动画化滚动手势

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滚动 →

在 Android 中，滚动通常是通过使用 ScrollView 类来实现的。将任何可能超出其容器边界的标准布局嵌套在 ScrollView 中，以提供由框架管理的可滚动视图。仅在特殊情况下才需要实现自定义滚动器。本文档介绍了如何使用 *滚动器* 对触摸手势做出响应来显示滚动效果。

您的应用可以使用滚动器（Scroller 或 OverScroller）来收集生成滚动动画所需的数据以响应触摸事件。它们很相似，但 OverScroller 还包括用于指示用户在平移或轻扫手势后何时到达内容边缘的方法。

  • 从 Android 12（API 级别 31）开始，视觉元素在拖动事件和轻扫事件上拉伸并弹回，并在轻扫事件上弹回。
  • 在 Android 11（API 级别 30）及更早版本中，边界在拖动或轻扫手势到边缘后会显示“辉光”效果。

本文档中的 InteractiveChart 示例使用 EdgeEffect 类来显示这些过度滚动效果。

您可以使用滚动器随着时间的推移动画化滚动，使用平台标准的滚动物理特性，如摩擦力、速度和其他特性。滚动器本身不绘制任何内容。滚动器会随着时间的推移跟踪滚动偏移量，但它们不会自动将这些位置应用到您的视图。您必须以使滚动动画看起来流畅的速率获取和应用新的坐标。

了解滚动术语

滚动是一个可以在 Android 中表示不同含义的词，具体取决于上下文。

滚动是移动视口（即您正在查看的内容的“窗口”）的通用过程。当滚动同时在x轴和y轴上进行时，称为平移。本文档中的 InteractiveChart 示例应用程序说明了两种不同类型的滚动，拖动和轻扫

  • 拖动：这是用户用手指在触摸屏上拖动时发生的滚动类型。您可以通过覆盖 onScroll() 在 GestureDetector.OnGestureListener 中来实现拖动。有关拖动的更多信息，请参阅 拖动和缩放。
  • 轻扫：这是用户快速拖动并抬起手指时发生的滚动类型。在用户抬起手指后，您通常希望继续移动视口，但减速直到视口停止移动。您可以通过覆盖 onFling() 在 GestureDetector.OnGestureListener 中并使用滚动器对象来实现轻扫。
  • 平移：沿x轴和y轴同时滚动称为平移。

通常将滚动器对象与轻扫手势结合使用，但您可以在任何需要 UI 对触摸事件做出响应显示滚动的上下文中使用它们。例如，您可以覆盖 onTouchEvent() 以直接处理触摸事件并产生滚动效果或响应这些触摸事件的“捕捉到页面”动画。

包含内置滚动实现的组件

以下 Android 组件包含对滚动和过度滚动行为的内置支持

如果您的应用需要支持在其他组件内滚动和过度滚动，请完成以下步骤

  1. 创建自定义基于触摸的滚动实现.
  2. 为了支持运行 Android 12 及更高版本的设备，实现拉伸过度滚动效果。

创建自定义基于触摸的滚动实现

本节介绍了如何创建自己的滚动器，如果您的应用使用不包含内置滚动和过度滚动支持的组件。

以下代码段来自 InteractiveChart 示例。它使用 GestureDetector 并覆盖 GestureDetector.SimpleOnGestureListener 方法 onFling()。它使用 OverScroller 来跟踪轻扫手势。如果用户在执行轻扫手势后到达内容边缘，则容器会指示用户何时到达内容的末尾。指示取决于设备运行的 Android 版本

  • 在 Android 12 及更高版本中，视觉元素会拉伸并弹回。
  • 在 Android 11 及更早版本中，视觉元素会显示辉光效果。

以下代码段的第一部分显示了 onFling() 的实现

Kotlin

// Viewport extremes. See currentViewport for a discussion of the viewport.
private val AXIS_X_MIN = -1f
private val AXIS_X_MAX = 1f
private val AXIS_Y_MIN = -1f
private val AXIS_Y_MAX = 1f

