Android深入探究自定义View之嵌套滑动的实现 Android深入探究自定义View之嵌套

本文主要探讨以下几个问题：

嵌套滑动设计目的
嵌套滑动的实现
嵌套滑动与事件分发机制

嵌套滑动设计目的不知道大家有没有注意过淘宝APP首页的二级联动，滑动的商品的时候上面类别也会滑动，滑动过程中类别模块停了商品还能继续滑动。也就是说滑动的是view，ViewGroup也会跟着滑动。如果用事件分发机制处理也能处理，但会及其麻烦。那用NestedScroll会咋样？
嵌套滑动的实现假设布局如下

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RecyclerView 实现了 NestedScrollingChild 接口，NestedScrollView 实现了 NestedScrollingParent，这是实现嵌套布局的基础

public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2, NestedScrollingChild3public class NestedScrollView extends FrameLayout implements NestedScrollingParent3, NestedScrollingChild3, ScrollingView

滑动屏幕时 RecyclerView 收到滑动事件，在 ACTION_DOWN 时

// RecyclerView.javaonTouchEvent函数 case MotionEvent.ACTION_DOWN: {mScrollPointerId = e.getPointerId(0); mInitialTouchX = mLastTouchX = (int) (e.getX() + 0.5f); mInitialTouchY = mLastTouchY = (int) (e.getY() + 0.5f); int nestedScrollAxis = ViewCompat.SCROLL_AXIS_NONE; if (canScrollHorizontally) {nestedScrollAxis |= ViewCompat.SCROLL_AXIS_HORIZONTAL; }if (canScrollVertically) {nestedScrollAxis |= ViewCompat.SCROLL_AXIS_VERTICAL; }// startNestedScroll(nestedScrollAxis, TYPE_TOUCH); } break;

继续深入

public boolean startNestedScroll(@ScrollAxis int axes, @NestedScrollType int type) {if (hasNestedScrollingParent(type)) {// Already in progressreturn true; }if (isNestedScrollingEnabled()) {ViewParent p = mView.getParent(); View child = mView; while (p != null) {if (ViewParentCompat.onStartNestedScroll(p, child, mView, axes, type)) {setNestedScrollingParentForType(type, p); ViewParentCompat.onNestedScrollAccepted(p, child, mView, axes, type); return true; }if (p instanceof View) {child = (View) p; }p = p.getParent(); }}return false; }

递归寻找NestedScrollingParent，然后回调 onStartNestedScroll 和 onNestedScrollAccepted 。onStartNestedScroll 决定了当前控件是否能接收到其内部View(非并非是直接子View)滑动时的参数；按下时确定其嵌套的父布局以及是否能收到后续事件。再看ACTION_MOVE事件

case MotionEvent.ACTION_MOVE: {if (dispatchNestedPreScroll(dx, dy, mScrollConsumed, mScrollOffset)) {dx -= mScrollConsumed[0]; dy -= mScrollConsumed[1]; vtev.offsetLocation(mScrollOffset[0], mScrollOffset[1]); } } break;

ACTION_MOVE 中调用了 dispatchNestedPreScroll 。dispatchNestedPreScroll 中会回调 onNestedPreScroll 方法，内部的 scrollByInternal 中还会回调 onNestedScroll 方法
整个流程如下

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onNestedPreScroll中，我们判断，如果是上滑且顶部控件未完全隐藏，则消耗掉dy，即consumed[1]=dy; 如果是下滑且内部View已经无法继续下拉，则消耗掉dy，即consumed[1]=dy，消耗掉的意思，就是自己去执行scrollBy，实际上就是我们的NestedScrollView 滑动。

public void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed, int type) {// 向上滑动。若当前topview可见，需要将topview滑动至不可见boolean hideTop = dy > 0 && getScrollY() < topView.getMeasuredHeight(); if (hideTop) {scrollBy(0, dy); //这个是被消费的距离，如果没有会被重复消费现象是父布局与子布局同时滑动，滑动的距离被消费两次consumed[1] = dy; } }

