Android NestedScrolling机制
NestedScrolling机制现在在App的作用越来越重要，许多很漂亮的交互都是基于NestedScrolling机制进行完成的。
NestedScrolling机制主要是能够让父View和子View在滚动时互相协调配合。其中有两个重要的类，分别是：
接口类
NestedScrollingParent(最新:NestedScrollingParent2)
NestedScrollingChild(最新:NestedScrollingChild2)
帮助类
NestedScrollingChildHelper
NestedScrollingParentHelper
父类继承NestedScrollingParent接口，而子类继承NestedScrollingChild接口，同时让父类包含子类，而不是自接父子关系，就搭起了NestedScrollingParent机制的基本骨架。
其主要流程是：
- 子类滑动，把滑动产生的事件和参数传给父类
- 父类根据子类传过来的参数进行各种交互操作，如变大缩小之类的
而NestedScrollingChildHelper和NestedScrollingParentHelper是两个帮助类，在实现NestedScrollingChild和NestedScrollingParent接口时，使用这两个帮助类可以简化我们的工作。
NestedScrollingChild 接口类
public interface NestedScrollingChild {
/**
* 设置嵌套滑动是否能用
*/
@Override
public void setNestedScrollingEnabled(boolean enabled);
/**
* 判断嵌套滑动是否可用
*/
@Override
public boolean isNestedScrollingEnabled();
/**
* 开始嵌套滑动
*
* @param axes 表示方向轴，有横向和竖向
*/
@Override
public boolean startNestedScroll(int axes);
/**
* 停止嵌套滑动
*/
@Override
public void stopNestedScroll();
/**
* 判断是否有父View 支持嵌套滑动
*/
@Override
public boolean hasNestedScrollingParent() ;
/**
* 滑行时调用
* @param velocityX x 轴上的滑动速率
* @param velocityY y 轴上的滑动速率
* @param consumed 是否被消费
* @return true if the nested scrolling parent consumed or otherwise reacted to the fling
*/
@Override
public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) ;
/**
* 进行滑行前调用
* @param velocityX x 轴上的滑动速率
* @param velocityY y 轴上的滑动速率
* @return true if a nested scrolling parent consumed the fling
*/
@Override
public boolean dispatchNestedPreFling(float velocityX, float velocityY) ;
/**
* 子view处理scroll后调用
* @param dxConsumed x轴上被消费的距离（横向）
* @param dyConsumed y轴上被消费的距离（竖向）
* @param dxUnconsumed x轴上未被消费的距离
* @param dyUnconsumed y轴上未被消费的距离
* @param offsetInWindow 子View的窗体偏移量
* @return true if the event was dispatched, false if it could not be dispatched.
*/
@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, @Nullable int[] offsetInWindow) ;
/**
* 在子View的onInterceptTouchEvent或者onTouch中，调用该方法通知父View滑动的距离
* @param dx x轴上滑动的距离
* @param dy y轴上滑动的距离
* @param consumed 父view消费掉的scroll长度
* @param offsetInWindow 子View的窗体偏移量
* @return 支持的嵌套的父View 是否处理了 滑动事件
*/
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow);
}
上面这个方法方法和代表的意思我都已经贴出来， 然后是只是一个接口类上面的方法要怎么实现呢，这时候就要用到上面的帮助类NestedScrollingChildHelper，一个完整的实现模板如下：
public class MyNestedScrollingChild extends LinearLayout implements NestedScrollingChild {
private NestedScrollingChildHelper mNestedScrollingChildHelper;
public MyNestedScrollingChild(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
mNestedScrollingChildHelper = new NestedScrollingChildHelper(this);
mNestedScrollingChildHelper.setNestedScrollingEnabled(true);
}
@Override
public void setNestedScrollingEnabled(boolean enabled) {
mNestedScrollingChildHelper.setNestedScrollingEnabled(enabled);
}
@Override
public boolean isNestedScrollingEnabled() {
return mNestedScrollingChildHelper.isNestedScrollingEnabled();
}
@Override
public boolean startNestedScroll(int axes) {
return mNestedScrollingChildHelper.startNestedScroll(axes);
}
@Override
public void stopNestedScroll() {
mNestedScrollingChildHelper.stopNestedScroll();
}
@Override
public boolean hasNestedScrollingParent() {
return mNestedScrollingChildHelper.hasNestedScrollingParent();
}
@Override
public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) {
return mNestedScrollingChildHelper.dispatchNestedFling(velocityX,velocityY,consumed);
