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

滑動屏幕時 RecyclerView 收到滑動事件，在 ACTION_DOWN 時

//	RecyclerView.java  onTouchEvent函數
 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;

繼續(xù)深入

    public boolean startNestedScroll(@ScrollAxis int axes, @NestedScrollType int type) {
        if (hasNestedScrollingParent(type)) {
            // Already in progress
            return 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;
    }

遞歸尋找NestedScrollingParent，然后回調 onStartNestedScroll 和 onNestedScrollAccepted 。onStartNestedScroll 決定了當前控件是否能接收到其內部View(非并非是直接子View)滑動時的參數；按下時確定其嵌套的父布局以及是否能收到后續(xù)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 中調用了 dispatchNestedPreScroll 。dispatchNestedPreScroll 中會回調 onNestedPreScroll 方法，內部的 scrollByInternal 中還會回調 onNestedScroll 方法

整個流程如下

在這里插入圖片描述

onNestedPreScroll中，我們判斷，如果是上滑且頂部控件未完全隱藏，則消耗掉dy，即consumed[1]=dy;如果是下滑且內部View已經無法繼續(xù)下拉，則消耗掉dy，即consumed[1]=dy，消耗掉的意思，就是自己去執(zhí)行scrollBy，實際上就是我們的NestedScrollView 滑動。

public void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed, int type) {
     // 向上滑動。若當前topview可見，需要將topview滑動至不可見
     boolean hideTop = dy > 0 && getScrollY() < topView.getMeasuredHeight();
     if (hideTop) {
         scrollBy(0, dy);
         //  這個是被消費的距離，如果沒有會被重復消費現象是父布局與子布局同時滑動，滑動的距離被消費兩次
         consumed[1] = dy;
     }
 }

整體代碼如下

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;
    /**
     * 用于判斷RecyclerView是否在fling
     */
    boolean isStartFling = false;
    /**
     * 記錄當前滑動的y軸加速度
     */
    private int velocityY = 0;

    private void init() {
        mFlingHelper = new FlingHelper(getContext());
        setOnScrollChangeListener(new View.OnScrollChangeListener() {
            @Override
            public 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情況下，記錄當前RecyclerView在y軸的偏移
                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);
        }
    }

    @Override
    public void fling(int velocityY) {
        super.fling(velocityY);
        if (velocityY <= 0) {
            this.velocityY = 0;
        } else {
            isStartFling = true;
            this.velocityY = velocityY;
        }
    }

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        topView = ((ViewGroup) getChildAt(0)).getChildAt(0);
        contentView = (ViewGroup) ((ViewGroup) getChildAt(0)).getChildAt(1);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // 調整contentView的高度為父容器高度，使之填充布局，避免父容器滾動后出現空白
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        ViewGroup.LayoutParams lp = contentView.getLayoutParams();
        lp.height = getMeasuredHeight();
        contentView.setLayoutParams(lp);
    }

    /**
     *          解決滑動沖突：RecyclerView在滑動之前會問下父布局是否需要攔截，父布局使用此方法
     */
    @Override
    public void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed, int type) {
        Log.e("NestedScrollLayout", getScrollY()+"::onNestedPreScroll::"+topView.getMeasuredHeight()+"::dy::"+dy);
        // 向上滑動。若當前topview可見，需要將topview滑動至不可見
        boolean hideTop = dy > 0 && getScrollY() < topView.getMeasuredHeight();
        if (hideTop) {
            scrollBy(0, dy);
            //  這個是被消費的距離，如果沒有會被重復消費，現象是父布局與子布局同時滑動
            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;
    }
}

嵌套滑動與事件分發(fā)機制

事件分發(fā)機制：子View首先得到事件處理權，處理過程中父View可以對其攔截，但是攔截了以后就無法再還給子View（本次手勢內）。
NestedScrolling 滑動機制：內部View在滾動的時候，首先將dx,dy交給NestedScrollingParent，NestedScrollingParent可對其進行部分消耗，剩余的部分還給內部View。

總結：嵌套布局要注意的有幾個方面