Android(安卓)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NestedScrollingChildHelperNestedScrollingParentHelper

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：

<?xml version="1.0" encoding="utf-8"?>

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

NestedScrollingChild 接口类

NestedScrollingParent 接口

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