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NestScrollView嵌套RecyclerView

andr_gale 人气:0

一.概述

本文主要实现淘宝首页嵌套滑动,中间tab吸顶效果,以及介绍NestScrollView嵌套RecyclerView处理滑动冲突的方法,淘宝首页的效果图如下:

二.开搞

首先我们通过一张图来分析下页面的布局结构:

先把最基础的页面搭出来,禁用Recycler滑动只需要重写onInterceptTouchEvent、onTouchEvent返回值都设为false即可:

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".activiy.ViewPagerActivity"
    android:background="#f2f2f2">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <com.aykj.nestscrolldemo.widget.NoScrollRecyclerView
            android:id="@+id/top_recycler_view"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"/>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

            <View
                android:layout_width="match_parent"
                android:layout_height="1px"
                android:background="#e0e0e0"/>

            <com.google.android.material.tabs.TabLayout
                android:id="@+id/tab_view"
                android:layout_width="match_parent"
                android:layout_height="wrap_content" />

            <View
                android:layout_width="match_parent"
                android:layout_height="1px"
                android:background="#e0e0e0"/>

            <androidx.viewpager.widget.ViewPager
                android:id="@+id/view_pager"
                android:layout_width="match_parent"
                android:layout_height="match_parent"/>

        </LinearLayout>

    </LinearLayout>

</ScrollView>
public class ViewPagerActivity extends AppCompatActivity {

    private List<String> topDatas = new ArrayList<>();
    private List<String> tabTitles = new ArrayList<>();
    ActivityViewPagerBinding viewBinding;
    private RecyclerAdapter topAdapter;
    private DividerItemDecoration divider;
    private TabFragmentAdapter pagerAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        viewBinding = ActivityViewPagerBinding.inflate(LayoutInflater.from(this));
        setContentView(viewBinding.getRoot());

        initDatas();
        initView();
    }

    private void initDatas() {
        topDatas.clear();
        for(int i=0; i<5; i++) {
            topDatas.add("top item " + (i + 1));
        }

        tabTitles.clear();
        tabTitles.add("tab1");
        tabTitles.add("tab2");
        tabTitles.add("tab3");
    }

    private void initView() {
        //init topRecycler
        divider = new DividerItemDecoration(this, LinearLayout.VERTICAL);
        divider.setDrawable(new ColorDrawable(Color.parseColor("#ffe0e0e0")));
        viewBinding.topRecyclerView.setLayoutManager(new LinearLayoutManager(this));
        viewBinding.topRecyclerView.addItemDecoration(divider);
        topAdapter = new RecyclerAdapter(this, topDatas);
        viewBinding.topRecyclerView.setAdapter(topAdapter);

        //initTabs with ViewPager
        pagerAdapter = new TabFragmentAdapter(getSupportFragmentManager(), tabTitles);
        viewBinding.viewPager.setAdapter(pagerAdapter);
        viewBinding.tabView.setupWithViewPager(viewBinding.viewPager);
        viewBinding.tabView.setTabMode(TabLayout.MODE_FIXED);
    }
}

可以看到ViewPager没有正常显示出来,这个时候可以重写ViewPager的onMeasure,重新测量ViewPager的宽高。也可以换用ViewPager2

public class CustomViewPager extends ViewPager {
  	...
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        //重写ViewPager的onMeasure
        int width = 0;
        int height = 0;
        for(int i=0; i<getChildCount(); i++) {
            View childView = getChildAt(0);
            measureChild(childView, widthMeasureSpec, heightMeasureSpec);
            width = Math.max(width, childView.getMeasuredWidth());
            height = Math.max(height, childView.getMeasuredHeight());
        }

        height += getPaddingTop() + getPaddingBottom();
        heightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }
}

从上面的效果图可以看到,ViewPager能正常显示出来了,但是在RecyclerView上滑动的时候发现,RecyclerView滑动完了之后,ScrollView才会滑动,并且ScrollView只滑动了一小段距离,这是因为首先ScrollView是不支持嵌套滑动的

ScrollView内部的第一个子View中所有子View的高度 = 顶部的RecyclerView高度 + TabLayout高度 + 底部RecyclerView中所有可见Item的高度

