NestedScrollView.java

1.触摸事件

 @Override
    public boolean onTouchEvent(MotionEvent ev) {
        initVelocityTrackerIfNotExists();

        MotionEvent vtev = MotionEvent.obtain(ev);

        final int actionMasked = MotionEventCompat.getActionMasked(ev);

        if (actionMasked == MotionEvent.ACTION_DOWN) {
            mNestedYOffset = 0;
        }
        vtev.offsetLocation(0, mNestedYOffset);

        switch (actionMasked) {
            case MotionEvent.ACTION_DOWN: {
                //通知nested开始滚动
                startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL);
                break;
            }
            case MotionEvent.ACTION_MOVE:
                //通知nested滚动
                if (dispatchNestedPreScroll(0, deltaY, mScrollConsumed, mScrollOffset)) {
                    deltaY -= mScrollConsumed[1];
                    vtev.offsetLocation(0, mScrollOffset[1]);
                    mNestedYOffset += mScrollOffset[1];
                }
                ...
                if (mIsBeingDragged) {
                    // Scroll to follow the motion event
                    ...
                    if (dispatchNestedScroll(0, scrolledDeltaY, 0, unconsumedY, mScrollOffset)) {
                        mLastMotionY -= mScrollOffset[1];
                        vtev.offsetLocation(0, mScrollOffset[1]);
                        mNestedYOffset += mScrollOffset[1];
                    } else if (canOverscroll) {
                        ...
                    }
                }
                break;
              case MotionEvent.ACTION_CANCEL:
              case MotionEvent.ACTION_UP:
                /* Release the drag */
                mIsBeingDragged = false;
                mActivePointerId = INVALID_POINTER;
                recycleVelocityTracker();
                if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0, getScrollRange())) {
                    ViewCompat.postInvalidateOnAnimation(this);
                }
                //通知nested滚动
                stopNestedScroll();
                break;
            
        }

        if (mVelocityTracker != null) {
            mVelocityTracker.addMovement(vtev);
        }
        vtev.recycle();
        return true;
    }

分析dispatchNestedScroll方法

@Override
    public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed,
            int dyUnconsumed, int[] offsetInWindow) {
        return mChildHelper.dispatchNestedScroll(dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed,
                offsetInWindow);
    }


NestedScrollViewHelper
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow) {
        if (isNestedScrollingEnabled() && mNestedScrollingParent != null) {
            if (dxConsumed != 0 || dyConsumed != 0 || dxUnconsumed != 0 || dyUnconsumed != 0) {
                int startX = 0;
                int startY = 0;
                if (offsetInWindow != null) {
                    mView.getLocationInWindow(offsetInWindow);
                    startX = offsetInWindow[0];
                    startY = offsetInWindow[1];
                }
//调用控制器的onNestedScroll
                ViewParentCompat.onNestedScroll(mNestedScrollingParent, mView, dxConsumed,
                        dyConsumed, dxUnconsumed, dyUnconsumed);

                if (offsetInWindow != null) {
                    mView.getLocationInWindow(offsetInWindow);
                    offsetInWindow[0] -= startX;
                    offsetInWindow[1] -= startY;
                }
                return true;
            } else if (offsetInWindow != null) {
                // No motion, no dispatch. Keep offsetInWindow up to date.
                offsetInWindow[0] = 0;
                offsetInWindow[1] = 0;
            }
        }
        return false;
    }

ViewParentCompatLollipop.java
public static void onNestedScroll(ViewParent parent, View target, int dxConsumed,
            int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
        try {
            parent.onNestedScroll(target, dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed);
        } catch (AbstractMethodError e) {
            Log.e(TAG, "ViewParent " + parent + " does not implement interface " +
                    "method onNestedScroll", e);
        }
    }

CoordinatorLayout.java

    public void onNestedScroll(View target, int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed) {
        final int childCount = getChildCount();
        boolean accepted = false;

        for (int i = 0; i < childCount; i++) {
            final View view = getChildAt(i);
            final LayoutParams lp = (LayoutParams) view.getLayoutParams();
//判断子View是否支持NestedScroll
            if (!lp.isNestedScrollAccepted()) {
                continue;
            }

            final Behavior viewBehavior = lp.getBehavior();
            if (viewBehavior != null) {
//调用Behavior.onNestedScroll,设置子View的偏移量
                viewBehavior.onNestedScroll(this, view, target, dxConsumed, dyConsumed,
                        dxUnconsumed, dyUnconsumed);
                accepted = true;
            }
        }

        if (accepted) {
//通知布局变话
            dispatchOnDependentViewChanged(true);
        }
    }

AppbarLayout#Behavior

@Override
        public void onNestedScroll(CoordinatorLayout coordinatorLayout, AppBarLayout child,
                View target, int dxConsumed, int dyConsumed,
                int dxUnconsumed, int dyUnconsumed) {
            if (dyUnconsumed < 0) {
                // If the scrolling view is scrolling down but not consuming, it's probably be at
                // the top of it's content
//向下滚动时，调整View位置
                scroll(coordinatorLayout, child, dyUnconsumed,
                        -child.getDownNestedScrollRange(), 0);
                // Set the expanding flag so that onNestedPreScroll doesn't handle any events
                mSkipNestedPreScroll = true;
            } else {
                // As we're no longer handling nested scrolls, reset the skip flag
                mSkipNestedPreScroll = false;
            }
        }

HeaderBehavior.java
//设置偏移量
final int scroll(CoordinatorLayout coordinatorLayout, V header,
            int dy, int minOffset, int maxOffset) {
        return setHeaderTopBottomOffset(coordinatorLayout, header,
                getTopBottomOffsetForScrollingSibling() - dy, minOffset, maxOffset);
    }


    int setHeaderTopBottomOffset(CoordinatorLayout parent, V header, int newOffset,
            int minOffset, int maxOffset) {
        final int curOffset = getTopAndBottomOffset();
        int consumed = 0;

        if (minOffset != 0 && curOffset >= minOffset && curOffset <= maxOffset) {
            // If we have some scrolling range, and we're currently within the min and max
            // offsets, calculate a new offset
            newOffset = MathUtils.constrain(newOffset, minOffset, maxOffset);
//计算有效偏移量，如果偏移量有变化则设置新的偏移量
            if (curOffset != newOffset) {
                setTopAndBottomOffset(newOffset);
                // Update how much dy we have consumed
                consumed = curOffset - newOffset;
            }
        }

        return consumed;
    }

public boolean setTopAndBottomOffset(int offset) {
        if (mViewOffsetHelper != null) {
            return mViewOffsetHelper.setTopAndBottomOffset(offset);
        } else {
            mTempTopBottomOffset = offset;
        }
        return false;
    }
//子View在布局时创建ViewOffsetHelper
@Override
    public boolean onLayoutChild(CoordinatorLayout parent, V child, int layoutDirection) {
        // First let lay the child out
        layoutChild(parent, child, layoutDirection);

        if (mViewOffsetHelper == null) {
            mViewOffsetHelper = new ViewOffsetHelper(child);
        }
        mViewOffsetHelper.onViewLayout();

        if (mTempTopBottomOffset != 0) {
            mViewOffsetHelper.setTopAndBottomOffset(mTempTopBottomOffset);
            mTempTopBottomOffset = 0;
        }
        if (mTempLeftRightOffset != 0) {
            mViewOffsetHelper.setLeftAndRightOffset(mTempLeftRightOffset);
            mTempLeftRightOffset = 0;
        }

        return true;
    }