Android RecyclerView 源码浅析
OverView
RecyclerView是Android5.0推出的新组件，可以认为是更加灵活强大的ListView，在日常开发中基本上已经取代了ListView成为长列表控件的首选。
其继承结构如下：
image.png
从继承结构可以看到，RecyclerView是ViewGroup的直接子类，这里就是RecyclerView与ListView第一点大不同，ListView和ViewGroup之间隔了两层，其实个人认为在继承结构上揭露了RecyclerView相比ListView的优点：由于RecyclerView这么简单的继承结构，说明了其主要功能都是通过组合来实现的，而ListView是通过继承实现的。
用法
通常来说，在业务代码中使用RecyclerView，主要有以下几个步骤：
- 创建RecyclerView实例
- 为RecyclerView设置LayoutManager，LayoutManager是一个抽象类，常用的实现类有LinearLayoutManager
- 如有必要，为RecyclerView设置ItemAnimator
- 如有必要，为RecyclerView设置ItemDecoration
- 继承RecyclerView.Adapter<VH extends ViewHolder>，实现getItemCount()/onCreateViewHolder(ViewGroup parent, int viewType)/onBindViewHolder(VH holder, int position)
- 在RecyclerView.Adapter中使用到的VH是ViewHolder的实现类，但ViewHolder虽然是一个抽象类，其中却没有需要我们实现的抽象方法，ViewHolder中关键的成员是itemView，即其承载的视图，可见性为public，可以直接获取
源码浅析
这部分跟随RecyclerView使用到的各个类的方法，看下这些关键方法是在何时被调用的
Adapter
-
public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType)
在源码中查找onCreateViewHolder(ViewGroup parent, int viewType)的调用，能得到下图
image.png
内一层是外一层的调用，最终可以看到主要的调用在LinearLayoutManager的onLayoutChildren()；直接的调用方法是在RecyclerView的tryGetViewHolderForPositionByDeadline(int position, boolean dryRun, long deadlineNs)中有这么1段：
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");
}
}
简单来说，就是在layout()过程，如果holder为null，就调用这个方法来创建一个ViewHolder
- public void onBindViewHolder(RecyclerView.ViewHolder holder, int position)
首先在RecyclerView.Adapter的bindViewHolder(VH holder, int position)中有直接调用：
public final void bindViewHolder(VH holder, int position) {
holder.mPosition = position;
if (hasStableIds()) {
holder.mItemId = getItemId(position);
}
holder.setFlags(ViewHolder.FLAG_BOUND,
ViewHolder.FLAG_BOUND | ViewHolder.FLAG_UPDATE | ViewHolder.FLAG_INVALID
| ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN);
TraceCompat.beginSection(TRACE_BIND_VIEW_TAG);
onBindViewHolder(holder, position, holder.getUnmodifiedPayloads());
holder.clearPayload();
final ViewGroup.LayoutParams layoutParams = holder.itemView.getLayoutParams();
if (layoutParams instanceof RecyclerView.LayoutParams) {
((LayoutParams) layoutParams).mInsetsDirty = true;
}
TraceCompat.endSection();
}
调用完之后从其itemView中取出LayoutParams来进行处理
然后再源码中查找最终的调用链，结果如下：
image.png
调用链中关键的一环是在RecyclerView的View getViewForPosition(int position, boolean dryRun)中
View getViewForPosition(int position, boolean dryRun) {
return tryGetViewHolderForPositionByDeadline(position, dryRun, FOREVER_NS).itemView;
}
里面先调用了tryGetViewHolderForPositionByDeadline(position, dryRun, FOREVER_NS)方法，在前面分析onCreateViewHolder()的时候已经看到了，这个方法里尝试获取holder，取不到就创建holder，创建后就进行onBindViewHolder()的操作，bind完之后在这里可以看到，返回的是holder.itemView，因此在布局过程中，就是从这里获得了正确的itemView
-
int getItemCount()
这个方法的调用，在源码里看都是直接调用
image.png
-
int getItemViewType(int position)
这个方法在源码中的被调用位置主要是RecyclerView的tryGetViewHolderForPositionByDeadline()函数，这个函数在上面也已经出现过了
final int type = mAdapter.getItemViewType(offsetPosition);
// 2) Find from scrap/cache via stable ids, if exists
if (mAdapter.hasStableIds()) {
holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),
type, dryRun);
if (holder != null) {
// update position
holder.mPosition = offsetPosition;
fromScrapOrHiddenOrCache = true;
}
}
通过这个方法拿到的type，就是在onCreateViewHolder()/onBindViewHolder()函数中的入参type
LayoutManager
- public abstract LayoutParams generateDefaultLayoutParams();
查找源码，这个方法被调用的位置为RecyclerView的同名函数中：
@Override
protected ViewGroup.LayoutParams generateDefaultLayoutParams() {
if (mLayout == null) {
throw new IllegalStateException("RecyclerView has no LayoutManager");
}
return mLayout.generateDefaultLayoutParams();
}
