在偶然的一次调试中,发现了RecyclerView的onCreateViewHolder和onBindViewHolder发生了多次调用:
而我的布局很简单:
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
android:id="@+id/first_column"
android:layout_width="100dp"
android:layout_height="match_parent"
android:text="first column"/>
android:id="@+id/second_column"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="1"
android:orientation="vertical">
android:id="@+id/recyclerview"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
```
```
public class MyActivityextends Activity {
private RecyclerViewmRecyclerView;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.my_layout);
initView();
setRecyclerView();
}
private void initView() {
mRecyclerView = (RecyclerView) findViewById(R.id.recyclerview);
}
private void setRecyclerView() {
mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
mRecyclerView.setAdapter(new RecyclerView.Adapter() {
@Override
public RecyclerView.ViewHolderonCreateViewHolder(ViewGroup parent, int viewType) {
Log.d("NQG", "onCreateViewHolder: ");
TextView textView =new TextView(MyActivity.this);
textView.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, 200));
return new RecyclerView.ViewHolder(textView) {};
}
@Override
public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
Log.d("NQG", "onBindViewHolder: " + position);
((TextView) holder.itemView).setText("" + position);
}
@Override
public int getItemCount() {
return 20;
}
});
}
}
```
这就很奇怪了,于是便从RecycleView开始下手,从onCreateViewHolder回溯,发现调用的地方在RecycleView.Recycler#tryGetViewHolderForPositionByDeadline():
```
ViewHoldertryGetViewHolderForPositionByDeadline(int position,
boolean dryRun, long deadlineNs) {
ViewHolder holder =null;
// 省略从缓存中取ViewHolder的相关代码
if (holder ==null) {
long start = getNanoTime();
if (deadlineNs !=FOREVER_NS
&& !mRecyclerPool.willCreateInTime(type, start, deadlineNs)) {
// abort - we have a deadline we can't meet
return null;
}
holder =mAdapter.createViewHolder(RecyclerView.this, type);
if (ALLOW_THREAD_GAP_WORK) {
// only bother finding nested RV if prefetching
RecyclerView innerView =findNestedRecyclerView(holder.itemView);
if (innerView !=null) {
holder.mNestedRecyclerView =new WeakReference<>(innerView);
}
}
long end = getNanoTime();
mRecyclerPool.factorInCreateTime(type, end - start);
if (DEBUG) {
Log.d(TAG, "tryGetViewHolderForPositionByDeadline created new ViewHolder");
}
}
// 省略部分代码
if (mState.isPreLayout() && holder.isBound()) {
// do not update unless we absolutely have to.
holder.mPreLayoutPosition = position;
}else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {
if (DEBUG && holder.isRemoved()) {
throw new IllegalStateException("Removed holder should be bound and it should"
+" come here only in pre-layout. Holder: " + holder
+ exceptionLabel());
}
final int offsetPosition =mAdapterHelper.findPositionOffset(position);
bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
}
```
注意到在第一次布局的时候,ViewHolder没有成功从RecycleView的缓存中取到过一次,每次都是new出来的,RecycleView的缓存失效?不应该的,
注意到在此时,onBindViewHolder的pos参数,显示每个item都执行了bind操作,猜想可能在RecycleView layout的时候,将所有item都进行了layout操作,虽然某些
view是无法显示下的,再回溯注意到tryGetViewHolderForPositionByDeadline是由LinearLayoutManager#next方法调用的,而next是在LinearLayoutManager#layoutChunk中调用的:
```
void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
LayoutState layoutState, LayoutChunkResult result) {
View view = layoutState.next(recycler);
if (view ==null) {
if (DEBUG && layoutState.mScrapList ==null) {
throw new RuntimeException("received null view when unexpected");
}
// if we are laying out views in scrap, this may return null which means there is
// no more items to layout.
