Scroll手势动画
最后更新于:2022-04-01 01:45:36
> 编写:[Andrwyw](https://github.com/Andrwyw) - 原文:[http://developer.android.com/training/gestures/scroll.html](http://developer.android.com/training/gestures/scroll.html)
在Android中,通常使用[ScrollView](http://developer.android.com/reference/android/widget/ScrollView.html)类来实现滚动(scroll)。任何可能超过父类边界的布局,都应该嵌套在[ScrollView](http://developer.android.com/reference/android/widget/ScrollView.html)中,来提供一个由系统框架管理的可滚动的view。仅在某些特殊情形下,我们才要实现一个自定义scroller。本节课程就描述了这样一个情形:使用scrollers显示滚动效果,以响应触摸手势。
为了收集数据来产生滚动动画,以响应一个触摸事件,你可以使用scrollers([Scroller](http://developer.android.com/reference/android/widget/Scroller.html)或者[OverScroller](http://developer.android.com/reference/android/widget/OverScroller.html))。这两个类很相似,但[OverScroller](http://developer.android.com/reference/android/widget/OverScroller.html)有一些函数,能在平移或快速滑动手势后,向用户指出已经达到内容尽头了。**InteractiveChart**例子使用了[EdgeEffect](http://developer.android.com/reference/android/widget/EdgeEffect.html)类(实际上是[EdgeEffectCompat](http://developer.android.com/reference/android/support/v4/widget/EdgeEffectCompat.html)类),在用户到达内容尽头时显示发光效果。
> **注意**:比起Scroller类,我们更推荐使用[OverScroller](http://developer.android.com/reference/android/widget/OverScroller.html)类来产生滚动动画。[OverScroller](http://developer.android.com/reference/android/widget/OverScroller.html)类为老设备提供了很好的向后兼容性。另外需要注意的是,仅当你要自己实现滚动时,才需要使用scrollers。如果你把布局嵌套在[ScrollView](http://developer.android.com/reference/android/widget/ScrollView.html)和[HorizontalScrollView](http://developer.android.com/reference/android/widget/HorizontalScrollView.html)中,它们会帮你把这些做好。
通过使用平台标准的滚动物理因素(摩擦、速度等),scroller被用来根据时间,产生滚动动画。实际上,scroller本身不会绘制任何东西。Scrollers只是随着时间的推移,帮你追踪滚动的偏移量,但它们不会自动地把这些位置应用到你的view上。你应该按一定频率,获取并应用这些新的坐标值,来让滚动动画更加顺滑。
### 理解术语Scrolling
在Android中,“Scrolling”这个词根据不同情景有着不同的含义。
**Scrolling**是指移动视窗(viewport)(指你正在看的内容所在的‘窗口’)的一般过程。当在x轴和y轴方向同时滚动时,就叫做平移(panning)。示例程序提供的**InteractiveChart**类,展示了两种不同类型的scrolling,拖拽与快速滑动。
- **拖拽**(dragging)是scrolling的一种类型,当用户在触摸屏上滑动手指时发生。重写[GestureDetector.OnGestureListener](http://developer.android.com/reference/android/view/GestureDetector.OnGestureListener.html)的[onScroll()](http://developer.android.com/reference/android/view/GestureDetector.OnGestureListener.html#onScroll(android.view.MotionEvent,android.view.MotionEvent,float,float))函数,经常用来实现简单的拖拽。关于拖拽的更多讨论,可以查看[**拖拽与缩放**](#)章节。
- **快速滑动**(fling)这种类型的scrolling,在用户快速拖拽后,抬起手指时发生。当用户抬高手指后,你通常想继续保持scrolling(移动视窗),但会一直减速直到视窗停止移动。通过重写[GestureDetector.OnGestureListener](http://developer.android.com/reference/android/view/GestureDetector.OnGestureListener.html)的[onFling()](http://developer.android.com/reference/android/view/GestureDetector.OnGestureListener.html#onFling(android.view.MotionEvent,android.view.MotionEvent,float,float))函数,使用scroller对象,可实现快速滑动。这种用法也就是本节课程的主题。
scroller对象通常会与快速滑动手势结合起来使用。但在任何你想让UI展示scrolling动画,以响应触摸事件的场景,都可以用它们来实现。比如,你可以重写[onTouchEvent()](http://developer.android.com/reference/android/view/View.html#onTouchEvent(android.view.MotionEvent))函数,直接处理触摸事件,并且产生一个scrolling效果或“对齐到页”动画(snapping to page),来响应这些触摸事件。
### 实现基于触摸的Scrolling
本节讲述如何使用scroller。下面的代码段来自InteractiveChart示例。它使用[GestureDetector][GestureDetector_url],并且重写了[GestureDetector.SimpleOnGestureListener](http://developer.android.com/reference/android/view/GestureDetector.SimpleOnGestureListener.html)的`onFling()`函数。它使用[OverScroller](http://developer.android.com/reference/android/widget/OverScroller.html)追踪快速滑动(fling)手势。快速滑动手势后,如果用户到达内容尽头了,应用会显示一种发光效果。
> 注意:**InteractiveChart**样例程序展示了一个可缩放、平移、滑动的表格。在接下来的代码段中,**mContentRect**表示view中的一块矩形坐标区域,该区域将被用来绘制表格。在任意给定的时间点,表格中某一部分会被绘制在这个区域内。**mCurrentViewport**表示当前在屏幕上可见的那一部分表格。因为像素偏移量通常当作整型处理,所以**mContentRect**是[Rect](http://developer.android.com/reference/android/graphics/Rect.html)类型的。因为图表的区域范围是数值型/浮点型值,所以**mCurrentViewport**是[RectF](http://developer.android.com/reference/android/graphics/RectF.html)类型。
代码段的第一部分展示了`onFling()`函数的实现:
~~~
// 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() {
@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;
}
...
@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);
}
~~~
当`onFling()`函数调用[postInvalidateOnAnimation()](http://developer.android.com/reference/android/support/v4/view/ViewCompat.html#postInvalidateOnAnimation(android.view.View))时,它会触发>computeScroll()来更新x、y的值。通常一个子view用scroller对象来产生滚动动画时会这样做,就像本例一样。
大多数views直接通过[scrollTo()](http://developer.android.com/reference/android/view/View.html#scrollTo(int,int))函数传递scroller对象的x、y坐标值。接下来的`computeScroll()`函数的实现中采用了一种不同的方式。它调用[computeScrollOffset()](http://developer.android.com/reference/android/widget/OverScroller.html#computeScrollOffset())函数来获得当前位置的x、y值。当满足边缘显示发光效果的条件时(图表已被放大显示,x或y值超过边界,并且app当前没有显示overscroll),这段代码会设置overscroll发光效果,并调用`postInvalidateOnAnimation()`函数来让view失效重绘:
~~~
// 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;
@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;
}
...
}
~~~
这是缩放部分的代码:
~~~
// 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);
}
~~~
这是上面代码段中调用过的**computeScrollSurfaceSize()**函数。他会计算当前可滚动的尺寸,以像素为单位。举例来说,如果整个图表区域都是可见的,它的值就简单地等于**mContentRect**的大小。如果图表在两个方向上都放大到200%,此函数返回的尺寸在水平、垂直方向上都会大两倍。
~~~
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()));
}
~~~
关于scroller用法的另一个示例,可查看[ViewPager](http://developer.android.com/reference/android/support/v4/view/ViewPager.html)类的[源代码](http://github.com/android/platform_frameworks_support/blob/master/v4/java/android/support/v4/view/ViewPager.java)。它用滚动来响应flings,并且使用滚动来实现“对齐到页”(snapping to page)动画。