这个高度只比ScrollView的高度大一点点导致的。为了实现嵌套滑动需要使用NestedScrollView,接下来把ScrollView替换成NestedScrollView:

整个页面可以滑完,看起来就像是两个Scroll被合并成一个了,如果单单只是实现上面的界面效果,我们完全可以使用一个RecyclerView即可,但是Tab没有吸顶,这是因为:

ScrollView内部的第一个子View中所有子View的高度 = 顶部的RecyclerView高度 + TabLayout高度 + 底部RecyclerView中所有Item的高度

要实现Tab吸顶,只需要重写NestedScrollView的onMeasue方法,将TabLayout的高度和ViewPager的高度之和设置为NestedScrollView的高度:

public class StickyScrollLayout extends NestedScrollView {
    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        int count = getChildCount();
        if(count == 1) {
            View firstChild = getChildAt(0);
            if(firstChild != null && firstChild instanceof ViewGroup) {
                int childCount = ((ViewGroup) firstChild).getChildCount();
                if(childCount > 1) {
                    topView = ((ViewGroup) firstChild).getChildAt(0);
                    contentView = ((ViewGroup) firstChild).getChildAt(1);
                }
            }
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        if(contentView != null) {
            ViewGroup.LayoutParams contentLayoutParams = contentView.getLayoutParams();
            contentLayoutParams.height = getMeasuredHeight();
            contentView.setLayoutParams(contentLayoutParams);
        }
    }
}

此时TabLayout可以吸顶了

三.处理嵌套滑动

从上图中可以看出,当我们在RecyclerView上向上滑动时,需要等RecyclerView滑动完,外部的NestedScrollView才开始滑动,而我们希望NestedScrollView中顶部的RecyclerView滑完之后,底部的RecyclerView才开始滑动,这是为什么呢?

查看NestedScrollView和RecyclerView的源码,可以知道NestedScrollView和RecyclerView分别实现了NestedScrollingParent3,NestedScrollingChild3接口,分别用来表示嵌套滑动的父View、嵌套滑动的子View,当我们的手指在RecyclerView上滑动时,滑动事件会从上往下分发至RecyclerView的onTouchEvent中,RecyclerView会依次响应ACTION_DOWN、ACTION_MOVE、ACTION_UP

RecyclerView在处理ACTION_DOWN时的关键代码如下:

public boolean onTouchEvent(MotionEvent e) {
  switch (action) {
    case MotionEvent.ACTION_DOWN: {
      if (canScrollHorizontally) {
        nestedScrollAxis |= ViewCompat.SCROLL_AXIS_HORIZONTAL;
      }
      if (canScrollVertically) {
        nestedScrollAxis |= ViewCompat.SCROLL_AXIS_VERTICAL;
      }
      startNestedScroll(nestedScrollAxis, TYPE_TOUCH);
    } break;
  }
  return true;
}

当手指按下屏幕时会调用其作为NestedScrollingChild的实现方法startNestedScroll,在startNestedScroll的具体实现中,会一级一级的往上查找是否有NestedScrollingParent,如果有,会调用NestedScrollingParent的onStartNestedScroll方法通知它我即将要开始滑动了,然后NestedScrollingParent会调用onNestedScrollAccepted继续传递给上层的NestedScrollingParent,此处的NestedScrollingParent整好由NestedScrollView来充当,而NestedScrollView的上层已经找不到NestedScrollingParent了,时间传给NestedScrollView之后就中断了。

紧接着处理一系列的ACTION_MOVE:

public boolean onTouchEvent(MotionEvent e) {
  switch (action) {
    case MotionEvent.ACTION_MOVE: {
      if (dispatchNestedPreScroll(
        canScrollHorizontally ? dx : 0,
        canScrollVertically ? dy : 0,
        mReusableIntPair, mScrollOffset, TYPE_TOUCH
      )) {
        dx -= mReusableIntPair[0];
        dy -= mReusableIntPair[1];
        // Updated the nested offsets
        mNestedOffsets[0] += mScrollOffset[0];
        mNestedOffsets[1] += mScrollOffset[1];
        // Scroll has initiated, prevent parents from intercepting
        getParent().requestDisallowInterceptTouchEvent(true);
      }
      
      if (scrollByInternal(
        canScrollHorizontally ? dx : 0,
        canScrollVertically ? dy : 0,
        e)) {
        getParent().requestDisallowInterceptTouchEvent(true);
      }
    } break;
  }
  return true;
}

