Android在Lollipop版本中引入了NestedScrolling——嵌套滾動機制。在Android的事件處理機制中，事件序列只能由父View和子View中的一個處理。在嵌套滾動機制中，子View處理事件前會將事件傳給父View處理，兩者協(xié)作配合處理事件。
在嵌套滾動機制中，父View需實現(xiàn)NestedScrollingParent接口，子View需要實現(xiàn)NestedScrollingChild接口。從Lollipop起View都已經(jīng)實現(xiàn)了NestedScrollingChild的方法。嵌套滾動過程如下：
上面提及的各個方法的具體用法請參考官方文檔。
Android為NestedScrollingChild提供了一個代理類NestedScrollingChildHelper。所以，NestedScrollingChild的最簡單的實現(xiàn)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，在適當(dāng)?shù)臅r機調(diào)用如下方法：
1.startNestedScroll
2.dispatchNestedPreScroll
3.dispatchNestedScroll
4.dispatchNestedPreFling
5.dispatchNestedFling
6.stopNestedScroll
Android為NestedScrollingParent提供了一個代理類NestedScrollingParentHelper。NestedScrollingParent的最簡單實現(xiàn)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方法從NestedScrollingChild向上查找愿意接收嵌套滾動事件的父View，如果找到了則調(diào)用父View的onNestedScrollAccepted方法。ViewParentCompat是父View的兼容類，該類會判斷版本，如果在Lollipop及以上則調(diào)用View自帶的方法。否則，調(diào)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; }調(diào)用父View的onNestedPreScroll方法并記錄滾動偏移量。參數(shù)offsetInWindow是一個長度為2的一位數(shù)組，記錄滾動的偏移量，用來修改Touch事件的坐標(biāo)，保證下次滾動的準(zhǔn)確性。dispatchNestedScroll方法也同理。
實現(xiàn)一個簡單的NestedScrollingParent。該View包含一個頭部View和RecyclerView。RecyclerView已經(jīng)實現(xiàn)了NestedScrollingChild接口方法。向上滾動時，如果頭部沒有完全收起，則向上滾動頭部。如果頭部收起才滾動RecyclerView。向下滾動時，如果頭部收起，則向下滾動頭部，否則滾動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 < headerH && 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 < headerH && 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 < headerH && 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(); } }}以上就是本文的全部內(nèi)容，希望對大家的學(xué)習(xí)有所幫助，也希望大家多多支持VEVB武林網(wǎng)。
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