在用recyclerview的时候，发现其onCreateViewHolder方法，既不是调用了一次，也不是调用了item个数的次数。因而想看看，RecyclerView的ViewHolder复用。
网上找到了一篇好的文章。
文章转自：http://www.jianshu.com/p/d7ec36aa8e4b
在复写RecyclerView.Adapter的时候，需要我们复写两个方法：
这两个方法从字面上看就是创建ViewHolder和绑定ViewHolder的意思，来看一下源码中对我们实现的这两个方法的注释：
/** * Called when RecyclerView needs a new {@link ViewHolder} of the given type to represent * an item. * * This new ViewHolder should be constructed with a new View that can represent the items * of the given type. You can either create a new View manually or inflate it from an XML * layout file. *
* The new ViewHolder will be used to display items of the adapter using * {@link #onBindViewHolder(ViewHolder, int, List)}. Since it will be re-used to display * different items in the data set, it is a good idea to cache references to sub views of * the View to avoid unnecessary {@link View#findViewById(int)} calls. * * @param parent The ViewGroup into which the new View will be added after it is bound to * an adapter position. * @param viewType The view type of the new View. * * @return A new ViewHolder that holds a View of the given view type. * @see #getItemViewType(int) * @see #onBindViewHolder(ViewHolder, int) */
当RecyclerView需要一个新的类型的item的ViewHolder的时候调用这个方法。
第二段描述是讲如何创建这个ViewHolder，跳过。
新的ViewHolder将会被用来通过adapter调用onBindViewHolder展示item。由于它将会被复用去展示在数据集中的不同items，所以缓存View的子view引用去避免不必要的对findViewById方法的调用是一个好主意。
看了上面的这段话，我产生了一个疑问，第一段话的意思仿佛是只有在需要新的类型的ViewHolder的时候才需要调用这个方法。如果是这样，的确可以从侧面说明他是以ViewHolder为单位来实现复用的。为了验证我的想法，我在onCreateViewHolder和onBindViewHolder方法中加入了计数的代码，看一下log：
log
从中可以看出并不是像我想的那样，只调用了一次，稍微想一下也很容易想明白，因为他是通过ViewHolder复用不假，我这里只有一种ViewType，上下滑动的时候需要的ViewHolder种类是只有一种，但是需要的ViewHolder对象数量并不止一个。所以在后面创建了5个ViewHolder之后，需要的数量够了，无论我怎么滑动，他都只需要复用以前创建的对象就行了。
在这里，感觉ViewHolder的类型和对象数量有点像Java中Class和对象的关系。Java中第一次将.class装载入JVM虚拟机的时候，会生成一个Class对象，以后所有这个类的对象都由Class生成。是不是有点像呢？
看到了这个log之后，我的第一反应是在这个ViewHolder对象的数量“够用”之后就停止调用onCreateViewHolder方法，看一下源码：
/** * This method calls {@link #onCreateViewHolder(ViewGroup, int)} to create a new * {@link ViewHolder} and initializes some private fields to be used by RecyclerView. * * @see #onCreateViewHolder(ViewGroup, int) */
public final VH createViewHolder(ViewGroup parent, int viewType) {
TraceCompat.beginSection(TRACE_CREATE_VIEW_TAG);
final VH holder = onCreateViewHolder(parent, viewType);
holder.mItemViewType = viewType;
TraceCompat.endSection();
return holder;
}可以发现这里并没有限制，那么是不是在调用这个createViewHolder方法的时候做了限制呢？
View getViewForPosition(int position, boolean dryRun) {
if (position <0 || position >= mState.getItemCount()) {
throw new IndexOutOfBoundsException("Invalid item position " + position
+ "(" + position + "). Item count:" + mState.getItemCount());
}
boolean fromScrap = false;
ViewHolder holder = null;
// 0) If there is a changed scrap, try to find from there
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrap = holder != null;
}
// 1) Find from scrap by position
if (holder == null) {
holder = getScrapViewForPosition(position, INVALID_TYPE, dryRun);
if (holder != null) {
if (!validateViewHolderForOffsetPosition(holder)) {
// recycle this scrap
if (!dryRun) {
// we would like to recycle this but need to make sure it is not used by
// animation logic etc.
