>. You should use
{@link android.support.v4.view.MotionEventCompat} to provide the best support for a wide range of
platforms. 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>

<pre>int action = MotionEventCompat.getActionMasked(event);
// Get the index of the pointer associated with the action.
int index = MotionEventCompat.getActionIndex(event);
int xPos = -1;
int yPos = -1;

Log.d(DEBUG_TAG,"The action is " + actionToString(action));
	    
if (event.getPointerCount() > 1) {
    Log.d(DEBUG_TAG,"Multitouch event"); 
    // The coordinates of the current screen contact, relative to 
    // the responding View or Activity.  
    xPos = (int)MotionEventCompat.getX(event, index);
    yPos = (int)MotionEventCompat.getY(event, index);

} else {
    // Single touch event
    Log.d(DEBUG_TAG,"Single touch event"); 
    xPos = (int)MotionEventCompat.getX(event, index);
    yPos = (int)MotionEventCompat.getY(event, index);
}
...

// Given an action int, returns a string description
public static String actionToString(int action) {
    switch (action) {
	        
        case MotionEvent.ACTION_DOWN: return "Down";
	case MotionEvent.ACTION_MOVE: return "Move";
	case MotionEvent.ACTION_POINTER_DOWN: return "Pointer Down";
	case MotionEvent.ACTION_UP: return "Up";
	case MotionEvent.ACTION_POINTER_UP: return "Pointer Up";
	case MotionEvent.ACTION_OUTSIDE: return "Outside";
	case MotionEvent.ACTION_CANCEL: return "Cancel";
    }
    return "";
}</pre>

 


<p>For more discussion of multi-touch and some examples, see the lesson <a href="scale.html">Dragging and Scaling</a>.