Android NestedScrolling機制

NestedScrolling機制現在在App的作用越來越重要，許多很漂亮的互動都是基於NestedScrolling機制進行完成的。

NestedScrolling機制主要是能夠讓父View和子View在滾動時互相協調配合。其中有兩個重要的類，分別是：

介面類
NestedScrollingParent(最新:NestedScrollingParent2)
NestedScrollingChild(最新:NestedScrollingChild2)

幫助類
NestedScrollingChildHelper
NestedScrollingParentHelper

父類繼承NestedScrollingParent介面，而子類繼承NestedScrollingChild介面，同時讓父類包含子類，而不是自接父子關係，就搭起了NestedScrollingParent機制的基本骨架。

其主要流程是：

1. 子類滑動，把滑動產生的事件和引數傳給父類
2. 父類根據子類傳過來的引數進行各種互動操作，如變大縮小之類的

而NestedScrollingChildHelper和NestedScrollingParentHelper是兩個幫助類，在實現NestedScrollingChild和NestedScrollingParent介面時，使用這兩個幫助類可以簡化我們的工作。

NestedScrollingChild 介面類

public interface NestedScrollingChild {

    /**
     * 設定巢狀滑動是否能用
     */
    @Override
    public void setNestedScrollingEnabled(boolean enabled);

    /**
     * 判斷巢狀滑動是否可用
     */
    @Override
    public boolean isNestedScrollingEnabled();

    /**
     * 開始巢狀滑動
     *
     * @param axes 表示方向軸，有橫向和豎向
     */
    @Override
    public boolean startNestedScroll(int axes);

    /**
     * 停止巢狀滑動
     */
    @Override
    public void stopNestedScroll();

    /**
     * 判斷是否有父View 支援巢狀滑動
     */
    @Override
    public boolean hasNestedScrollingParent() ;

    /**
     * 滑行時呼叫
     * @param velocityX x 軸上的滑動速率
     * @param velocityY y 軸上的滑動速率
     * @param consumed 是否被消費
     * @return  true if the nested scrolling parent consumed or otherwise reacted to the fling
     */
    @Override
    public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) ;

    /**
     * 進行滑行前呼叫
     * @param velocityX x 軸上的滑動速率
     * @param velocityY y 軸上的滑動速率
     * @return true if a nested scrolling parent consumed the fling
     */
    @Override
    public boolean dispatchNestedPreFling(float velocityX, float velocityY) ;

    /**
     * 子view處理scroll後呼叫
     * @param dxConsumed x軸上被消費的距離（橫向）
     * @param dyConsumed y軸上被消費的距離（豎向）
     * @param dxUnconsumed x軸上未被消費的距離
     * @param dyUnconsumed y軸上未被消費的距離
     * @param offsetInWindow 子View的窗體偏移量
     * @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，呼叫該方法通知父View滑動的距離
     * @param dx  x軸上滑動的距離
     * @param dy  y軸上滑動的距離
     * @param consumed 父view消費掉的scroll長度
     * @param offsetInWindow   子View的窗體偏移量
     * @return 支援的巢狀的父View 是否處理了 滑動事件
     */
    @Override
    public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow);

}

上面這個方法方法和代表的意思我都已經貼出來， 然後是隻是一個介面類上面的方法要怎麼實現呢，這時候就要用到上面的幫助類NestedScrollingChildHelper，一個完整的實現模板如下：

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 介面

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();
}

從上面的程式碼可以看出NestedScrollingChild的方法執行之後就會回撥父View的各個方法，從方法名也知道作用和NestedScrollingChild的用作大同小異。當子View執行startNestedScroll時，就會回撥父View的onStartNestedScroll、onNestedScrollAccepted方法，當子View執行dispatchNestedPreScroll方法時，就會回撥父View的onNestedPreScroll，當子View執行dispatchNestedScroll方法時，就會回撥父View的onNestedScroll方法，由此類推，dispatchNestedPreFling回撥父View的onNestedPreFling方法，dispatchNestedFling回撥父View的onNestedFling方法，等。

同時也有幾個介面是需要幫助類進行實現的，模板程式碼如下：

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();
    }

}

最後總結，子View通過startNestedScroll()發起巢狀滑動，同時父View也會回撥自己的onStartNestedScroll()方法，接著子View每次在滾動前都會呼叫dispatchNestedPreScroll()方法,父View的onNestedPreScroll()也會操作，父View決定是否熬滑動，然後才是子View自己滑動，之後子View也可以呼叫上面的其它方法做相應的處理，最後呼叫stopNestedScroll()結束。

最後舉一個例項吧

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;
    }

    //限制滾動範圍
    @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);
    }

}

首先給出NestedScrollingChild子View,重點看一下onTouchEvent()方法,當MotionEvent.ACTION_MOVE時,不斷的呼叫startNestedScroll()和dispatchNestedPreScroll()向父View傳送直接，然後滾動通過scrollBy()滾動觸發事件的View,這就是最核心的程式碼了，接著看父View程式碼如下：

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()對應這個父View的onNestedScroll()如上面的程式碼，通過View.setTranslationY()來滑動整個子View，consumed[1]=dy;表示子View和滑動的View一起滑。最後看一下佈局檔案：

<?xml version="1.0" encoding="utf-8"?>
<com.jack.meituangoodsdetails.view.MyNestedScrollParent
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="170dp">

        <ImageView
            android:id="@+id/foodIV"
            android:layout_width="match_parent"
            android:layout_height="170dp"
            android:src="@mipmap/food_bg"
            android:scaleType="fitXY"/>

        <TextView
            android:id="@+id/titleTV"
            android:layout_width="match_parent"
            android:layout_height="50dp"
            android:gravity="center"
            android:textColor="@android:color/white"
            android:background="@color/color2e8b57"
            android:text="MyTitle"/>

    </FrameLayout>

    <com.jack.meituangoodsdetails.view.MyNestedScrollChild
        android:id="@+id/scroll_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">
        <TextView
            android:id="@+id/scrollText"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:gravity="center"
            android:text="123\n456\n789\n111\n222\n333\n444\n555\n666\n777\n888\n999\n14\n12\n13\n44\n55\n66\n77\n88\n99\n11\n22\n33\n44\n55\n66\n77\n88\n99\n77\n88\n88\n8\n88\n88\n" />
    </com.jack.meituangoodsdetails.view.MyNestedScrollChild>

</com.jack.meituangoodsdetails.view.MyNestedScrollParent>

父View包裹子View，以達成依賴關係。

講了NestedScrolling,就有必要講解CoordinatorLayout.Behavior，下回講吧，最後奉上原始碼吧https://github.com/jack921/MeiTuanGoodsDetails