}
@Override
public boolean dispatchNestedPreFling(float velocityX, float velocityY) {
return mNestedScrollingChildHelper.dispatchNestedPreFling(velocityX,velocityY);
}
@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, @Nullable int[] offsetInWindow) {
return mNestedScrollingChildHelper.dispatchNestedScroll(dxConsumed,dyConsumed,dxUnconsumed,dyUnconsumed,offsetInWindow);
}
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow) {
return dispatchNestedPreScroll(dx,dy,consumed,offsetInWindow);
}
}
NestedScrollingParent 接口
public interface NestedScrollingParent {
@Override
public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes);
@Override
public void onStopNestedScroll(View child);
@Override
public void onNestedScrollAccepted(View child, View target, int axes);
@Override
public void onNestedScroll(View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed);
@Override
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed);
@Override
public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed);
@Override
public boolean onNestedPreFling(View target, float velocityX, float velocityY);
@Override
public int getNestedScrollAxes();
}
从上面的代码可以看出NestedScrollingChild的方法执行之后就会回调父View的各个方法，从方法名也知道作用和NestedScrollingChild的用作大同小异。当子View执行startNestedScroll时，就会回调父View的onStartNestedScroll、onNestedScrollAccepted方法，当子View执行dispatchNestedPreScroll方法时，就会回调父View的onNestedPreScroll，当子View执行dispatchNestedScroll方法时，就会回调父View的onNestedScroll方法，由此类推，dispatchNestedPreFling回调父View的onNestedPreFling方法，dispatchNestedFling回调父View的onNestedFling方法，等。
同时也有几个接口是需要帮助类进行实现的，模板代码如下：
public class MyNestedScrollingParent extends LinearLayout implements NestedScrollingParent {
private NestedScrollingParentHelper mNestedScrollingParentHelper;
public MyNestedScrollingParent(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
mNestedScrollingParentHelper = new NestedScrollingParentHelper(this);
}
@Override
public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes) {
return super.onStartNestedScroll(child, target, nestedScrollAxes);
}
@Override
public void onNestedScroll(View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
super.onNestedScroll(target, dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed);
}
@Override
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed) {
super.onNestedPreScroll(target, dx, dy, consumed);
}
@Override
public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed) {
return super.onNestedFling(target, velocityX, velocityY, consumed);
}
@Override
public boolean onNestedPreFling(View target, float velocityX, float velocityY) {
return super.onNestedPreFling(target, velocityX, velocityY);
}
@Override
public void onStopNestedScroll(View child) {
mNestedScrollingParentHelper.onStopNestedScroll(child);
}
@Override
public void onNestedScrollAccepted(View child, View target, int axes) {
mNestedScrollingParentHelper.onNestedScrollAccepted(child,target,axes);
}
@Override
public int getNestedScrollAxes() {
return mNestedScrollingParentHelper.getNestedScrollAxes();
}
}
最后总结，子View通过startNestedScroll()发起嵌套滑动，同时父View也会回调自己的onStartNestedScroll()方法，接着子View每次在滚动前都会调用dispatchNestedPreScroll()方法,父View的onNestedPreScroll()也会操作，父View决定是否熬滑动，然后才是子View自己滑动，之后子View也可以调用上面的其它方法做相应的处理，最后调用stopNestedScroll()结束。
最后举一个实例吧
public class MyNestedScrollChild extends LinearLayout implements NestedScrollingChild2 {
private NestedScrollingChildHelper mNestedScrollingChildHelper;