holder.addFlags(ViewHolder.FLAG_INVALID);
if (holder.isScrap()) {
removeDetachedView(holder.itemView, false);
holder.unScrap();
} else if (holder.wasReturnedFromScrap()) {
holder.clearReturnedFromScrapFlag();
}
recycleViewHolderInternal(holder);
}
holder = null;
} else {
fromScrap = true;
}
}
}
if (holder == null) {
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
if (offsetPosition <0 || offsetPosition >= mAdapter.getItemCount()) {
throw new IndexOutOfBoundsException("Inconsistency detected. Invalid item "
+ "position " + position + "(offset:" + offsetPosition + ")."
+ "state:" + mState.getItemCount());
}
final int type = mAdapter.getItemViewType(offsetPosition);
// 2) Find from scrap via stable ids, if exists
if (mAdapter.hasStableIds()) {
holder = getScrapViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);
if (holder != null) {
// update position
holder.mPosition = offsetPosition;
fromScrap = true;
}
}
if (holder == null && mViewCacheExtension != null) {
// We are NOT sending the offsetPosition because LayoutManager does not
// know it.
final View view = mViewCacheExtension
.getViewForPositionAndType(this, position, type);
if (view != null) {
holder = getChildViewHolder(view);
if (holder == null) {
throw new IllegalArgumentException("getViewForPositionAndType returned"
+ " a view which does not have a ViewHolder");
} else if (holder.shouldIgnore()) {
throw new IllegalArgumentException("getViewForPositionAndType returned"
+ " a view that is ignored. You must call stopIgnoring before"
+ " returning this view.");
}
}
}
if (holder == null) { // fallback to recycler
// try recycler.
// Head to the shared pool.
if (DEBUG) {
Log.d(TAG, "getViewForPosition(" + position + ") fetching from shared "
+ "pool");
}
holder = getRecycledViewPool().getRecycledView(type);
if (holder != null) {
holder.resetInternal();
if (FORCE_INVALIDATE_DISPLAY_LIST) {
invalidateDisplayListInt(holder);
}
}
}
if (holder == null) {
holder = mAdapter.createViewHolder(RecyclerView.this, type);
if (DEBUG) {
Log.d(TAG, "getViewForPosition created new ViewHolder");
}
}
}
// This is very ugly but the only place we can grab this information
// before the View is rebound and returned to the LayoutManager for post layout ops.
// We don't need this in pre-layout since the VH is not updated by the LM.
if (fromScrap && !mState.isPreLayout() && holder
.hasAnyOfTheFlags(ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST)) {
holder.setFlags(0, ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);
if (mState.mRunSimpleAnimations) {
int changeFlags = ItemAnimator
.buildAdapterChangeFlagsForAnimations(holder);
changeFlags |= ItemAnimator.FLAG_APPEARED_IN_PRE_LAYOUT;
final ItemHolderInfo info = mItemAnimator.recordPreLayoutInformation(mState,
holder, changeFlags, holder.getUnmodifiedPayloads());
recordAnimationInfoIfBouncedHiddenView(holder, info);
}
}
boolean bound = false;
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);
}
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
holder.mOwnerRecyclerView = RecyclerView.this;
mAdapter.bindViewHolder(holder, offsetPosition);
attachAccessibilityDelegate(holder.itemView);
bound = true;
if (mState.isPreLayout()) {
holder.mPreLayoutPosition = position;
}
}
final ViewGroup.LayoutParams lp = holder.itemView.getLayoutParams();
final LayoutParams rvLayoutParams;
if (lp == null) {
rvLayoutParams = (LayoutParams) generateDefaultLayoutParams();
holder.itemView.setLayoutParams(rvLayoutParams);
} else if (!checkLayoutParams(lp)) {
rvLayoutParams = (LayoutParams) generateLayoutParams(lp);
holder.itemView.setLayoutParams(rvLayoutParams);
} else {
rvLayoutParams = (LayoutParams) lp;
}
rvLayoutParams.mViewHolder = holder;
rvLayoutParams.mPendingInvalidate = fromScrap && bound;
return holder.itemView;
}可以看出的确是有条件的。当然，在此不具体分析，不然可能会深入细节无法自拔。
说实话，上面分析完，我也有点没方向，因为毕竟整个RecyclerView一万多行代码在那，不知道看哪了，不过好在网上有篇文曾干过和我差不多的事 RecyclerView源码分析 ，前人指了条路，跟着看一下源码好了。
Recycler：
/** * A Recycler is responsible for managing scrapped or detached item views for reuse. * * A "scrapped" view is a view that is still attached to its parent RecyclerView but * that has been marked for removal or reuse.
