前言
最近去小米之家体验了下小米9，发现MIUI有一个挺特别的列表动画效果，在系统上的各种应用上都能见到它的身影。
ScrollView，然后重写onInterceptTouchEvent方法和OnTouchEvent方法，计算手指滑动距离来缩放内部控件。 这种方式适合对View触摸分发机制比较熟悉的同学，代码比较复杂，看了下现有的库也都没能实现MIUI中Fling状态的弹性效果。正好最近看了下NestedScrolling的相关知识，发现能很好地实现这些效果，所以就让我们来看看吧。
预备知识
需要先了解下NestedScrollChild和NestedScrollParent，所谓的NestedScrolling机制是这样的：内部NestedScrollingChild在滚动的时候，预先将dx,dy通过NestedScrollingChildHelper传递给NestedScrollingParent，NestedScrollingParent可先对其进行部分消耗，Parent处理完后，再将剩余的部分还给内部NestedScrollingChild处理，最后再把剩下的dx,dy再给Parent做最后处理，这样一次触摸滑动事件将可以由多个控件共同消耗处理，这样就可以很方便解决之前一次触摸滑动事件只能被一个控件响应而产生的嵌套滑动问题。
先看下NestedScrollParent：
public interface NestedScrollingParent {public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes);public void onNestedScrollAccepted(View child, View target, int nestedScrollAxes);public void onStopNestedScroll(View target);public void onNestedScroll(View target, int dxConsumed, int dyConsumed,int dxUnconsumed, int dyUnconsumed);public void onNestedPreScroll(View target, int dx, int dy, int[] consumed);public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed);public boolean onNestedPreFling(View target, float velocityX, float velocityY);public int getNestedScrollAxes();
}
复制代码先看下NestedScrollingChild
public interface NestedScrollingChild {void setNestedScrollingEnabled(boolean enabled);boolean isNestedScrollingEnabled();boolean startNestedScroll(int axes);void stopNestedScroll();boolean hasNestedScrollingParent();boolean dispatchNestedScroll(int dxConsumed, int dyConsumed,int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow);boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow);boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed);boolean dispatchNestedPreFling(float velocityX, float velocityY);}
复制代码可以看到parent和child的api命名很类似，是成对出现的，确实，它们之前存在发起方和接收方的事件调用关系，都是由child先响应滑动触摸实现，通过NestedScrollingChildHelper分发给parent。
弹性列表实现
为方便解析，我们先只实现下滑的弹性动画：
//子view，需事先NestedScrollingChildprivate var childView: View? = nullprivate val mNestedScrollingParentHelper: NestedScrollingParentHelperprivate var offsetScale = 0fprivate var flingScale = 0fprivate var consumedDy = 0set(value) {field = if (value > 0) {0} else {value}}//是否是Fling滑动private var filing = false//判定滑动的最小距离private var touchSlop: Int = 0private var animator: ValueAnimator? = nullinit {mNestedScrollingParentHelper = NestedScrollingParentHelper(this)touchSlop = ViewConfiguration.get(context).scaledTouchSlop}override fun onFinishInflate() {super.onFinishInflate()childView = getChildAt(0)}/*** 滚动开始*/override fun onStartNestedScroll(child: View, target: View, nestedScrollAxes: Int): Boolean {filing = falseconsumedDy = 0return child === childView && ViewCompat.SCROLL_AXIS_VERTICAL == nestedScrollAxes}/*** 先于child滚动*/override fun onNestedPreScroll(target: View, dx: Int, dy: Int, consumed: IntArray) {if (childView!!.scrollY == 0 && (dy < 0 || consumedDy < 0)) {consumedDy += dyif (Math.abs(consumedDy) > touchSlop) {//计算缩放值,最大放大1.3倍offsetScale = (1.3 - 600f / (2000 + Math.pow(Math.abs(consumedDy).toDouble(), 2.0))).toFloat()startBouncingTop()//存放消耗的距离，child会接收consumed[1] = dy}}}override fun onNestedScrollAccepted(child: View, target: View, nestedScrollAxes: Int) {mNestedScrollingParentHelper.onNestedScrollAccepted(child, target, nestedScrollAxes)}/*** 先于child处理Fling*/override fun onNestedPreFling(target: View, velocityX: Float, velocityY: Float): Boolean {if (velocityY < 0 && childView!!.scrollY == 0) {filing = trueconsumedDy = (consumedDy + velocityY).toInt()flingScale = (0.3 - 600f / (2000 + Math.pow(Math.abs(consumedDy).toDouble(), 2.0))).toFloat()return true}return false}override fun onNestedFling(target: View, velocityX: Float, velocityY: Float, consumed: Boolean): Boolean {return filing}override fun onStopNestedScroll(target: View) {mNestedScrollingParentHelper.onStopNestedScroll(target)backBouncing(filing)}override fun getNestedScrollAxes(): Int {return mNestedScrollingParentHelper.nestedScrollAxes}/*** 进行回弹*/private fun backBouncing(filing: Boolean) {//初始化if (animator != null && animator!!.isRunning) {animator!!.cancel()animator = null}if (filing) {animator = ValueAnimator.ofFloat(offsetScale, flingScale, 0f)animator!!.duration = 400} else {animator = ValueAnimator.ofFloat(offsetScale, 0f)animator!!.duration = 250}animator!!.interpolator = OvershootInterpolator()animator!!.addUpdateListener {offsetScale = it.animatedValue as FloatstartBouncingTop()}animator!!.start()}/*** 从顶部开始滑动*/private fun startBouncingTop() {childView!!.pivotY = 0fchildView!!.pivotX = 0fchildView!!.scaleY = offsetScale}
复制代码弹性效果
参考文章
Android 8.0 NestedScrollingChild2与NestedScrollingParent2实现RecyclerView阻尼回弹效果
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