private int[] offset=new int[2];
private int[] consumed=new int[2];
private TextView scrollText;
private int showHeight;
private int lastY;
private boolean srcollTop=false;
public MyNestedScrollChild(Context context) {
super(context);
}
public MyNestedScrollChild(Context context, AttributeSet attrs) {
super(context, attrs);
setBackgroundColor(context.getResources().getColor(R.color.colorffffff));
}
@Override
protected void onFinishInflate() {
super.onFinishInflate();
scrollText=(TextView)getChildAt(0);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
showHeight = getMeasuredHeight();
heightMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
public boolean canChildScrollUp() {
return srcollTop;
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch(event.getAction()){
case MotionEvent.ACTION_DOWN:
lastY=(int)event.getRawY();
break;
case MotionEvent.ACTION_MOVE:
int y=(int)(event.getRawY());
int dy=y-lastY;
lastY=y;
if(startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL)
&&dispatchNestedPreScroll(0,dy,consumed,offset)){
int remain = dy - consumed[1];
if (remain != 0) {
scrollBy(0, -remain);
}
}else{
scrollBy(0, -dy);
}
break;
}
return true;
}
//限制滚动范围
@Override
public void scrollTo(int x, int y) {
int maxY = getMeasuredHeight()- showHeight;
if (y > maxY) {
y = maxY;
srcollTop=false;
}else if (y < 0) {
y = 0;
srcollTop=true;
}else{
srcollTop=false;
}
super.scrollTo(x, y);
}
public NestedScrollingChildHelper getNestedScrollingChildHelper(){
if(mNestedScrollingChildHelper==null){
mNestedScrollingChildHelper=new NestedScrollingChildHelper(this);
mNestedScrollingChildHelper.setNestedScrollingEnabled(true);
}
return mNestedScrollingChildHelper;
}
@Override
public boolean startNestedScroll(int axes, int type) {
return getNestedScrollingChildHelper().startNestedScroll(axes,type);
}
@Override
public void stopNestedScroll(int type) {
getNestedScrollingChildHelper().stopNestedScroll(type);
}
@Override
public boolean hasNestedScrollingParent(int type) {
return getNestedScrollingChildHelper().hasNestedScrollingParent(type);
}
@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed,
int dyUnconsumed, @Nullable int[] offsetInWindow, int type) {
return getNestedScrollingChildHelper().dispatchNestedScroll(dxConsumed,dyConsumed,
dxUnconsumed,dyUnconsumed,offsetInWindow,type);
}
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow, int type) {
return getNestedScrollingChildHelper().dispatchNestedPreScroll(dx,dy,consumed,offsetInWindow,type);
}
@Override
public void setNestedScrollingEnabled(boolean enabled) {
getNestedScrollingChildHelper().setNestedScrollingEnabled(enabled);
}
@Override
public boolean isNestedScrollingEnabled() {
return getNestedScrollingChildHelper().isNestedScrollingEnabled();
}
@Override
public boolean startNestedScroll(int axes) {
return getNestedScrollingChildHelper().startNestedScroll(axes);
}
@Override
public void stopNestedScroll() {
getNestedScrollingChildHelper().stopNestedScroll();
}
@Override
public boolean hasNestedScrollingParent() {
return getNestedScrollingChildHelper().hasNestedScrollingParent();
}
@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed,
@Nullable int[] offsetInWindow) {
return getNestedScrollingChildHelper().dispatchNestedScroll(dxConsumed,dyConsumed,dxUnconsumed,dyUnconsumed,offsetInWindow);
}
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow) {