RecyclerView接收到ACTION_MOVE后,首先会调用其作为NestedScrollingChild的实现方法dispatchNestedPreScroll,在dispatchNestedPreScroll的具体实现中,会一级一级的往上查找是否有NestedScrollingParent,如果有,会调用NestedScrollingParent的dispatchNestedPreScroll,紧接着调用NestedScrollView的onNestedPreScroll,来告诉NestedScrollView我即将要滑动 xxx 距离,你需不需要滑动,在NestedScrollView的onNestedPreScroll方法中并不会去响应滑动,又会把自己作为一个NestedScrollingChild,把事件继续往上传递,而在NestedScrollView的上层已经没有可以处理嵌套滑动的NestedScrollingParent了

@Override
public void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed,
int type) {
	dispatchNestedPreScroll(dx, dy, consumed, null, type);
}

具体的事件传递流程如下图:

因此我们可以重写NestedScrollView的onNestedPreScroll方法来使NestedScrollView滑动

public class StickyNestedScrollLayout extends NestedScrollView {
  
  	@Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        int count = getChildCount();
        if(count == 1) {
            View firstChild = getChildAt(0);
            if(firstChild != null && firstChild instanceof ViewGroup) {
                int childCount = ((ViewGroup) firstChild).getChildCount();
                if(childCount > 1) {
                    topView = ((ViewGroup) firstChild).getChildAt(0);
                    contentView = ((ViewGroup) firstChild).getChildAt(1);
                }
            }
        }
    }
  
    @Override
    public void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed, int type) {
        boolean topIsShow = getScrollY() >=0 && getScrollY() < topView.getHeight();
        if(topIsShow) {
            scrollBy(0, dy);
        } else {
          super.onNestedPreScroll(target, dx, dy, consumed, type);
        }
    }
}

此时NestedScrollView能滑动了,但是NestedScrollView滑动的同时,RecyclerView也会跟着滑动,这是为什么呢?

在RecyclerView的dispatchNestedPreScroll方法具体实现中,有这样一段代码

public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed,
            @Nullable int[] offsetInWindow, @NestedScrollType int type) {
  if (isNestedScrollingEnabled()) {
      consumed[0] = 0;
      consumed[1] = 0;
      ViewParentCompat.onNestedPreScroll(parent, mView, dx, dy, consumed, type);
    	//consumed[0]、consumed[1]的值仍为0
      return consumed[0] != 0 || consumed[1] != 0;//返回false
    }
  }
  return false;
}

再结合RecyclerView的ACTION_MOVE来看:

public boolean onTouchEvent(MotionEvent e) {
  switch (action) {
    case MotionEvent.ACTION_MOVE: {
      if (dispatchNestedPreScroll(
        canScrollHorizontally ? dx : 0,
        canScrollVertically ? dy : 0,
        mReusableIntPair, mScrollOffset, TYPE_TOUCH
      )) {
        //dispatchNestedPreScroll返回了false,此处的if语句不会执行,因此RecyclerView也会滑动
        dx -= mReusableIntPair[0];
        dy -= mReusableIntPair[1];
        // Updated the nested offsets
        mNestedOffsets[0] += mScrollOffset[0];
        mNestedOffsets[1] += mScrollOffset[1];
        // Scroll has initiated, prevent parents from intercepting
        getParent().requestDisallowInterceptTouchEvent(true);
      }
      
      if (scrollByInternal(
        canScrollHorizontally ? dx : 0,
        canScrollVertically ? dy : 0,
        e)) {
        getParent().requestDisallowInterceptTouchEvent(true);
      }
    } break;
  }
  return true;
}

因此,我们,在NestedScrollView的onNestedPreScroll方法中,处理完滑动后,通过consumed告诉RecyclerView我滑动了多少,这样