// The current viewport. This rectangle represents the visible chart
// domain and range. The viewport is the part of the app that the
// user manipulates via touch gestures.
private val currentViewport = 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 must be drawn.
private lateinit var contentRect: Rect

private lateinit var scroller: OverScroller
private lateinit var scrollerStartViewport: RectF
...
private val gestureListener = object : GestureDetector.SimpleOnGestureListener() {

    override fun onDown(e: MotionEvent): Boolean {
        // Initiates the decay phase of any active edge effects.
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.S) {
            releaseEdgeEffects()
        }
        scrollerStartViewport.set(currentViewport)
        // Aborts any active scroll animations and invalidates.
        scroller.forceFinished(true)
        ViewCompat.postInvalidateOnAnimation(this@InteractiveLineGraphView)
        return true
    }
    ...
    override fun onFling(
            e1: MotionEvent,
            e2: MotionEvent,
            velocityX: Float,
            velocityY: Float
    ): Boolean {
        fling((-velocityX).toInt(), (-velocityY).toInt())
        return true
    }
}

private fun fling(velocityX: Int, velocityY: Int) {
    // Initiates the decay phase of any active edge effects.
    // On Android 12 and later, the edge effect (stretch) must
    // continue.
    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.S) {
            releaseEdgeEffects()
    }
    // Flings use math in pixels, as opposed to math based on the viewport.
    val surfaceSize: Point = computeScrollSurfaceSize()
    val (startX: Int, startY: Int) = scrollerStartViewport.run {
        set(currentViewport)
        (surfaceSize.x * (left - AXIS_X_MIN) / (AXIS_X_MAX - AXIS_X_MIN)).toInt() to
                (surfaceSize.y * (AXIS_Y_MAX - bottom) / (AXIS_Y_MAX - AXIS_Y_MIN)).toInt()
    }
    // Before flinging, stops the current animation.
    scroller.forceFinished(true)
    // Begins the animation.
    scroller.fling(
            // Current scroll position.
            startX,
            startY,
            velocityX,
            velocityY,
            /*
             * Minimum and maximum scroll positions. The minimum scroll
             * position is generally 0 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 is 800 pixels.
             */
            0, surfaceSize.x - contentRect.width(),
            0, surfaceSize.y - contentRect.height(),
            // The edges of the content. This comes into play when using
            // the EdgeEffect class to draw "glow" overlays.
            contentRect.width() / 2,
            contentRect.height() / 2
    )
    // Invalidates to trigger computeScroll().
    ViewCompat.postInvalidateOnAnimation(this)
}

Java

// Viewport extremes. See currentViewport for a discussion of the viewport.
private static final float AXIS_X_MIN = -1f;
private static final float AXIS_X_MAX = 1f;
private static final float AXIS_Y_MIN = -1f;
private static final float AXIS_Y_MAX = 1f;

// The current viewport. This rectangle represents the visible chart
// domain and range. The viewport is the part of the app that the
// user manipulates via touch gestures.
private RectF currentViewport =
  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 must be drawn.
private final Rect contentRect = new Rect();

private final OverScroller scroller;
private final RectF scrollerStartViewport =
  new RectF(); // Used only for zooms and flings.
...
private final GestureDetector.SimpleOnGestureListener gestureListener
        = new GestureDetector.SimpleOnGestureListener() {
    @Override
    public boolean onDown(MotionEvent e) {
         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.S) {
            releaseEdgeEffects();
        }
        scrollerStartViewport.set(currentViewport);
        scroller.forceFinished(true);
        ViewCompat.postInvalidateOnAnimation(InteractiveLineGraphView.this);
        return true;
    }
...
    @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.
    // On Android 12 and later, the edge effect (stretch) must
    // continue.
    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.S) {
            releaseEdgeEffects();
    }
    // Flings use math in pixels, as opposed to math based on the viewport.
    Point surfaceSize = computeScrollSurfaceSize();
    scrollerStartViewport.set(currentViewport);
    int startX = (int) (surfaceSize.x * (scrollerStartViewport.left -
            AXIS_X_MIN) / (
            AXIS_X_MAX - AXIS_X_MIN));
    int startY = (int) (surfaceSize.y * (AXIS_Y_MAX -
            scrollerStartViewport.bottom) / (
            AXIS_Y_MAX - AXIS_Y_MIN));
    // Before flinging, stops the current animation.
    scroller.forceFinished(true);
    // Begins the animation.
    scroller.fling(
            // Current scroll position.
            startX,
            startY,
            velocityX,
            velocityY,
            /*
             * Minimum and maximum scroll positions. The minimum scroll
             * position is generally 0 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 is 800 pixels.
             */
            0, surfaceSize.x - contentRect.width(),
            0, surfaceSize.y - contentRect.height(),
            // The edges of the content. This comes into play when using
            // the EdgeEffect class to draw "glow" overlays.
            contentRect.width() / 2,
            contentRect.height() / 2);
    // Invalidates to trigger computeScroll().
    ViewCompat.postInvalidateOnAnimation(this);
}

当 onFling() 调用 postInvalidateOnAnimation() 时，它会触发 computeScroll() 来更新x和y的值。这通常在视图子项使用滚动器对象动画化滚动时完成，如前面的示例所示。