* * Typical use of a Recycler by a {@link LayoutManager} will be to obtain views for * an adapter's data set representing the data at a given position or item ID. * If the view to be reused is considered "dirty" the adapter will be asked to rebind it. * If not, the view can be quickly reused by the LayoutManager with no further work. * Clean views that have not {@link android.view.View#isLayoutRequested() requested layout} * may be repositioned by a LayoutManager without remeasurement.
*/Recycler负责管理废弃（scrapped）或者分离（detach）的item。
scrapped指的是仍然在RecyclerView上但是已经被标记了移除或者复用。
一个对Recycler的经典的使用时LayoutManager，它通过Recycler为adapter的数据集的特定位置获取一个view。如果这个view将被复用但被认为是“dirty”的，那么这个adapter将调用方法重新绑定它。如果不是，这个view可以迅速的被LayoutManager复用而不用进一步的处理。Clean view无需调用request layout，不需要重新测量就能复用。
RecycledViewPool：
/** * RecycledViewPool lets you share Views between multiple RecyclerViews. * * If you want to recycle views across RecyclerViews, create an instance of RecycledViewPool * and use {@link RecyclerView#setRecycledViewPool(RecycledViewPool)}. *
* RecyclerView automatically creates a pool for itself if you don't provide one. * */
public static class RecycledViewPool {
private SparseArray> mScrap =
new SparseArray>();
private SparseIntArray mMaxScrap = new SparseIntArray();
private int mAttachCount = 0;
private static final int DEFAULT_MAX_SCRAP = 5;
public void clear() {
mScrap.clear();
}
public void setMaxRecycledViews(int viewType, int max) {
mMaxScrap.put(viewType, max);
final ArrayList scrapHeap = mScrap.get(viewType);
if (scrapHeap != null) {
while (scrapHeap.size() > max) {
scrapHeap.remove(scrapHeap.size() - 1);
}
}
}
public ViewHolder getRecycledView(int viewType) {
final ArrayList scrapHeap = mScrap.get(viewType);
if (scrapHeap != null && !scrapHeap.isEmpty()) {
final int index = scrapHeap.size() - 1;
final ViewHolder scrap = scrapHeap.get(index);
scrapHeap.remove(index);
return scrap;
}
return null;
}
int size() {
int count = 0;
for (int i = 0; i viewHolders = mScrap.valueAt(i);
if (viewHolders != null) {
count += viewHolders.size();
}
}
return count;
}
public void putRecycledView(ViewHolder scrap) {
final int viewType = scrap.getItemViewType();
final ArrayList scrapHeap = getScrapHeapForType(viewType);
if (mMaxScrap.get(viewType) <= scrapHeap.size()) {
return;
}
if (DEBUG && scrapHeap.contains(scrap)) {
throw new IllegalArgumentException("this scrap item already exists");
}
scrap.resetInternal();
scrapHeap.add(scrap);
}
void attach(Adapter adapter) {
mAttachCount++;
}
void detach() {
mAttachCount--;
}
