以动画方式显示滚动手势

试用 Compose 方式
Jetpack Compose 是推荐用于 Android 的界面工具包。了解如何在 Compose 中使用触控和输入功能。

在 Android 中,滚动通常通过 ScrollView 类实现。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。只有在特殊情况下才需要实现自定义滚动条。本文档介绍了如何使用“滚动条”显示响应轻触手势的滚动效果。

您的应用可以使用滚动条(ScrollerOverScroller)收集生成滚动动画以响应触摸事件所需的数据。二者很相似,但 OverScroller 还包含一些方法,用于在用户做出平移或快速滑动手势后到达内容边缘时向其显示相关信息。

  • 从 Android 12(API 级别 31)开始,视觉元素会在发生拖动事件时拉伸和弹回,在发生快速滑动事件时快速滑动和弹回。
  • 在 Android 11(API 级别 30)及更低版本中,向边缘拖动或滑动手势后,边界会显示“发光”效果。

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

您可以使用滚动条为一段时间内的滚动添加动画效果,并采用平台标准的滚动物理特性(例如摩擦力、速度和其他特性)。滚动条本身不会绘制任何内容。滚动条会随着时间的推移为您跟踪滚动偏移,但不会自动将这些位置应用到视图。获取和应用新坐标时,必须采用使滚动动画看起来平滑的速率。

了解滚动术语

“滚动”这个词在 Android 中可能具有不同的含义,具体取决于上下文。

滚动是移动视口(即您正在查看的内容“窗口”)的一般过程。如果同时在 x 轴和 y 轴上滚动,称为“平移”。本文档中的 InteractiveChart 示例应用展示了两种不同类型的滚动,即拖动和快速:

  • 拖动:这是用户在触摸屏上拖动手指时发生的滚动类型。您可以通过替换 GestureDetector.OnGestureListener 中的 onScroll() 来实现拖动。 如需详细了解如何拖动,请参阅拖动和缩放
  • 滑动:用户快速拖动并抬起手指时发生的一种滚动。在用户抬起手指后,您通常需要继续移动视口,但要减速,直到视口停止移动。您可以通过替换 GestureDetector.OnGestureListener 中的 onFling() 并使用滚动条对象来实现快滑。
  • 平移:同时沿 x 轴和 y 轴滚动称为“平移”。

将滚动条对象与快滑手势结合使用很常见,但如果您希望界面显示滚动以响应轻触事件,可以在任何上下文中使用它们。例如,您可以替换 onTouchEvent() 以直接处理触摸事件,并生成滚动效果或“对准页面”动画来响应这些触摸事件。

包含内置滚动实现的组件

以下 Android 组件内置了对滚动和滚动行为的支持:

如果您的应用需要在另一个组件内支持滚动和滚动,请完成以下步骤:

  1. 创建自定义触摸滚动实现
  2. 如需支持搭载 Android 12 及更高版本的设备,请实现拉伸滚动效果

创建自定义触摸滚动实现

本部分介绍了在应用使用的组件不内置对滚动和滚动回弹的支持的情况下,应如何创建您自己的滚动条。

以下代码段来自 InteractiveChart 示例。它使用 GestureDetector 并替换 GestureDetector.SimpleOnGestureListener 方法 onFling()。它使用 OverScroller 跟踪滑动手势。如果用户在执行快滑手势后到达内容边缘,容器会指明用户到达内容末尾的时间。具体指示取决于设备搭载的 Android 版本:

  • 在 Android 12 及更高版本中,视觉元素会拉伸和弹跳。
  • 在 Android 11 及更低版本中,视觉元素会显示发光效果。

以下代码段的第一部分展示了 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);
}

onFling() 调用 postInvalidateOnAnimation() 时,它会触发 computeScroll() 以更新 x 和 y 的值。这通常在子视图使用滚动条对象为滚动添加动画效果时完成,如上例所示。

大多数视图会将滚动条对象的 xy 位置直接传递给 scrollTo()。以下 computeScroll() 实现采用了不同的方法:它会调用 computeScrollOffset() 来获取 x 和 y 的当前位置。当满足显示滚动“发光”边缘效果的条件时,即显示内容放大,x 或 y 超出边界,并且应用尚未显示滚动回弹,代码会设置滚动发光效果并调用 postInvalidateOnAnimation() 以触发视图的失效。

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;
        }
        ...
    }

以下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);
}

这是在上述代码段中调用的 computeScrollSurfaceSize() 方法。它会计算当前的可滚动 surface 大小(以像素为单位)。例如,如果整个图表区域都是可见的,这就是 mContentRect 的当前大小。如果图表在两个方向上均放大 200%,则返回的大小在水平和垂直方向上均为原来的两倍。

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()));
}

如需查看滚动条用法的其他示例,请参阅 ViewPager 类的源代码。它会滚动以响应快滑,并使用滚动来实现“对准页面”动画。

实现拉伸滚动效果

从 Android 12 开始,EdgeEffect 添加了以下 API,用于实现拉伸滚动效果:

  • getDistance()
  • onPullDistance()

为了通过拉伸滚动提供最佳用户体验,请执行以下操作:

  1. 如果拉伸动画在用户轻触内容时生效,请将轻触注册为“捕获”。用户停止动画并再次开始操控拉伸。
  2. 当用户沿拉伸的相反方向移动手指时,松开拉伸直至其完全消失,然后开始滚动。
  3. 当用户在拉伸过程中快速滑动时,快速滑动 EdgeEffect 可增强拉伸效果。

捕捉动画

当用户捕获进行中的拉伸动画时,EdgeEffect.getDistance() 会返回 0。此条件表示必须通过触摸动作操纵拉伸。在大多数容器中,捕获都在 onInterceptTouchEvent() 中检测到,如以下代码段所示:

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 mIsBeingDraggedtrue 时,onInterceptTouchEvent() 会返回 true,因此,在子项有机会使用事件之前,使用事件就已足够。

释放滚动效果

请务必在滚动之前释放拉伸效果,以防止将拉伸应用于滚动内容。以下代码示例采用了此最佳做法:

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);
      }
            ...

当用户进行拖动时,请先消耗 EdgeEffect 拉取距离,然后再将触摸事件传递给嵌套滚动容器或拖动滚动。在前面的代码示例中,当显示边缘效果且可以通过动作释放时,getDistance() 会返回一个正值。当触摸事件释放拉伸时,它首先被 EdgeEffect 消耗,以便在显示其他效果(例如嵌套滚动)之前将其完全释放。您可以使用 getDistance() 了解释放当前效果所需的拉取距离。

onPull() 不同,onPullDistance() 会返回所传递的增量的消耗量。从 Android 12 开始,如果在 getDistance()0 时向 onPull()onPullDistance() 传递负 deltaDistance 值,则拉伸效果不会改变。在 Android 11 及更低版本中,onPull() 允许总距离的负值显示发光效果。

停用滚动回弹

您可以在布局文件中或以编程方式选择停用滚动回弹。

如需在布局文件中停用,请设置 android:overScrollMode,如以下示例所示:

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

要以编程方式停用,请使用如下代码:

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: