training/gestures/viewgroup.jd

page.title=Managing Touch Events in a ViewGroup
parent.title=Using Touch Gestures
parent.link=index.html

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@jd:body

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<!-- table of contents -->
<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#intercept">Intercept Touch Events in a ViewGroup</a></li>
  <li><a href="#vc">Use ViewConfiguration Constants</a></li>
  <li><a href="#delegate">Extend a Child View's Touchable Area</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>

<p>Handling touch events in a {@link android.view.ViewGroup} takes special care,
because it's common for a {@link android.view.ViewGroup} to have children that
are targets for different touch events than the {@link android.view.ViewGroup}
itself. To make sure that each view correctly receives the touch events intended
for it, override the {@link android.view.ViewGroup#onInterceptTouchEvent
onInterceptTouchEvent()} method.</p>

<h2 id="intercept">Intercept Touch Events in a ViewGroup</h2>

<p>The {@link android.view.ViewGroup#onInterceptTouchEvent onInterceptTouchEvent()}
method is called whenever a touch event is detected on the surface of a
{@link android.view.ViewGroup}, including on the surface of its children. If
{@link android.view.ViewGroup#onInterceptTouchEvent onInterceptTouchEvent()}
returns {@code true}, the {@link android.view.MotionEvent} is intercepted,
meaning it will be not be passed on to the child, but rather to the
{@link android.view.View#onTouchEvent onTouchEvent()} method of the parent.</p>

<p>The {@link android.view.ViewGroup#onInterceptTouchEvent onInterceptTouchEvent()}
method gives a parent the chance to see any touch event before its children do.
If you return {@code true} from
{@link android.view.ViewGroup#onInterceptTouchEvent onInterceptTouchEvent()},
the child view that was previously handling touch events
receives an {@link android.view.MotionEvent#ACTION_CANCEL}, and the events from that
point forward are sent to the parent's
{@link android.view.View#onTouchEvent onTouchEvent()} method for the usual handling.
{@link android.view.ViewGroup#onInterceptTouchEvent onInterceptTouchEvent()} can also
return {@code false} and simply spy on events as they travel down the view hierarchy
to their usual targets, which will handle the events with their own
{@link android.view.View#onTouchEvent onTouchEvent()}.


<p>In the following snippet, the class {@code MyViewGroup} extends
{@link android.view.ViewGroup}.
{@code MyViewGroup} contains multiple child views. If you drag your finger across
a child view horizontally, the child view should no longer get touch events, and
{@code MyViewGroup} should handle touch events by scrolling its contents. However,
if you press buttons in the child view, or scroll the child view vertically,
the parent shouldn't intercept those touch events, because the child is the
intended target. In those cases,
{@link android.view.ViewGroup#onInterceptTouchEvent onInterceptTouchEvent()} should
return {@code false}, and {@code MyViewGroup}'s
{@link android.view.View#onTouchEvent onTouchEvent()} won't be called.</p>

<pre>public class MyViewGroup extends ViewGroup {

    private int mTouchSlop;

    ...

    ViewConfiguration vc = ViewConfiguration.get(view.getContext());
    mTouchSlop = vc.getScaledTouchSlop();

    ...

    &#64;Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        /*
         * This method JUST determines whether we want to intercept the motion.
         * If we return true, onTouchEvent will be called and we do the actual
         * scrolling there.
         */


        final int action = MotionEventCompat.getActionMasked(ev);

        // Always handle the case of the touch gesture being complete.
        if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
            // Release the scroll.
            mIsScrolling = false;
            return false; // Do not intercept touch event, let the child handle it
        }

        switch (action) {
            case MotionEvent.ACTION_MOVE: {
                if (mIsScrolling) {
                    // We're currently scrolling, so yes, intercept the
                    // touch event!
                    return true;
                }

                // If the user has dragged her finger horizontally more than
                // the touch slop, start the scroll

                // left as an exercise for the reader
                final int xDiff = calculateDistanceX(ev);

                // Touch slop should be calculated using ViewConfiguration
                // constants.
                if (xDiff > mTouchSlop) {
                    // Start scrolling!
                    mIsScrolling = true;
                    return true;
                }
                break;
            }
            ...
        }

        // In general, we don't want to intercept touch events. They should be
        // handled by the child view.
        return false;
    }

    &#64;Override
    public boolean onTouchEvent(MotionEvent ev) {
        // Here we actually handle the touch event (e.g. if the action is ACTION_MOVE,
        // scroll this container).
        // This method will only be called if the touch event was intercepted in
        // onInterceptTouchEvent
        ...
    }
}</pre>

<p>Note that {@link android.view.ViewGroup} also provides a
{@link android.view.ViewGroup#requestDisallowInterceptTouchEvent requestDisallowInterceptTouchEvent()} method.
The {@link android.view.ViewGroup} calls this method when a child does not want the parent and its
ancestors to intercept touch events with
{@link android.view.ViewGroup#onInterceptTouchEvent onInterceptTouchEvent()}.
</p>

<h2 id="vc">Use ViewConfiguration Constants</h2>

<p>The above snippet uses the current {@link android.view.ViewConfiguration} to initialize
a variable called {@code mTouchSlop}. You can use the {@link
android.view.ViewConfiguration} class to access common distances, speeds, and
times used by the Android system.</p>