/** * Detaches the old adapter and attaches the new one. * * RecycledViewPool will clear its cache if it has only one adapter attached and the new * adapter uses a different ViewHolder than the oldAdapter. * * @param oldAdapter The previous adapter instance. Will be detached. * @param newAdapter The new adapter instance. Will be attached. * @param compatibleWithPrevious True if both oldAdapter and newAdapter are using the same * ViewHolder and view types. */
void onAdapterChanged(Adapter oldAdapter, Adapter newAdapter, boolean compatibleWithPrevious) {
if (oldAdapter != null) {
detach();
}
if (!compatibleWithPrevious && mAttachCount == 0) {
clear();
}
if (newAdapter != null) {
attach(newAdapter);
}
}
private ArrayList getScrapHeapForType(int viewType) {
ArrayList scrap = mScrap.get(viewType);
if (scrap == null) {
scrap = new ArrayList<>();
mScrap.put(viewType, scrap);
if (mMaxScrap.indexOfKey(viewType) <0) {
mMaxScrap.put(viewType, DEFAULT_MAX_SCRAP);
}
}
return scrap;
}
} 老规矩，先看注释：RecycledViewPool让你在多个RecyclerView之间共享View。如果你想要在RecyclerView间循环利用view，创建一个RecyclerViewPool的实例然后调用setRecyclerViewPool方法。
如果你不提供一个那么RecyclerView将为他自己自动的创建一个RecycledViewPool。
接下来看下里面的代码，内部有一个SparseArray一个SparseIntArray，看到这终于感觉快看到点子上了，毕竟看起来就像是两个放东西的容器，应该是离真相不远了。先看下SparseArray是个啥，点进去看注释第一句就是 SparseArrays map integers to Objects ，这是一个integer和对象的映射，他内部也是有两个数组一个是integer作为键的int[] mKeys的int类型的数组，另外一个是Object[] mValues的对象数组。而结合他在RecycledViewPool中的定义 SparseArray
而SparseIntArray则是Integer映射Integer，在这可以结合setMaxRecycledViews方法中的第一行代码 mMaxScrap.put(viewType,max)，可以看出这是表明了一种ViewType对应的可保存对象集合的最大尺寸。
大致了解了下RecycledViewPool，然后回头去看一下之前被我跳过的getViewForPosition：
View getViewForPosition(int position, boolean dryRun) {
if (position <0 || position >= mState.getItemCount()) {
throw new IndexOutOfBoundsException("Invalid item position " + position
+ "(" + position + "). Item count:" + mState.getItemCount());
}
boolean fromScrap = false;
ViewHolder holder = null;
// 0) If there is a changed scrap, try to find from there
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrap = holder != null;
}
// 1) Find from scrap by position
if (holder == null) {
holder = getScrapViewForPosition(position, INVALID_TYPE, dryRun);
if (holder != null) {
if (!validateViewHolderForOffsetPosition(holder)) {
// recycle this scrap
if (!dryRun) {
// we would like to recycle this but need to make sure it is not used by
// animation logic etc.
holder.addFlags(ViewHolder.FLAG_INVALID);
if (holder.isScrap()) {
removeDetachedView(holder.itemView, false);
holder.unScrap();
} else if (holder.wasReturnedFromScrap()) {
holder.clearReturnedFromScrapFlag();
}
recycleViewHolderInternal(holder);
}
holder = null;
} else {
fromScrap = true;
}
}
}
if (holder == null) {
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
if (offsetPosition <0 || offsetPosition >= mAdapter.getItemCount()) {
throw new IndexOutOfBoundsException("Inconsistency detected. Invalid item "
+ "position " + position + "(offset:" + offsetPosition + ")."