- public void onLayoutChildren(Recycler recycler, State state)
这个方法本身并不是abstract函数，但在LayoutManager中必须重写：
public void onLayoutChildren(Recycler recycler, State state) {
Log.e(TAG, "You must override onLayoutChildren(Recycler recycler, State state) ");
}
在源码中查找其调用，可以看到被调用的地方主要有两处：
首先是RecyclerView的dispatchLayoutStep1()函数
private void dispatchLayoutStep1() {
mState.assertLayoutStep(State.STEP_START);
mState.mIsMeasuring = false;
eatRequestLayout();
mViewInfoStore.clear();
onEnterLayoutOrScroll();
processAdapterUpdatesAndSetAnimationFlags();
saveFocusInfo();
mState.mTrackOldChangeHolders = mState.mRunSimpleAnimations && mItemsChanged;
mItemsAddedOrRemoved = mItemsChanged = false;
mState.mInPreLayout = mState.mRunPredictiveAnimations;
mState.mItemCount = mAdapter.getItemCount();
findMinMaxChildLayoutPositions(mMinMaxLayoutPositions);
if (mState.mRunSimpleAnimations) {
// Step 0: Find out where all non-removed items are, pre-layout
int count = mChildHelper.getChildCount();
for (int i = 0; i < count; ++i) {
final ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));
if (holder.shouldIgnore() || (holder.isInvalid() && !mAdapter.hasStableIds())) {
continue;
}
final ItemHolderInfo animationInfo = mItemAnimator
.recordPreLayoutInformation(mState, holder,
ItemAnimator.buildAdapterChangeFlagsForAnimations(holder),
holder.getUnmodifiedPayloads());
mViewInfoStore.addToPreLayout(holder, animationInfo);
if (mState.mTrackOldChangeHolders && holder.isUpdated() && !holder.isRemoved()
&& !holder.shouldIgnore() && !holder.isInvalid()) {
long key = getChangedHolderKey(holder);
// This is NOT the only place where a ViewHolder is added to old change holders
// list. There is another case where:
// * A VH is currently hidden but not deleted
// * The hidden item is changed in the adapter
// * Layout manager decides to layout the item in the pre-Layout pass (step1)
// When this case is detected, RV will un-hide that view and add to the old
// change holders list.
mViewInfoStore.addToOldChangeHolders(key, holder);
}
}
}
if (mState.mRunPredictiveAnimations) {
// Step 1: run prelayout: This will use the old positions of items. The layout manager
// is expected to layout everything, even removed items (though not to add removed
// items back to the container). This gives the pre-layout position of APPEARING views
// which come into existence as part of the real layout.
// Save old positions so that LayoutManager can run its mapping logic.
saveOldPositions();
final boolean didStructureChange = mState.mStructureChanged;
mState.mStructureChanged = false;
// temporarily disable flag because we are asking for previous layout
mLayout.onLayoutChildren(mRecycler, mState);
mState.mStructureChanged = didStructureChange;
for (int i = 0; i < mChildHelper.getChildCount(); ++i) {
final View child = mChildHelper.getChildAt(i);
final ViewHolder viewHolder = getChildViewHolderInt(child);
if (viewHolder.shouldIgnore()) {
continue;
}
if (!mViewInfoStore.isInPreLayout(viewHolder)) {
int flags = ItemAnimator.buildAdapterChangeFlagsForAnimations(viewHolder);
boolean wasHidden = viewHolder
.hasAnyOfTheFlags(ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);
if (!wasHidden) {
flags |= ItemAnimator.FLAG_APPEARED_IN_PRE_LAYOUT;
}
final ItemHolderInfo animationInfo = mItemAnimator.recordPreLayoutInformation(
mState, viewHolder, flags, viewHolder.getUnmodifiedPayloads());
if (wasHidden) {
recordAnimationInfoIfBouncedHiddenView(viewHolder, animationInfo);
} else {
mViewInfoStore.addToAppearedInPreLayoutHolders(viewHolder, animationInfo);
}
}
}
// we don't process disappearing list because they may re-appear in post layout pass.