大多数视图将滚动器对象的x和y位置直接传递给 scrollTo()。以下 computeScroll() 的实现采用了不同的方法：它调用 computeScrollOffset() 以获取x和y的当前位置。当满足显示过度滚动“辉光”边缘效果的条件时（即，显示已放大，x或y超出范围，并且应用尚未显示过度滚动），代码会设置过度滚动辉光效果并调用 postInvalidateOnAnimation() 以触发视图上的无效。

Kotlin

// Edge effect/overscroll tracking objects.
private lateinit var edgeEffectTop: EdgeEffect
private lateinit var edgeEffectBottom: EdgeEffect
private lateinit var edgeEffectLeft: EdgeEffect
private lateinit var edgeEffectRight: EdgeEffect

private var edgeEffectTopActive: Boolean = false
private var edgeEffectBottomActive: Boolean = false
private var edgeEffectLeftActive: Boolean = false
private var edgeEffectRightActive: Boolean = false

override fun computeScroll() {
    super.computeScroll()

    var needsInvalidate = false

    // The scroller isn't finished, meaning a fling or
    // programmatic pan operation is active.
    if (scroller.computeScrollOffset()) {
        val surfaceSize: Point = computeScrollSurfaceSize()
        val currX: Int = scroller.currX
        val currY: Int = scroller.currY

        val (canScrollX: Boolean, canScrollY: Boolean) = currentViewport.run {
            (left > AXIS_X_MIN || right < AXIS_X_MAX) to (top > AXIS_Y_MIN || bottom < AXIS_Y_MAX)
        }

        /*
         * If you are zoomed in, currX or currY is
         * outside of bounds, and you aren't already
         * showing overscroll, then render the overscroll
         * glow edge effect.
         */
        if (canScrollX
                && currX < 0
                && edgeEffectLeft.isFinished
                && !edgeEffectLeftActive) {
            edgeEffectLeft.onAbsorb(scroller.currVelocity.toInt())
            edgeEffectLeftActive = true
            needsInvalidate = true
        } else if (canScrollX
                && currX > surfaceSize.x - contentRect.width()
                && edgeEffectRight.isFinished
                && !edgeEffectRightActive) {
            edgeEffectRight.onAbsorb(scroller.currVelocity.toInt())
            edgeEffectRightActive = true
            needsInvalidate = true
        }

        if (canScrollY
                && currY < 0
                && edgeEffectTop.isFinished
                && !edgeEffectTopActive) {
            edgeEffectTop.onAbsorb(scroller.currVelocity.toInt())
            edgeEffectTopActive = true
            needsInvalidate = true
        } else if (canScrollY
                && currY > surfaceSize.y - contentRect.height()
                && edgeEffectBottom.isFinished
                && !edgeEffectBottomActive) {
            edgeEffectBottom.onAbsorb(scroller.currVelocity.toInt())
            edgeEffectBottomActive = true
            needsInvalidate = true
        }
        ...
    }
}

Java

// Edge effect/overscroll tracking objects.
private EdgeEffectCompat edgeEffectTop;
private EdgeEffectCompat edgeEffectBottom;
private EdgeEffectCompat edgeEffectLeft;
private EdgeEffectCompat edgeEffectRight;

private boolean edgeEffectTopActive;
private boolean edgeEffectBottomActive;
private boolean edgeEffectLeftActive;
private boolean edgeEffectRightActive;

@Override
public void computeScroll() {
    super.computeScroll();

    boolean needsInvalidate = false;

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

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

        /*
         * If you are zoomed in, currX or currY is
         * outside of bounds, and you aren't already
         * showing overscroll, then render the overscroll
         * glow edge effect.
         */
        if (canScrollX
                && currX < 0
                && edgeEffectLeft.isFinished()
                && !edgeEffectLeftActive) {
            edgeEffectLeft.onAbsorb((int)mScroller.getCurrVelocity());
            edgeEffectLeftActive = true;
            needsInvalidate = true;
        } else if (canScrollX
                && currX > (surfaceSize.x - contentRect.width())
                && edgeEffectRight.isFinished()
                && !edgeEffectRightActive) {
            edgeEffectRight.onAbsorb((int)mScroller.getCurrVelocity());
            edgeEffectRightActive = true;
            needsInvalidate = true;
        }

        if (canScrollY
                && currY < 0
                && edgeEffectTop.isFinished()
                && !edgeEffectTopActive) {
            edgeEffectRight.onAbsorb((int)mScroller.getCurrVelocity());
            edgeEffectTopActive = true;
            needsInvalidate = true;
        } else if (canScrollY
                && currY > (surfaceSize.y - contentRect.height())
                && edgeEffectBottom.isFinished()
                && !edgeEffectBottomActive) {
            edgeEffectRight.onAbsorb((int)mScroller.getCurrVelocity());
            edgeEffectBottomActive = true;
            needsInvalidate = true;
        }
        ...
    }

