Android在Lollipop版本中引入了NestedScrolling——嵌套滚动机制。在Android的事件处理机制中，事件序列只能由父View和子View中的一个处理。在嵌套滚动机制中，子View处理事件前会将事件传给父View处理，两者协作配合处理事件。
在嵌套滚动机制中，父View需实现NestedScrollingParent接口，子View需要实现NestedScrollingChild接口。从Lollipop起View都已经实现了NestedScrollingChild的方法。嵌套滚动过程如下：
上面提及的各个方法的具体用法请参考官方文档。
Android为NestedScrollingChild提供了一个代理类NestedScrollingChildHelper。所以，NestedScrollingChild的最简单的实现如下。
public class NestedScrollingChildView extends FrameLayout implements NestedScrollingChild {
private final NestedScrollingChildHelper mChildHelper;
public NestedScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
mChildHelper = new NestedScrollingChildHelper(this);
}
@Override
public void setNestedScrollingEnabled(boolean enabled) {
mChildHelper.setNestedScrollingEnabled(enabled);
}
@Override
public boolean isNestedScrollingEnabled() {
return mChildHelper.isNestedScrollingEnabled();
}
@Override
public boolean startNestedScroll(int axes) {
return mChildHelper.startNestedScroll(axes);
}
@Override
public void stopNestedScroll() {
mChildHelper.stopNestedScroll();
}
@Override
public boolean hasNestedScrollingParent() {
return mChildHelper.hasNestedScrollingParent();
}
@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed,
int dyUnconsumed, int[] offsetInWindow) {
return mChildHelper.dispatchNestedScroll(dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed,
offsetInWindow);
}
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) {
return mChildHelper.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow);
}
@Override
public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) {
return mChildHelper.dispatchNestedFling(velocityX, velocityY, consumed);
}
@Override
public boolean dispatchNestedPreFling(float velocityX, float velocityY) {
return mChildHelper.dispatchNestedPreFling(velocityX, velocityY);
}
}
然后，在适当的时机调用如下方法：
1.startNestedScroll
2.dispatchNestedPreScroll
3.dispatchNestedScroll
4.dispatchNestedPreFling
5.dispatchNestedFling
6.stopNestedScroll
Android为NestedScrollingParent提供了一个代理类NestedScrollingParentHelper。NestedScrollingParent的最简单实现如下。
public class NestedScrollView extends FrameLayout implements NestedScrollingParent {
private final NestedScrollingParentHelper mParentHelper;
public NestedScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
mParentHelper = new NestedScrollingParentHelper(this);
}
@Override
public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes) {
...
return (nestedScrollAxes & ViewCompat.SCROLL_AXIS_VERTICAL) != 0;
}
@Override
public void onNestedScrollAccepted(View child, View target, int axes) {
mParentHelper.onNestedScrollAccepted(child, target, axes);
...
}
@Override
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed) {
...
}
@Override
public void onNestedScroll(View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
...
}
@Override
public boolean onNestedPreFling(View target, float velocityX, float velocityY) {
...
return false;
}
@Override
public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed) {
...