+ "state:" + mState.getItemCount());
}
final int type = mAdapter.getItemViewType(offsetPosition);
// 2) Find from scrap via stable ids, if exists
if (mAdapter.hasStableIds()) {
holder = getScrapViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);
if (holder != null) {
// update position
holder.mPosition = offsetPosition;
fromScrap = true;
}
}
if (holder == null && mViewCacheExtension != null) {
// We are NOT sending the offsetPosition because LayoutManager does not
// know it.
final View view = mViewCacheExtension
.getViewForPositionAndType(this, position, type);
if (view != null) {
holder = getChildViewHolder(view);
if (holder == null) {
throw new IllegalArgumentException("getViewForPositionAndType returned"
+ " a view which does not have a ViewHolder");
} else if (holder.shouldIgnore()) {
throw new IllegalArgumentException("getViewForPositionAndType returned"
+ " a view that is ignored. You must call stopIgnoring before"
+ " returning this view.");
}
}
}
if (holder == null) { // fallback to recycler
// try recycler.
// Head to the shared pool.
if (DEBUG) {
Log.d(TAG, "getViewForPosition(" + position + ") fetching from shared "
+ "pool");
}
holder = getRecycledViewPool().getRecycledView(type);
if (holder != null) {
holder.resetInternal();
if (FORCE_INVALIDATE_DISPLAY_LIST) {
invalidateDisplayListInt(holder);
}
}
}
if (holder == null) {
holder = mAdapter.createViewHolder(RecyclerView.this, type);
if (DEBUG) {
Log.d(TAG, "getViewForPosition created new ViewHolder");
}
}
}
// This is very ugly but the only place we can grab this information
// before the View is rebound and returned to the LayoutManager for post layout ops.
// We don't need this in pre-layout since the VH is not updated by the LM.
if (fromScrap && !mState.isPreLayout() && holder
.hasAnyOfTheFlags(ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST)) {
holder.setFlags(0, ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);
if (mState.mRunSimpleAnimations) {
int changeFlags = ItemAnimator
.buildAdapterChangeFlagsForAnimations(holder);
changeFlags |= ItemAnimator.FLAG_APPEARED_IN_PRE_LAYOUT;
final ItemHolderInfo info = mItemAnimator.recordPreLayoutInformation(mState,
holder, changeFlags, holder.getUnmodifiedPayloads());
recordAnimationInfoIfBouncedHiddenView(holder, info);
}
}
boolean bound = false;
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);
}
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
holder.mOwnerRecyclerView = RecyclerView.this;
mAdapter.bindViewHolder(holder, offsetPosition);
attachAccessibilityDelegate(holder.itemView);
bound = true;
if (mState.isPreLayout()) {
holder.mPreLayoutPosition = position;
}
}
final ViewGroup.LayoutParams lp = holder.itemView.getLayoutParams();
final LayoutParams rvLayoutParams;
if (lp == null) {
rvLayoutParams = (LayoutParams) generateDefaultLayoutParams();
holder.itemView.setLayoutParams(rvLayoutParams);
} else if (!checkLayoutParams(lp)) {
rvLayoutParams = (LayoutParams) generateLayoutParams(lp);
holder.itemView.setLayoutParams(rvLayoutParams);
} else {
rvLayoutParams = (LayoutParams) lp;
}
rvLayoutParams.mViewHolder = holder;
rvLayoutParams.mPendingInvalidate = fromScrap && bound;
return holder.itemView;
}在具体分析这个方法之前，先给出这个类内部几个参数的大致意思：
private ArrayList
private ArrayList
private ArrayList
private ViewCacheExtension mViewCacheExtension 开发者控制的ViewHolder缓存
private RecycledViewPool mRecyclerPool 提供复用ViewHolder池。
可以看到源码中已经给了我们步骤提示：
如此一来经历了以上的步骤，一个ViewHolder便会先从缓存中读取，如果都无法匹配到，则会新创建一个。如此便实现了ViewHolder的复用。
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