clearOldPositions();
} else {
clearOldPositions();
}
onExitLayoutOrScroll();
resumeRequestLayout(false);
mState.mLayoutStep = State.STEP_LAYOUT;
}
其次是dispatchLayoutStep2()中：
private void dispatchLayoutStep2() {
eatRequestLayout();
onEnterLayoutOrScroll();
mState.assertLayoutStep(State.STEP_LAYOUT | State.STEP_ANIMATIONS);
mAdapterHelper.consumeUpdatesInOnePass();
mState.mItemCount = mAdapter.getItemCount();
mState.mDeletedInvisibleItemCountSincePreviousLayout = 0;
// Step 2: Run layout
mState.mInPreLayout = false;
mLayout.onLayoutChildren(mRecycler, mState);
mState.mStructureChanged = false;
mPendingSavedState = null;
// onLayoutChildren may have caused client code to disable item animations; re-check
mState.mRunSimpleAnimations = mState.mRunSimpleAnimations && mItemAnimator != null;
mState.mLayoutStep = State.STEP_ANIMATIONS;
onExitLayoutOrScroll();
resumeRequestLayout(false);
}
ItemDecoration
- onDraw(Canvas c, RecyclerView parent, RecyclerView.State state)
这个函数作用在于绘制分割线，在源码中被调用的地方在RecyclerView中：
@Override
public void onDraw(Canvas c) {
super.onDraw(c);
final int count = mItemDecorations.size();
for (int i = 0; i < count; i++) {
mItemDecorations.get(i).onDraw(c, this, mState);
}
}
其实就是RecyclerView的onDraw()函数中进行了分割线的绘制
-
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state)
这个函数是，在每一项itemView绘制的时候，通过控制outRect的left/top/right/bottom值，来确定itemView四周留出来的范围，效果类似于padding或margin
在源码中被调用的地方有：
image.png
可以看到，主要是和measure有关的函数，很容易理解，因为就是在measure过程确定一个itemView占据多少空间
ItemAnimator
ItemAnimator主要用于对item的插入、修改、删除动画处理主要有animateApprearance/animateDisappearance/animateChange/animatePersistence这几个在对应动作的函数
-
public abstract boolean animateAppearance(@NonNull ViewHolder viewHolder, @Nullable ItemHolderInfo preLayoutInfo, @NonNull ItemHolderInfo postLayoutInfo);
在源码中查找animateAppearance的调用链为：
image.png
最终又是在熟悉的dispatchLayoutStep3()中被调用
// Step 4: Process view info lists and trigger animations
mViewInfoStore.process(mViewInfoProcessCallback);
但这一行并不是很能看出怎么就调用了animateAppearance，看下再上一步的调用：
image.png
可以看到在调用process()函数的时候，是有根据record.flags来判断应该执行的是哪个处理
-
public abstract boolean animateDisappearance(@NonNull ViewHolder viewHolder, @NonNull ItemHolderInfo preLayoutInfo, @Nullable ItemHolderInfo postLayoutInfo)
在源码中查找其调用位置：
image.png
可以看到跟animateAppearance一样，最终都是被dispatchLayoutStep3()调用
-
public abstract boolean animateChange(@NonNull ViewHolder oldHolder,
@NonNull ViewHolder newHolder,
@NonNull ItemHolderInfo preLayoutInfo, @NonNull ItemHolderInfo postLayoutInfo)
这个函数的调用链和上述一致，就不重复贴图了
-
public abstract boolean animatePersistence(@NonNull ViewHolder viewHolder,
@NonNull ItemHolderInfo preLayoutInfo, @NonNull ItemHolderInfo postLayoutInfo);
这个函数的调用链依然和上述一致