return false;
}
@Override
public void onStopNestedScroll(View child) {
mParentHelper.onStopNestedScroll(child);
}
@Override
public int getNestedScrollAxes() {
return mParentHelper.getNestedScrollAxes();
}
}
public boolean startNestedScroll(int axes) {
if (hasNestedScrollingParent()) {
// Already in progress
return true;
}
if (isNestedScrollingEnabled()) {
ViewParent p = mView.getParent();
View child = mView;
while (p != null) {
if (ViewParentCompat.onStartNestedScroll(p, child, mView, axes)) {
mNestedScrollingParent = p;
ViewParentCompat.onNestedScrollAccepted(p, child, mView, axes);
return true;
}
if (p instanceof View) {
child = (View) p;
}
p = p.getParent();
}
}
return false;
}
startNestedScroll方法从NestedScrollingChild向上查找愿意接收嵌套滚动事件的父View，如果找到了则调用父View的onNestedScrollAccepted方法。ViewParentCompat是父View的兼容类，该类会判断版本，如果在Lollipop及以上则调用View自带的方法。否则，调用NestedScrollingParent的接口方法。
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) {
if (isNestedScrollingEnabled() && mNestedScrollingParent != null) {
if (dx != 0 || dy != 0) {
int startX = 0;
int startY = 0;
if (offsetInWindow != null) {
mView.getLocationInWindow(offsetInWindow);
startX = offsetInWindow[0];
startY = offsetInWindow[1];
}
if (cOnsumed== null) {
if (mTempNestedScrollCOnsumed== null) {
mTempNestedScrollCOnsumed= new int[2];
}
cOnsumed= mTempNestedScrollConsumed;
}
consumed[0] = 0;
consumed[1] = 0;
ViewParentCompat.onNestedPreScroll(mNestedScrollingParent, mView, dx, dy, consumed);
if (offsetInWindow != null) {
mView.getLocationInWindow(offsetInWindow);
offsetInWindow[0] -= startX;
offsetInWindow[1] -= startY;
}
return consumed[0] != 0 || consumed[1] != 0;
} else if (offsetInWindow != null) {
offsetInWindow[0] = 0;
offsetInWindow[1] = 0;
}
}
return false;
}
调用父View的onNestedPreScroll方法并记录滚动偏移量。参数offsetInWindow是一个长度为2的一位数组，记录滚动的偏移量，用来修改Touch事件的坐标，保证下次滚动的准确性。dispatchNestedScroll方法也同理。
实现一个简单的NestedScrollingParent。该View包含一个头部View和RecyclerView。RecyclerView已经实现了NestedScrollingChild接口方法。向上滚动时，如果头部没有完全收起，则向上滚动头部。如果头部收起才滚动RecyclerView。向下滚动时，如果头部收起，则向下滚动头部，否则滚动RecyclerView。
public class HeaderLayout extends LinearLayout implements NestedScrollingParent {
private NestedScrollingParentHelper mParentHelper;
private int headerH;
private ScrollerCompat mScroller;
private boolean resetH = false;
public HeaderLayout(Context context) {
this(context, null);
}
public HeaderLayout(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public HeaderLayout(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
mParentHelper = new NestedScrollingParentHelper(this);
mScroller = ScrollerCompat.create(this.getContext());
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
headerH = getChildAt(0).getMeasuredHeight();
}
@Override
public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes) {
return (nestedScrollAxes & ViewCompat.SCROLL_AXIS_VERTICAL) != 0;
}
@Override
public void onNestedScrollAccepted(View child, View target, int axes) {
mParentHelper.onNestedScrollAccepted(child, target, axes);
}
@Override
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed) {
int scrollY = getScrollY();
if (dy > 0 && scrollY = 0) {
int cOnsumedY= Math.min(dy, headerH - scrollY);
consumed[1] = consumedY;
scrollBy(0, consumedY);
if (!resetH) {
resetH = true;
int w = getWidth();
int h = getHeight() + headerH;
setLayoutParams(new FrameLayout.LayoutParams(w, h));
}
} else if (dy <0 && scrollY == headerH) {
consumed[1] = dy;
scrollBy(0, dy);
} else if (dy <0 && scrollY 0) {
int cOnsumedY= Math.max(dy, -scrollY);
consumed[1] = consumedY;
scrollBy(0, consumedY);
}
}
@Override
public void onNestedScroll(View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
}
@Override
public boolean onNestedPreFling(View target, float velocityX, float velocityY) {
int scrollY = getScrollY();
if (velocityY > 0 && scrollY 0) {
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
}
mScroller.fling(0, scrollY, (int)velocityX, (int)velocityY, 0, 0, 0, headerH);
ViewCompat.postInvalidateOnAnimation(this);
return true;
}
return false;
}
@Override
public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed) {
return false;
}
@Override
public void onStopNestedScroll(View child) {
mParentHelper.onStopNestedScroll(child);
}
@Override
public int getNestedScrollAxes() {
return mParentHelper.getNestedScrollAxes();
}
@Override
public void computeScroll() {
if (mScroller.computeScrollOffset()) {
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
postInvalidate();
}
}
}
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