training/gestures/scroll.jd

page.title=Animating a Scroll Gesture
parent.title=Using Touch Gestures
parent.link=index.html

trainingnavtop=true
next.title=Handling Multi-Touch Gestures
next.link=multi.html

@jd:body

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

<!-- table of contents -->
<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#term">Understand Scrolling Terminology</a></li>
  <li><a href="#scroll">Implement Touch-Based Scrolling</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>
</div>

</div>
</div>

<p>In Android, scrolling is typically achieved by using the
{@link android.widget.ScrollView}
class. Any standard layout that might extend beyond the bounds of its container should be
nested in a {@link android.widget.ScrollView} to provide a scrollable view that's
managed by the framework. Implementing a custom scroller should only be
necessary for special scenarios. This lesson describes such a scenario: displaying
a scrolling effect in response to touch gestures using <em>scrollers</em>.


<p>You can use scrollers ({@link android.widget.Scroller} or {@link
android.widget.OverScroller}) to collect the data you need to produce a
scrolling animation in response to a touch event. They are similar, but
{@link android.widget.OverScroller}
includes methods for indicating to users that they've reached the content edges
after a pan or fling gesture. The {@code InteractiveChart} sample
uses the {@link android.widget.EdgeEffect} class
(actually the {@link android.support.v4.widget.EdgeEffectCompat} class)
to display a "glow" effect when users reach the content edges.</p>

<p class="note"><strong>Note:</strong> We recommend that you
use {@link android.widget.OverScroller} rather than {@link
android.widget.Scroller} for scrolling animations.
{@link android.widget.OverScroller} provides the best backward
compatibility with older devices.
<br />
Also note that you generally only need to use scrollers
when implementing scrolling yourself. {@link android.widget.ScrollView} and
{@link android.widget.HorizontalScrollView} do all of this for you if you nest your
layout within them.
</p>


<p>A scroller is used  to animate scrolling over time, using platform-standard
scrolling physics (friction, velocity, etc.). The scroller itself doesn't
actually draw anything. Scrollers track scroll offsets for you over time, but
they don't  automatically apply those positions to your view. It's your
responsibility to get and apply new coordinates at a rate that will make the
scrolling animation look smooth.</p>


<h2 id="term">Understand Scrolling Terminology</h2>

<p>"Scrolling" is a word that can take on different meanings in Android, depending on the context.</p>

<p><strong>Scrolling</strong> is the general process of moving the viewport (that is, the 'window'
of content you're looking at). When scrolling is in both the x and y axes, it's called
<em>panning</em>. The sample application provided with this class, {@code InteractiveChart}, illustrates
two different types of scrolling, dragging and flinging:</p>
<ul>
    <li><strong>Dragging</strong> is the type of scrolling that occurs when a user drags her
finger across the touch screen. Simple dragging is often implemented by overriding
{@link android.view.GestureDetector.OnGestureListener#onScroll onScroll()} in
{@link android.view.GestureDetector.OnGestureListener}. For more discussion of dragging, see
<a href="dragging.jd">Dragging and Scaling</a>.</li>

    <li><strong>Flinging</strong> is the type of scrolling that occurs when a user
drags and lifts her finger quickly. After the user lifts her finger, you generally
want to keep scrolling (moving the viewport), but decelerate until the viewport stops moving.
Flinging can be implemented by overriding
{@link android.view.GestureDetector.OnGestureListener#onFling onFling()}
in {@link android.view.GestureDetector.OnGestureListener}, and by using
a scroller object. This is the use
case that is the topic of this lesson.</li>
</ul>

<p>It's common to use scroller objects
in conjunction with a fling gesture, but they
can be used in pretty much any context where you want the UI to display
scrolling in response to a touch event. For example, you could override
{@link android.view.View#onTouchEvent onTouchEvent()} to process touch
events directly, and produce a scrolling effect or a "snapping to page" animation
in response to those touch events.</p>


<h2 id="#scroll">Implement Touch-Based Scrolling</h2>

<p>This section describes how to use a scroller.
The snippet shown below comes from the {@code InteractiveChart} sample
provided with this class.
It uses a
{@link android.view.GestureDetector}, and overrides the
{@link android.view.GestureDetector.SimpleOnGestureListener} method
{@link android.view.GestureDetector.OnGestureListener#onFling onFling()}.
It uses {@link android.widget.OverScroller} to track the fling gesture.
If the user reaches the content edges
after the fling gesture, the app displays a "glow" effect.
</p>