这是执行实际缩放的代码部分

Kotlin

lateinit var zoomer: Zoomer
val zoomFocalPoint = PointF()
...
// If a zoom is in progress—either programmatically
// or through double touch—this performs the zoom.
if (zoomer.computeZoom()) {
    val newWidth: Float = (1f - zoomer.currZoom) * scrollerStartViewport.width()
    val newHeight: Float = (1f - zoomer.currZoom) * scrollerStartViewport.height()
    val pointWithinViewportX: Float =
            (zoomFocalPoint.x - scrollerStartViewport.left) / scrollerStartViewport.width()
    val pointWithinViewportY: Float =
            (zoomFocalPoint.y - scrollerStartViewport.top) / scrollerStartViewport.height()
    currentViewport.set(
            zoomFocalPoint.x - newWidth * pointWithinViewportX,
            zoomFocalPoint.y - newHeight * pointWithinViewportY,
            zoomFocalPoint.x + newWidth * (1 - pointWithinViewportX),
            zoomFocalPoint.y + newHeight * (1 - pointWithinViewportY)
    )
    constrainViewport()
    needsInvalidate = true
}
if (needsInvalidate) {
    ViewCompat.postInvalidateOnAnimation(this)
}

Java

// Custom object that is functionally similar to Scroller.
Zoomer zoomer;
private PointF zoomFocalPoint = new PointF();
...
// If a zoom is in progress—either programmatically
// or through double touch—this performs the zoom.
if (zoomer.computeZoom()) {
    float newWidth = (1f - zoomer.getCurrZoom()) *
            scrollerStartViewport.width();
    float newHeight = (1f - zoomer.getCurrZoom()) *
            scrollerStartViewport.height();
    float pointWithinViewportX = (zoomFocalPoint.x -
            scrollerStartViewport.left)
            / scrollerStartViewport.width();
    float pointWithinViewportY = (zoomFocalPoint.y -
            scrollerStartViewport.top)
            / scrollerStartViewport.height();
    currentViewport.set(
            zoomFocalPoint.x - newWidth * pointWithinViewportX,
            zoomFocalPoint.y - newHeight * pointWithinViewportY,
            zoomFocalPoint.x + newWidth * (1 - pointWithinViewportX),
            zoomFocalPoint.y + newHeight * (1 - pointWithinViewportY));
    constrainViewport();
    needsInvalidate = true;
}
if (needsInvalidate) {
    ViewCompat.postInvalidateOnAnimation(this);
}

这是在前面代码段中调用的 computeScrollSurfaceSize() 方法。它计算当前可滚动表面大小（以像素为单位）。例如，如果整个图表区域可见，则这是 mContentRect 的当前大小。如果图表在水平和垂直方向上放大 200%，则返回的大小是水平和垂直方向的两倍。

Kotlin

private fun computeScrollSurfaceSize(): Point {
    return Point(
            (contentRect.width() * (AXIS_X_MAX - AXIS_X_MIN) / currentViewport.width()).toInt(),
            (contentRect.height() * (AXIS_Y_MAX - AXIS_Y_MIN) / currentViewport.height()).toInt()
    )
}

Java

private Point computeScrollSurfaceSize() {
    return new Point(
            (int) (contentRect.width() * (AXIS_X_MAX - AXIS_X_MIN)
                    / currentViewport.width()),
            (int) (contentRect.height() * (AXIS_Y_MAX - AXIS_Y_MIN)
                    / currentViewport.height()));
}

另一个滚动器用法的示例，请参阅ViewPager类的源代码。它响应抛掷进行滚动，并使用滚动来实现“捕捉到页面”动画。

实现拉伸过度滚动效果

从 Android 12 开始，EdgeEffect 添加了以下 API 用于实现拉伸过度滚动效果

  • getDistance()
  • onPullDistance()

为了提供最佳的拉伸过度滚动用户体验，请执行以下操作

  1. 当用户触摸内容时，拉伸动画生效，将触摸注册为“捕捉”。用户停止动画并开始再次操控拉伸。
  2. 当用户沿与拉伸相反的方向移动手指时，释放拉伸直到完全消失，然后开始滚动。
  3. 当用户在拉伸期间抛掷时，抛掷 EdgeEffect 以增强拉伸效果。