result.mFinished =true;
return;
}
LayoutParams params = (LayoutParams) view.getLayoutParams();
if (layoutState.mScrapList ==null) {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addView(view);
}else {
addView(view, 0);
}
}
// 省略之后代码
```
很明显layoutChunk方法会根据一些参数,将item add到RecycleView中,而layoutChunk是由LinearLayoutManager#fill方法调用的:
```
int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
RecyclerView.State state, boolean stopOnFocusable) {
// max offset we should set is mFastScroll + available
final int start = layoutState.mAvailable;
if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
// TODO ugly bug fix. should not happen
if (layoutState.mAvailable <0) {
layoutState.mScrollingOffset += layoutState.mAvailable;
}
recycleByLayoutState(recycler, layoutState);
}
int remainingSpace = layoutState.mAvailable + layoutState.mExtra;
LayoutChunkResult layoutChunkResult =mLayoutChunkResult;
while ((layoutState.mInfinite || remainingSpace >0) && layoutState.hasMore(state)) {
layoutChunkResult.resetInternal();
if (VERBOSE_TRACING) {
TraceCompat.beginSection("LLM LayoutChunk");
}
layoutChunk(recycler, state, layoutState, layoutChunkResult);
if (VERBOSE_TRACING) {
TraceCompat.endSection();
}
if (layoutChunkResult.mFinished) {
break;
}
layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;
/**
* Consume the available space if:
* * layoutChunk did not request to be ignored
* * OR we are laying out scrap children
* * OR we are not doing pre-layout
*/
if (!layoutChunkResult.mIgnoreConsumed ||mLayoutState.mScrapList !=null
|| !state.isPreLayout()) {
layoutState.mAvailable -= layoutChunkResult.mConsumed;
// we keep a separate remaining space because mAvailable is important for recycling
remainingSpace -= layoutChunkResult.mConsumed;
}
if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
layoutState.mScrollingOffset += layoutChunkResult.mConsumed;
if (layoutState.mAvailable <0) {
layoutState.mScrollingOffset += layoutState.mAvailable;
}
recycleByLayoutState(recycler, layoutState);
}
if (stopOnFocusable && layoutChunkResult.mFocusable) {
break;
}
}
if (DEBUG) {
validateChildOrder();
}
return start - layoutState.mAvailable;
}
```
其中while执行条件为:
1.layoutState.mInfinite为true或者RecycleView剩余的空间大于0
2.layoutState.hasMore(state)为true
先看第二个条件,layoutState.hasMore(state)代码如下:
```
/**
* @return true if there are more items in the data adapter
*/
boolean hasMore(RecyclerView.State state) {
return mCurrentPosition >=0 &&mCurrentPosition < state.getItemCount();
}
```
很明显,判断是否到了最后一个item.
DeBug到此处:
注意while循环中的layoutState.mInfinite变量为true,这会造成将所有item view都add到RecycleView中,这就解释了为何调用
onCreateViewHolder和onBindViewHolder的次数与Adapter#getItemCount()值一致了.
那么接下来,问题来了,为何layoutState.mInfinite值为true呢?注意到LinearLayoutManager#fill方法由LinearLayoutManager#onLayoutChildren调用,其中有如下代码:
mLayoutState.mInfinite = resolveIsInfinite();
boolean resolveIsInfinite() {
return mOrientationHelper.getMode() == View.MeasureSpec.UNSPECIFIED
&&mOrientationHelper.getEnd() ==0;
}
最终是根据RecycleView.LayoutManager#mHeightMode == View.MeasureSpec.UNSPECIFIED判断的,而mHeightMode赋值的地方一处在设置LayoutManager的时候
,另一处在调用RecycleView.LayoutManager#setMeasureSpecs的地方,在RecycleView#onMeasure中有如下代码:
很明显,传入onMeasure的heightSpec为0.
可能会奇怪了,为啥最终显示的效果是正确的呢?这是因为进行了多次测量后,skipMeasure变量值为true,便不会走RecycleView.LayoutManager#setMeasureSpecs流程,
也不会再走onLayoutChildren等接下来的流程了.
再分析为何传入onMeasure的heightSpec为0,在布局中RecycleView的父布局为LinearLayout,recyclerview的heightSpec由父布局measure时传入,
在LinearLayout#measureHorizontal中有如下代码:
此时child就是RecycleView的父布局,注意到heightMeasureSpec,明显是有值的,但为何得到的值为0呢?那就要看View#makeSafeMeasureSpec方法了:
/**
* Like {@link #makeMeasureSpec(int, int)}, but any spec with a mode of UNSPECIFIED
* will automatically get a size of 0. Older apps expect this.
*
* @hide internal use only for compatibility with system widgets and older apps
*/
public static int makeSafeMeasureSpec(int size, int mode) {
if (sUseZeroUnspecifiedMeasureSpec && mode ==UNSPECIFIED) {
return 0;
}
return makeMeasureSpec(size, mode);
}
很明显View#sUseZeroUnspecifiedMeasureSpec变量为true了,返回0了,注意到sUseZeroUnspecifiedMeasureSpec赋值的地方:
// In M and newer, our widgets can pass a "hint" value in the size
// for UNSPECIFIED MeasureSpecs. This lets child views of scrolling containers
// know what the expected parent size is going to be, so e.g. list items can size
// themselves at 1/3 the size of their container. It breaks older apps though,
// specifically apps that use some popular open source libraries.
sUseZeroUnspecifiedMeasureSpec = targetSdkVersion < Build.VERSION_CODES.M;
噢,原来只要targetSdkVersion小于AndroidM就会为true,参考工程中AndroidManifest.xml:
uses-sdk android:minSdkVersion="17" android:targetSdkVersion="19"
的确为true,这下问题的来龙去脉就理清了.
解决这个问题,我目前想到了两个办法:
1).更改App的targetSdkVersion为M及以上(不适合我对应的场景)
2).将其LinearLayout父布局改为match_parent/match_parent(PS:RecycleView的直接或者间接LinearLayout父布局均不能带weight)