<p class="note"><strong>Note:</strong> The {@code InteractiveChart} sample app displays a
chart that you can zoom, pan, scroll, and so on. In the following snippet,
{@code mContentRect} represents the rectangle coordinates within the view that the chart
will be drawn into. At any given time, a subset of the total chart domain and range are drawn
into this rectangular area.
{@code mCurrentViewport} represents the portion of the chart that is currently
visible in the screen. Because pixel offsets are generally treated as integers,
{@code mContentRect} is of the type {@link android.graphics.Rect}. Because the
graph domain and range are decimal/float values, {@code mCurrentViewport} is of
the type {@link android.graphics.RectF}.</p>

<p>The first part of the snippet shows the implementation of
{@link android.view.GestureDetector.OnGestureListener#onFling onFling()}:</p>

<pre>// The current viewport. This rectangle represents the currently visible
// chart domain and range. The viewport is the part of the app that the
// user manipulates via touch gestures.
private RectF mCurrentViewport =
        new RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX);

// The current destination rectangle (in pixel coordinates) into which the
// chart data should be drawn.
private Rect mContentRect;

private OverScroller mScroller;
private RectF mScrollerStartViewport;
...
private final GestureDetector.SimpleOnGestureListener mGestureListener
        = new GestureDetector.SimpleOnGestureListener() {
    &#64;Override
    public boolean onDown(MotionEvent e) {
        // Initiates the decay phase of any active edge effects.
        releaseEdgeEffects();
        mScrollerStartViewport.set(mCurrentViewport);
        // Aborts any active scroll animations and invalidates.
        mScroller.forceFinished(true);
        ViewCompat.postInvalidateOnAnimation(InteractiveLineGraphView.this);
        return true;
    }
    ...
    &#64;Override
    public boolean onFling(MotionEvent e1, MotionEvent e2,
            float velocityX, float velocityY) {
        fling((int) -velocityX, (int) -velocityY);
        return true;
    }
};

private void fling(int velocityX, int velocityY) {
    // Initiates the decay phase of any active edge effects.
    releaseEdgeEffects();
    // Flings use math in pixels (as opposed to math based on the viewport).
    Point surfaceSize = computeScrollSurfaceSize();
    mScrollerStartViewport.set(mCurrentViewport);
    int startX = (int) (surfaceSize.x * (mScrollerStartViewport.left -
            AXIS_X_MIN) / (
            AXIS_X_MAX - AXIS_X_MIN));
    int startY = (int) (surfaceSize.y * (AXIS_Y_MAX -
            mScrollerStartViewport.bottom) / (
            AXIS_Y_MAX - AXIS_Y_MIN));
    // Before flinging, aborts the current animation.
    mScroller.forceFinished(true);
    // Begins the animation
    mScroller.fling(
            // Current scroll position
            startX,
            startY,
            velocityX,
            velocityY,
            /*
             * Minimum and maximum scroll positions. The minimum scroll
             * position is generally zero and the maximum scroll position
             * is generally the content size less the screen size. So if the
             * content width is 1000 pixels and the screen width is 200
             * pixels, the maximum scroll offset should be 800 pixels.
             */
            0, surfaceSize.x - mContentRect.width(),
            0, surfaceSize.y - mContentRect.height(),
            // The edges of the content. This comes into play when using
            // the EdgeEffect class to draw "glow" overlays.
            mContentRect.width() / 2,
            mContentRect.height() / 2);
    // Invalidates to trigger computeScroll()
    ViewCompat.postInvalidateOnAnimation(this);
}</pre>

<p>When {@link android.view.GestureDetector.OnGestureListener#onFling onFling()} calls
{@link android.support.v4.view.ViewCompat#postInvalidateOnAnimation postInvalidateOnAnimation()},
it triggers
{@link android.view.View#computeScroll computeScroll()} to update the values for x and y.
This is typically be done when a view child is animating a scroll using a scroller object, as in this example. </p>

<p>Most views pass the scroller object's x and y position directly to
{@link android.view.View#scrollTo scrollTo()}.
The following implementation of {@link android.view.View#computeScroll computeScroll()}
takes a different approach&mdash;it calls
{@link android.widget.OverScroller#computeScrollOffset computeScrollOffset()} to get the current
location of x and y. When the criteria for displaying an overscroll "glow" edge effect are met
(the display is zoomed in, x or y is out of bounds, and the app isn't already showing an overscroll),
the code sets up the overscroll glow effect and calls
{@link android.support.v4.view.ViewCompat#postInvalidateOnAnimation postInvalidateOnAnimation()}
to trigger an invalidate on the view:</p>

<pre>// Edge effect / overscroll tracking objects.
private EdgeEffectCompat mEdgeEffectTop;
private EdgeEffectCompat mEdgeEffectBottom;
private EdgeEffectCompat mEdgeEffectLeft;
private EdgeEffectCompat mEdgeEffectRight;

private boolean mEdgeEffectTopActive;
private boolean mEdgeEffectBottomActive;
private boolean mEdgeEffectLeftActive;
private boolean mEdgeEffectRightActive;