RecyclerView会重新设置dx、dy的值,因此RecyclerView就不会跟着滑动了

public class StickyNestedScrollLayout extends NestedScrollView {
    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        int count = getChildCount();
        if(count == 1) {
            View firstChild = getChildAt(0);
            if(firstChild != null && firstChild instanceof ViewGroup) {
                int childCount = ((ViewGroup) firstChild).getChildCount();
                if(childCount > 1) {
                    topView = ((ViewGroup) firstChild).getChildAt(0);
                    contentView = ((ViewGroup) firstChild).getChildAt(1);
                }
            }
        }
    }

    @Override
    public void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed, int type) {
        boolean topIsShow = getScrollY() >=0 && getScrollY() < topView.getHeight();
        if(topIsShow) {
            scrollBy(0, dy);
            //告诉RecyclerView,我滑动了多少距离
            consumed[1] = dy;
        } else {
            super.onNestedPreScroll(target, dx, dy, consumed, type);
        }
    }
}

四.实现惯性滑动

实现思路:

记录父控件惯性滑动的速度判断NestedScrollView是否滚动到底部,若滚动到底部,判断子控件是否需要继续滚动滚动将惯性滑动的速度转化成距离,计算子控件应滑的距离 = 惯性距离 - 父控件已滑动距离,并将子控件应滑的距离转化成速度交给子控件进行惯性滑动

1.记录父控件惯性滑动的速度

public void fling(int velocityY) {
  super.fling(velocityY);
  if (velocityY <= 0) {
  	mVelocityY = 0;
  } else {
  	mVelocityY = velocityY;
  }
}

2.判断NestedScrollView是否滚动到底部,若滚动到底部,判断子控件是否需要继续滚动

@Override
protected void onScrollChanged(int scrollX, int scrollY, int oldScrollX, int oldScrollY) {
  super.onScrollChanged(scrollX, scrollY, oldScrollX, oldScrollY);
  /*
         * scrollY == 0 即还未滚动
         * scrollY == getChildAt(0).getMeasuredHeight() - v.getMeasuredHeight()即滚动到底部了
         */
  //判断NestedScrollView是否滚动到底部,若滚动到底部,判断子控件是否需要继续滚动
  if (scrollY == getChildAt(0).getMeasuredHeight() - this.getMeasuredHeight()) {
    dispatchChildFling();
  }
  //累计自身滚动的距离
  mConsumedY += scrollY - oldScrollY;
}

3.将惯性滑动的速度转化成距离,计算子控件应滑的距离 = 惯性距离 - 父控件已滑动距离,并将子控件应滑的距离转化成速度交给子控件进行惯性滑动

private void dispatchChildFling() {
    if(mFlingHelper == null) {
      mFlingHelper = new FlingHelper(getContext());
    }
    if (mVelocityY != 0) {
        //将惯性滑动速度转化成距离
        double distance = mFlingHelper.getSplineFlingDistance(mVelocityY);
        //计算子控件应该滑动的距离 = 惯性滑动距离 - 已滑距离
        if (distance > mConsumedY) {
            RecyclerView recyclerView = getChildRecyclerView(mContentView);
            if (recyclerView != null) {
                //将剩余滑动距离转化成速度交给子控件进行惯性滑动
                int velocityY = mFlingHelper.getVelocityByDistance(distance - mConsumedY);
                recyclerView.fling(0, velocityY);
            }
        }
    }

    mConsumedY = 0;
    mVelocityY = 0;
}

//递归获取子控件RecyclerView
private RecyclerView getChildRecyclerView(ViewGroup viewGroup) {
  for (int i = 0; i < viewGroup.getChildCount(); i++) {
    View view = viewGroup.getChildAt(i);
    if (view instanceof RecyclerView && Objects.requireNonNull(((RecyclerView) view).getLayoutManager()).canScrollVertically()) {
      return (RecyclerView) view;
    } else if (viewGroup.getChildAt(i) instanceof ViewGroup) {
      RecyclerView childRecyclerView = getChildRecyclerView((ViewGroup) viewGroup.getChildAt(i));
      if (childRecyclerView != null && Objects.requireNonNull((childRecyclerView).getLayoutManager()).canScrollVertically()) {
        return childRecyclerView;
      }
    }
  }
  return null;
}

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