捕捉动画

当用户捕捉到活动的拉伸动画时，EdgeEffect.getDistance() 返回 0。此条件表示拉伸必须由触摸动作操控。在大多数容器中，捕捉是在 onInterceptTouchEvent() 中检测到的，如下面的代码片段所示

Kotlin

override fun onInterceptTouchEvent(ev: MotionEvent): Boolean {
  ...
  when (action and MotionEvent.ACTION_MASK) {
    MotionEvent.ACTION_DOWN ->
      ...
      isBeingDragged = EdgeEffectCompat.getDistance(edgeEffectBottom) > 0f ||
          EdgeEffectCompat.getDistance(edgeEffectTop) > 0f
      ...
  }
  return isBeingDragged
}

Java

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
  ...
  switch (action & MotionEvent.ACTION_MASK) {
    case MotionEvent.ACTION_DOWN:
      ...
      isBeingDragged = EdgeEffectCompat.getDistance(edgeEffectBottom) > 0
          || EdgeEffectCompat.getDistance(edgeEffectTop) > 0;
      ...
  }
}

在前面的示例中，当 mIsBeingDragged 为 true 时，onInterceptTouchEvent() 返回 true，因此在子元素有机会消费事件之前消费该事件就足够了。

释放过度滚动效果

在滚动之前释放拉伸效果非常重要，以防止拉伸应用于滚动内容。以下代码示例应用了此最佳实践

Kotlin

override fun onTouchEvent(ev: MotionEvent): Boolean {
  val activePointerIndex = ev.actionIndex

  when (ev.getActionMasked()) {
    MotionEvent.ACTION_MOVE ->
      val x = ev.getX(activePointerIndex)
      val y = ev.getY(activePointerIndex)
      var deltaY = y - lastMotionY
      val pullDistance = deltaY / height
      val displacement = x / width

      if (deltaY < 0f && EdgeEffectCompat.getDistance(edgeEffectTop) > 0f) {
        deltaY -= height * EdgeEffectCompat.onPullDistance(edgeEffectTop,
            pullDistance, displacement);
      }
      if (deltaY > 0f && EdgeEffectCompat.getDistance(edgeEffectBottom) > 0f) {
        deltaY += height * EdgeEffectCompat.onPullDistance(edgeEffectBottom,
            -pullDistance, 1 - displacement);
      }
      ...
  }

Java

@Override
public boolean onTouchEvent(MotionEvent ev) {

  final int actionMasked = ev.getActionMasked();

  switch (actionMasked) {
    case MotionEvent.ACTION_MOVE:
      final float x = ev.getX(activePointerIndex);
      final float y = ev.getY(activePointerIndex);
      float deltaY = y - lastMotionY;
      float pullDistance = deltaY / getHeight();
      float displacement = x / getWidth();

      if (deltaY < 0 && EdgeEffectCompat.getDistance(edgeEffectTop) > 0) {
        deltaY -= getHeight() * EdgeEffectCompat.onPullDistance(edgeEffectTop,
            pullDistance, displacement);
      }
      if (deltaY > 0 && EdgeEffectCompat.getDistance(edgeEffectBottom) > 0) {
        deltaY += getHeight() * EdgeEffectCompat.onPullDistance(edgeEffectBottom,
            -pullDistance, 1 - displacement);
      }
            ...

当用户正在拖动时，在将触摸事件传递到嵌套滚动容器或拖动滚动之前，先消耗 EdgeEffect 的拉伸距离。在前面的代码示例中，当显示边缘效果时，getDistance() 返回正值，并且可以通过运动释放。当触摸事件释放拉伸时，它首先会被 EdgeEffect 消耗，以便在显示其他效果（如嵌套滚动）之前完全释放。您可以使用 getDistance() 来了解释放当前效果需要多少拉伸距离。

与 onPull() 不同，onPullDistance() 返回传递的增量的消耗量。从 Android 12 开始，如果在 getDistance() 为 0 时将负 deltaDistance 值传递给 onPull() 或 onPullDistance()，则拉伸效果不会改变。在 Android 11 及更早版本中，onPull() 允许负值总距离显示辉光效果。

选择退出过度滚动

您可以在布局文件中或以编程方式选择退出过度滚动。

要在布局文件中选择退出，请设置 android:overScrollMode，如下例所示

<MyCustomView android:overScrollMode="never">
    ...
</MyCustomView>

要以编程方式选择退出，请使用以下类似代码

Kotlin

customView.overScrollMode = View.OVER_SCROLL_NEVER

Java

customView.setOverScrollMode(View.OVER_SCROLL_NEVER);

其他资源

请参阅以下相关资源