&#64;Override
public void computeScroll() {
    super.computeScroll();

    boolean needsInvalidate = false;

    // The scroller isn't finished, meaning a fling or programmatic pan
    // operation is currently active.
    if (mScroller.computeScrollOffset()) {
        Point surfaceSize = computeScrollSurfaceSize();
        int currX = mScroller.getCurrX();
        int currY = mScroller.getCurrY();

        boolean canScrollX = (mCurrentViewport.left > AXIS_X_MIN
                || mCurrentViewport.right < AXIS_X_MAX);
        boolean canScrollY = (mCurrentViewport.top > AXIS_Y_MIN
                || mCurrentViewport.bottom < AXIS_Y_MAX);

        /*
         * If you are zoomed in and currX or currY is
         * outside of bounds and you're not already
         * showing overscroll, then render the overscroll
         * glow edge effect.
         */
        if (canScrollX
                && currX < 0
                && mEdgeEffectLeft.isFinished()
                && !mEdgeEffectLeftActive) {
            mEdgeEffectLeft.onAbsorb((int)
                    OverScrollerCompat.getCurrVelocity(mScroller));
            mEdgeEffectLeftActive = true;
            needsInvalidate = true;
        } else if (canScrollX
                && currX > (surfaceSize.x - mContentRect.width())
                && mEdgeEffectRight.isFinished()
                && !mEdgeEffectRightActive) {
            mEdgeEffectRight.onAbsorb((int)
                    OverScrollerCompat.getCurrVelocity(mScroller));
            mEdgeEffectRightActive = true;
            needsInvalidate = true;
        }

        if (canScrollY
                && currY < 0
                && mEdgeEffectTop.isFinished()
                && !mEdgeEffectTopActive) {
            mEdgeEffectTop.onAbsorb((int)
                    OverScrollerCompat.getCurrVelocity(mScroller));
            mEdgeEffectTopActive = true;
            needsInvalidate = true;
        } else if (canScrollY
                && currY > (surfaceSize.y - mContentRect.height())
                && mEdgeEffectBottom.isFinished()
                && !mEdgeEffectBottomActive) {
            mEdgeEffectBottom.onAbsorb((int)
                    OverScrollerCompat.getCurrVelocity(mScroller));
            mEdgeEffectBottomActive = true;
            needsInvalidate = true;
        }
        ...
    }</pre>

<p>Here is the section of the code that performs the actual zoom:</p>

<pre>// Custom object that is functionally similar to Scroller
Zoomer mZoomer;
private PointF mZoomFocalPoint = new PointF();
...

// If a zoom is in progress (either programmatically or via double
// touch), performs the zoom.
if (mZoomer.computeZoom()) {
    float newWidth = (1f - mZoomer.getCurrZoom()) *
            mScrollerStartViewport.width();
    float newHeight = (1f - mZoomer.getCurrZoom()) *
            mScrollerStartViewport.height();
    float pointWithinViewportX = (mZoomFocalPoint.x -
            mScrollerStartViewport.left)
            / mScrollerStartViewport.width();
    float pointWithinViewportY = (mZoomFocalPoint.y -
            mScrollerStartViewport.top)
            / mScrollerStartViewport.height();
    mCurrentViewport.set(
            mZoomFocalPoint.x - newWidth * pointWithinViewportX,
            mZoomFocalPoint.y - newHeight * pointWithinViewportY,
            mZoomFocalPoint.x + newWidth * (1 - pointWithinViewportX),
            mZoomFocalPoint.y + newHeight * (1 - pointWithinViewportY));
    constrainViewport();
    needsInvalidate = true;
}
if (needsInvalidate) {
    ViewCompat.postInvalidateOnAnimation(this);
}
</pre>

<p>This is the {@code computeScrollSurfaceSize()} method that's called in the above snippet. It
computes the current scrollable surface size, in pixels. For example, if the entire chart area is visible,
this is simply the current size of {@code mContentRect}. If the chart is zoomed in 200% in both directions,
the returned size will be twice as large horizontally and vertically.</p>

<pre>private Point computeScrollSurfaceSize() {
    return new Point(
            (int) (mContentRect.width() * (AXIS_X_MAX - AXIS_X_MIN)
                    / mCurrentViewport.width()),
            (int) (mContentRect.height() * (AXIS_Y_MAX - AXIS_Y_MIN)
                    / mCurrentViewport.height()));
}</pre>

<p>For another example of scroller usage, see the
<a href="http://github.com/android/platform_frameworks_support/blob/master/v4/java/android/support/v4/view/ViewPager.java">source code</a> for the
{@link android.support.v4.view.ViewPager} class. It scrolls in response to flings,
and uses scrolling to implement the "snapping to page" animation.</p>