<p>"Touch slop" refers to the distance in pixels a user's touch can wander
before the gesture is interpreted as scrolling. Touch slop is typically used to
prevent accidental scrolling when the user is performing some other touch
operation, such as touching on-screen elements.</p>

<p>Two other commonly used {@link android.view.ViewConfiguration} methods are
{@link android.view.ViewConfiguration#getScaledMinimumFlingVelocity getScaledMinimumFlingVelocity()}
and {@link android.view.ViewConfiguration#getScaledMaximumFlingVelocity getScaledMaximumFlingVelocity()}.
These methods  return the minimum and maximum velocity (respectively) to initiate a fling,
as measured in pixels per second. For example:</p>

<pre>ViewConfiguration vc = ViewConfiguration.get(view.getContext());
private int mSlop = vc.getScaledTouchSlop();
private int mMinFlingVelocity = vc.getScaledMinimumFlingVelocity();
private int mMaxFlingVelocity = vc.getScaledMaximumFlingVelocity();

...

case MotionEvent.ACTION_MOVE: {
    ...
    float deltaX = motionEvent.getRawX() - mDownX;
    if (Math.abs(deltaX) > mSlop) {
        // A swipe occurred, do something
    }

...

case MotionEvent.ACTION_UP: {
    ...
    } if (mMinFlingVelocity <= velocityX && velocityX <= mMaxFlingVelocity
            && velocityY < velocityX) {
        // The criteria have been satisfied, do something
    }
}</pre>


<h2 id="delegate">Extend a Child View's Touchable Area</h2>

<p>Android provides the {@link android.view.TouchDelegate} class to make it possible
for a parent to extend the touchable area of a child view beyond the child's bounds.

This is useful when the child has to be small, but should have a larger touch region. You can
also use this approach to shrink the child's touch region if need be.</p>

<p>In the following example, an {@link android.widget.ImageButton} is the
"delegate view" (that is, the child whose touch area the parent will extend).
Here is the layout file:</p>

<pre>
&lt;RelativeLayout xmlns:android=&quot;http://schemas.android.com/apk/res/android";
     android:id=&quot;@+id/parent_layout&quot;
     android:layout_width=&quot;match_parent&quot;
     android:layout_height=&quot;match_parent&quot;
     tools:context=&quot;.MainActivity&quot; &gt;

     &lt;ImageButton android:id=&quot;@+id/button&quot;
          android:layout_width=&quot;wrap_content&quot;
          android:layout_height=&quot;wrap_content&quot;
          android:background=&quot;@null&quot;
          android:src=&quot;@drawable/icon&quot; /&gt;
&lt;/RelativeLayout&gt;
</pre>

<p>The snippet below does the following:</p>

<ul>
<li>Gets the parent view and posts a {@link java.lang.Runnable} on the UI thread. This ensures that the parent lays out its children before calling the {@link android.view.View#getHitRect getHitRect()} method. The {@link android.view.View#getHitRect getHitRect()} method gets the child's hit rectangle (touchable area) in the parent's coordinates.</li>
<li>Finds the {@link android.widget.ImageButton} child view and calls {@link android.view.View#getHitRect getHitRect()} to get the bounds of the child's touchable area.</li>
<li>Extends the bounds of the {@link android.widget.ImageButton}'s hit rectangle.</li>
<li>Instantiates a {@link android.view.TouchDelegate}, passing in the expanded hit rectangle and the {@link android.widget.ImageButton} child view as parameters.</li>
<li>Sets the {@link android.view.TouchDelegate} on the parent view, such that touches within the touch delegate bounds are routed to the child.</li>

</ul>

In its capacity as touch delegate for the {@link android.widget.ImageButton} child view, the
parent view will receive all touch events. If the touch event occurred within the child's hit
rectangle, the parent will pass the touch
event to the child for handling.</p>


<pre>
public class MainActivity extends Activity {

    &#64;Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // Get the parent view
        View parentView = findViewById(R.id.parent_layout);

        parentView.post(new Runnable() {
            // Post in the parent's message queue to make sure the parent
            // lays out its children before you call getHitRect()
            &#64;Override
            public void run() {
                // The bounds for the delegate view (an ImageButton
                // in this example)
                Rect delegateArea = new Rect();
                ImageButton myButton = (ImageButton) findViewById(R.id.button);
                myButton.setEnabled(true);
                myButton.setOnClickListener(new View.OnClickListener() {
                    &#64;Override
                    public void onClick(View view) {
                        Toast.makeText(MainActivity.this,
                                "Touch occurred within ImageButton touch region.",
                                Toast.LENGTH_SHORT).show();
                    }
                });

                // The hit rectangle for the ImageButton
                myButton.getHitRect(delegateArea);

                // Extend the touch area of the ImageButton beyond its bounds
                // on the right and bottom.
                delegateArea.right += 100;
                delegateArea.bottom += 100;

                // Instantiate a TouchDelegate.
                // "delegateArea" is the bounds in local coordinates of
                // the containing view to be mapped to the delegate view.
                // "myButton" is the child view that should receive motion
                // events.
                TouchDelegate touchDelegate = new TouchDelegate(delegateArea,
                        myButton);

                // Sets the TouchDelegate on the parent view, such that touches
                // within the touch delegate bounds are routed to the child.
                if (View.class.isInstance(myButton.getParent())) {
                    ((View) myButton.getParent()).setTouchDelegate(touchDelegate);
                }
            }
        });
    }
}</pre>