整体代码如下

public class NestedScrollLayout extends NestedScrollView {private View topView; private ViewGroup contentView; private static final String TAG = "NestedScrollLayout"; public NestedScrollLayout(Context context) {this(context, null); init(); }public NestedScrollLayout(Context context, @Nullable AttributeSet attrs) {this(context, attrs, 0); init(); }public NestedScrollLayout(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {this(context, attrs, defStyleAttr, 0); init(); }public NestedScrollLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {super(context, attrs, defStyleAttr); init(); }private FlingHelper mFlingHelper; int totalDy = 0; /*** 用于判断RecyclerView是否在fling*/boolean isStartFling = false; /*** 记录当前滑动的y轴加速度*/private int velocityY = 0; private void init() {mFlingHelper = new FlingHelper(getContext()); setOnScrollChangeListener(new View.OnScrollChangeListener() {@Overridepublic void onScrollChange(View v, int scrollX, int scrollY, int oldScrollX, int oldScrollY) {if (isStartFling) {totalDy = 0; isStartFling = false; }if (scrollY == 0) {Log.e(TAG, "TOP SCROLL"); // refreshLayout.setEnabled(true); }if (scrollY == (getChildAt(0).getMeasuredHeight() - v.getMeasuredHeight())) {Log.e(TAG, "BOTTOM SCROLL"); dispatchChildFling(); }//在RecyclerView fling情况下，记录当前RecyclerView在y轴的偏移totalDy += scrollY - oldScrollY; }}); }private void dispatchChildFling() {if (velocityY != 0) {Double splineFlingDistance = mFlingHelper.getSplineFlingDistance(velocityY); if (splineFlingDistance > totalDy) {childFling(mFlingHelper.getVelocityByDistance(splineFlingDistance - Double.valueOf(totalDy))); }}totalDy = 0; velocityY = 0; }private void childFling(int velY) {RecyclerView childRecyclerView = getChildRecyclerView(contentView); if (childRecyclerView != null) {childRecyclerView.fling(0, velY); }}@Overridepublic void fling(int velocityY) {super.fling(velocityY); if (velocityY <= 0) {this.velocityY = 0; } else {isStartFling = true; this.velocityY = velocityY; }}@Overrideprotected void onFinishInflate() {super.onFinishInflate(); topView = ((ViewGroup) getChildAt(0)).getChildAt(0); contentView = (ViewGroup) ((ViewGroup) getChildAt(0)).getChildAt(1); }@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {// 调整contentView的高度为父容器高度，使之填充布局，避免父容器滚动后出现空白super.onMeasure(widthMeasureSpec, heightMeasureSpec); ViewGroup.LayoutParams lp = contentView.getLayoutParams(); lp.height = getMeasuredHeight(); contentView.setLayoutParams(lp); }/***解决滑动冲突：RecyclerView在滑动之前会问下父布局是否需要拦截，父布局使用此方法*/@Overridepublic void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed, int type) {Log.e("NestedScrollLayout", getScrollY()+"::onNestedPreScroll::"+topView.getMeasuredHeight()+"::dy::"+dy); // 向上滑动。若当前topview可见，需要将topview滑动至不可见boolean hideTop = dy > 0 && getScrollY() < topView.getMeasuredHeight(); if (hideTop) {scrollBy(0, dy); //这个是被消费的距离，如果没有会被重复消费，现象是父布局与子布局同时滑动consumed[1] = dy; }}private RecyclerView getChildRecyclerView(ViewGroup viewGroup) {for (int i = 0; i < viewGroup.getChildCount(); i++) {View view = viewGroup.getChildAt(i); if (view instanceof RecyclerView && view.getClass() == NestedLogRecyclerView.class) {return (RecyclerView) viewGroup.getChildAt(i); } else if (viewGroup.getChildAt(i) instanceof ViewGroup) {ViewGroup childRecyclerView = getChildRecyclerView((ViewGroup) viewGroup.getChildAt(i)); if (childRecyclerView instanceof RecyclerView) {return (RecyclerView) childRecyclerView; }}continue; }return null; }}

嵌套滑动与事件分发机制

事件分发机制：子View首先得到事件处理权，处理过程中父View可以对其拦截，但是拦截了以后就无法再还给子View（本次手势内）。
NestedScrolling 滑动机制：内部View在滚动的时候，首先将dx,dy交给NestedScrollingParent，NestedScrollingParent可对其进行部分消耗，剩余的部分还给内部View。

总结：嵌套布局要注意的有几个方面