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一、问题描述
使用NestedScrollView嵌套RecyclerView导致RecyclerView复用失效，RecyclerView会将所有数据一次性全部加载。
布局文件如下：
<androidx.core.widget.NestedScrollView xmlns:android="http://schemas.android.com/apk/res/android"xmlns:app="http://schemas.android.com/apk/res-auto"android:layout_width="match_parent"android:layout_height="match_parent"><androidx.recyclerview.widget.RecyclerViewandroid:id="@+id/recycler_view"android:layout_width="match_parent"android:layout_height="match_parent"android:layout_below="@id/tool_bar" />
</androidx.core.widget.NestedScrollView>
Adapter的onBindViewHolder打印日志代码如下：
public void onBindViewHolder(final ViewHolder holder, final int position) {Log.d("tag", "onBindViewHolder>>" + holder.itemView.toString());}
日志如下
onBindViewHolder>>android.widget.LinearLayout{5853c0e
onBindViewHolder>>android.widget.LinearLayout{a7e243c
onBindViewHolder>>android.widget.LinearLayout{95d0b1a
onBindViewHolder>>android.widget.LinearLayout{e274828
onBindViewHolder>>android.widget.LinearLayout{cb0bee6
onBindViewHolder>>android.widget.LinearLayout{4bcbed4
onBindViewHolder>>android.widget.LinearLayout{a39e372
onBindViewHolder>>android.widget.LinearLayout{a90f440
onBindViewHolder>>android.widget.LinearLayout{cbec4be
onBindViewHolder>>android.widget.LinearLayout{cb1146c
onBindViewHolder>>android.widget.LinearLayout{31e6eca
onBindViewHolder>>android.widget.LinearLayout{9d10b58
onBindViewHolder>>android.widget.LinearLayout{91cad96
onBindViewHolder>>android.widget.LinearLayout{5f78504
onBindViewHolder>>android.widget.LinearLayout{ee0d22
onBindViewHolder>>android.widget.LinearLayout{ce9ed70
onBindViewHolder>>android.widget.LinearLayout{983d96e
onBindViewHolder>>android.widget.LinearLayout{f58709c
onBindViewHolder>>android.widget.LinearLayout{d981e7a
onBindViewHolder>>android.widget.LinearLayout{6c9fa88
我们在Adapter中加载20条数据，RecylerView就所有数据一次性显示完了。这样在数据量小不会有问题，数据量大时就会造成卡顿或者OOM。
二、原因分析
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {super.onMeasure(widthMeasureSpec, heightMeasureSpec);...}
NestedScrollView的onMeasure方法如上。会调用父类的onMeasure。NestedScrollView是继承FrameLayout，因此会调用FrameLayout的onMeasure。FrameLayout的onMeasure代码如下：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {int count = getChildCount();for (int i = 0; i < count; i++) {final View child = getChildAt(i);if (mMeasureAllChildren || child.getVisibility() != GONE) {measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);}} }
FrameLayout的onMeasure会循环调用measureChildWithMargins测量子View。
因为NestedScrollView重写了measureChildWithMargins，因此我们应该看NestedScrollView的measureChildWithMargins：
protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed,int parentHeightMeasureSpec, int heightUsed) {final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,getPaddingLeft() + getPaddingRight() + lp.leftMargin + lp.rightMargin+ widthUsed, lp.width);final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(lp.topMargin + lp.bottomMargin, MeasureSpec.UNSPECIFIED);child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}
这里将高度的测量模式指定为 MeasureSpec.UNSPECIFIED。一般情况topMargin和bottomMargin指定为0，因此高度的测量值是0。
RecyclerView的measure和layout最终都交给LayoutManager完成。上面例子使用的LayoutManager是LinearLayoutManager,RecyclerView的measure和layout最终会执行到LinearLayoutManager的fill方法。
int fill(RecyclerView.Recycler recycler, LayoutState layoutState,RecyclerView.State state, boolean stopOnFocusable) {...while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {layoutChunkResult.resetInternal();if (RecyclerView.VERBOSE_TRACING) {TraceCompat.beginSection("LLM LayoutChunk");}layoutChunk(recycler, state, layoutState, layoutChunkResult);...return start - layoutState.mAvailable;}
fill方法在while循环中设置子item的布局。layoutState.hasMore(state)是当还有item就为true，layoutState.mInfinite的赋值如下：
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {...mLayoutState.mInfinite = resolveIsInfinite();...fill(recycler, mLayoutState, state, false);...
}
resolveIsInfinite源码如下：
boolean resolveIsInfinite() {return mOrientationHelper.getMode() == View.MeasureSpec.UNSPECIFIED&& mOrientationHelper.getEnd() == 0;}
getMode和getEnd的源码如下：
public static OrientationHelper createVerticalHelper(RecyclerView.LayoutManager layoutManager) {return new OrientationHelper(layoutManager) {...@Overridepublic int getEnd() {return mLayoutManager.getHeight();}@Overridepublic int getMode() {return mLayoutManager.getHeightMode();}...};}
实际调用mLayoutManager的方法。
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2 {public abstract static class LayoutManager {...@Pxpublic int getHeight() {return mHeight;}public int getHeightMode() {return mHeightMode;}...}
}
在RecyclerView的onMeasure会调用setMeasureSpecs：
protected void onMeasure(int widthSpec, int heightSpec) {...mLayout.setMeasureSpecs(widthSpec, heightSpec);...
}
setMeasureSpecs源码如下：
void setMeasureSpecs(int wSpec, int hSpec) {mWidth = MeasureSpec.getSize(wSpec);mWidthMode = MeasureSpec.getMode(wSpec);if (mWidthMode == MeasureSpec.UNSPECIFIED && !ALLOW_SIZE_IN_UNSPECIFIED_SPEC) {mWidth = 0;}mHeight = MeasureSpec.getSize(hSpec);mHeightMode = MeasureSpec.getMode(hSpec);if (mHeightMode == MeasureSpec.UNSPECIFIED && !ALLOW_SIZE_IN_UNSPECIFIED_SPEC) {mHeight = 0;}}
在这里会初始化mHeight和mHeightMode，即使用父控件NestedScrollView传入的测量高度进行赋值，因此上面例子得到的mHeight =0，mHeightMode =MeasureSpec.UNSPECIFIED。所以上面的mLayoutState.mInfinite会返回true，在fill中加载子item时就会一直加载所有的item。
三、如何解决
要解决上面问题，只需要NestedScrollView测量RecyclerView不使用MeasureSpec.UNSPECIFIED模式即可。因此可以重写measureChildWithMargins。
@Overrideprotected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) {child.measure(parentWidthMeasureSpec, parentHeightMeasureSpec);}
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