return getNestedScrollingChildHelper().dispatchNestedPreScroll(dx,dy,consumed,offsetInWindow);
}
@Override
public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) {
return getNestedScrollingChildHelper().dispatchNestedFling(velocityX,velocityY,consumed);
}
@Override
public boolean dispatchNestedPreFling(float velocityX, float velocityY) {
return getNestedScrollingChildHelper().dispatchNestedPreFling(velocityX,velocityY);
}
}
首先给出NestedScrollingChild子View,重点看一下onTouchEvent()方法,当MotionEvent.ACTION_MOVE时,不断的调用startNestedScroll()和dispatchNestedPreScroll()向父View发送直接，然后滚动通过scrollBy()滚动触发事件的View,这就是最核心的代码了，接着看父View代码如下：
public class MyNestedScrollParent extends LinearLayout implements NestedScrollingParent2 {
private NestedScrollingParentHelper mNestedScrollingParentHelper;
private MyNestedScrollChild scrollChildView;
private ImageView foodIV;
private TextView titleTV;
private int imageHeight;
private int titleHeight;
private int imageMargin;
private int scrollY;
public MyNestedScrollParent(Context context) {
this(context,null);
}
public MyNestedScrollParent(Context context, AttributeSet attrs) {
super(context, attrs);
mNestedScrollingParentHelper=new NestedScrollingParentHelper(this);
}
@Override
protected void onFinishInflate() {
super.onFinishInflate();
FrameLayout frameLayout=(FrameLayout) getChildAt(0);
scrollChildView=(MyNestedScrollChild) getChildAt(1);
foodIV=frameLayout.findViewById(R.id.foodIV);
titleTV=frameLayout.findViewById(R.id.titleTV);
foodIV.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
imageHeight=foodIV.getHeight();
}
});
titleTV.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
titleHeight=titleTV.getHeight();
}
});
}
@Override
public boolean onStartNestedScroll(@NonNull View child, @NonNull View target, int axes, int type) {
if(target instanceof MyNestedScrollChild) {
return true;
}
return false;
}
@Override
public void onNestedScrollAccepted(@NonNull View child, @NonNull View target, int axes, int type) {
mNestedScrollingParentHelper.onNestedScrollAccepted(child,target,axes,type);
}
@Override
public void onStopNestedScroll(@NonNull View target, int type) {
mNestedScrollingParentHelper.onStopNestedScroll(target,type);
}
@Override
public int getNestedScrollAxes() {
return mNestedScrollingParentHelper.getNestedScrollAxes();
}
@Override
public void onStopNestedScroll(@NonNull View target) {
mNestedScrollingParentHelper.onStopNestedScroll(target);
}
@Override
public void onNestedScroll(@NonNull View target, int dxConsumed, int dyConsumed,
int dxUnconsumed, int dyUnconsumed, int type) {
}
@Override
public void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed, int type) {
imageMargin=titleHeight-imageHeight;
scrollY+=dy;
if(scrollY<=imageMargin){
scrollY=imageMargin;
scrollChildView.setTranslationY(scrollY);
}else{
if(dy<0){
//上滑
consumed[1]=dy;
scrollChildView.setTranslationY(scrollY);
}else{
//下滑
if(!scrollChildView.canChildScrollUp()){
scrollY-=dy;
}
if(scrollY>=0){
scrollY=0;
}
scrollChildView.setTranslationY(scrollY);
}
}
}
@Override
public boolean onStartNestedScroll(@NonNull View child, @NonNull View target, int axes) {
return false;
}
}
前面的子View的dispatchNestedPreScroll()对应这个父View的onNestedScroll()如上面的代码，通过View.setTranslationY()来滑动整个子View，consumed[1]=dy;表示子View和滑动的View一起滑。最后看一下布局文件：
父View包裹子View，以达成依赖关系。
讲了NestedScrolling,就有必要讲解CoordinatorLayout.Behavior，下回讲吧，最后奉上源码吧https://github.com/jack921/MeiTuanGoodsDetails