Android中的RecyclerView源码分析

RecyclerView组件

Adapter

提供数据以及数据变更通知

LayoutManager

布局管理，负责测绘、指定item位置、item回收、

ItemAnimator

Item变更动画

关键实现

1、ViewHolder复用

三层缓存

第一层：Recycler中的mCachedViews

第二层：ViewCacheExtension 由开发者实现的缓存策略，可通过setViewCacheExtension设置到RecyclerView中

第三层：RecycledViewPool中的mScrap 可在多个RecyclerView共享的View缓存池

Scrapped View（废弃的View）：一个View虽然仍附属在它的父视图RecyclerView上，但是已经被标记为已使用或已迁移。这样的View就叫做Scrapped View。

获取一个View的过程

//RecyclerView.Recycler
final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();
private ArrayList<ViewHolder> mChangedScrap = null;

final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>();

private final List<ViewHolder>
mUnmodifiableAttachedScrap = Collections.unmodifiableList(mAttachedScrap);
//所有获取View调用的入口
public View getViewForPosition(int position) {
//调用内部实现
//参数 position 获取一个View的位置
//     false   不进行预检操作  dryrun
return getViewForPosition(position, false);
}

View getViewForPosition(int position, boolean dryRun) {
//参数检查
...
//如果处于提前布局状态，尝试从mChangedScrap列表寻找
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrap = holder != null;
}
//如果没找到，则从mAttachedScrap列表中寻找
holder = getScrapViewForPosition(position, INVALID_TYPE, dryRun);
//如果没找到，或者找到的ViewHolder和OffsetPosition不匹
//配，则重新计算OffsetPosition。并且根据stable ids在
//mAttachedScrap中寻找(如果mAdapter存在stableIds)。
if (mAdapter.hasStableIds()) {
holder = getScrapViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);
if (holder != null) {
// update position
holder.mPosition = offsetPosition;
fromScrap = true;
}
}
//如果还找不到，则从mViewCacheExtension中寻找，
//前提是开发者为RecyclerView设置了ViewCacheExtension
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);
....
....
//如果没有设置ViewCacheExtension,肯定还是找不到的，
//那么就会从RecycledViewPool中寻找。
//因为RecycledViewPool是多个RecyclerView共享的，如果找到合适的ViewHolder，要先更新需要显示的信息。
holder = getRecycledViewPool().getRecycledView(type);
...
...
//最后一层缓存也找不到的话，只能新建了
holder = mAdapter.createViewHolder(RecyclerView.this, type);
...
//填充数据
mAdapter.bindViewHolder(holder, offsetPosition);

...
return holder.itemView;
}

2、数据变更通知（观察者模式）

观察者AdapterDataObserver，具体实现为RecyclerViewDataObserver，当数据源发生变更时，及时响应界面变化

public static abstract class AdapterDataObserver {
public void onChanged() {
// Do nothing
}

public void onItemRangeChanged(int positionStart, int itemCount) {
// do nothing
}

public void onItemRangeChanged(int positionStart, int itemCount, Object payload) {
// fallback to onItemRangeChanged(positionStart, itemCount) if app
// does not override this method.
onItemRangeChanged(positionStart, itemCount);
}

public void onItemRangeInserted(int positionStart, int itemCount) {
// do nothing
}

public void onItemRangeRemoved(int positionStart, int itemCount) {
// do nothing
}

public void onItemRangeMoved(int fromPosition, int toPosition, int itemCount) {
// do nothing
}
}

被观察者AdapterDataObservable，内部持有观察者AdapterDataObserver集合

static class AdapterDataObservable extends Observable<AdapterDataObserver> {
public boolean hasObservers() {
return !mObservers.isEmpty();
}

public void notifyChanged() {
// since onChanged() is implemented by the app, it could do anything, including
// removing itself from {@link mObservers} - and that could cause problems if
// an iterator is used on the ArrayList {@link mObservers}.
// to avoid such problems, just march thru the list in the reverse order.
for (int i = mObservers.size() - 1; i >= 0; i--) {
mObservers.get(i).onChanged();
}
}

public void notifyItemRangeChanged(int positionStart, int itemCount) {
notifyItemRangeChanged(positionStart, itemCount, null);
}

public void notifyItemRangeChanged(int positionStart, int itemCount, Object payload) {
// since onItemRangeChanged() is implemented by the app, it could do anything, including
// removing itself from {@link mObservers} - and that could cause problems if
// an iterator is used on the ArrayList {@link mObservers}.
// to avoid such problems, just march thru the list in the reverse order.
for (int i = mObservers.size() - 1; i >= 0; i--) {
mObservers.get(i).onItemRangeChanged(positionStart, itemCount, payload);
}
}

public void notifyItemRangeInserted(int positionStart, int itemCount) {
// since onItemRangeInserted() is implemented by the app, it could do anything,
// including removing itself from {@link mObservers} - and that could cause problems if
// an iterator is used on the ArrayList {@link mObservers}.
// to avoid such problems, just march thru the list in the reverse order.
for (int i = mObservers.size() - 1; i >= 0; i--) {
mObservers.get(i).onItemRangeInserted(positionStart, itemCount);
}
}

public void notifyItemRangeRemoved(int positionStart, int itemCount) {
// since onItemRangeRemoved() is implemented by the app, it could do anything, including
// removing itself from {@link mObservers} - and that could cause problems if
// an iterator is used on the ArrayList {@link mObservers}.
// to avoid such problems, just march thru the list in the reverse order.
for (int i = mObservers.size() - 1; i >= 0; i--) {
mObservers.get(i).onItemRangeRemoved(positionStart, itemCount);
}
}

public void notifyItemMoved(int fromPosition, int toPosition) {
for (int i = mObservers.size() - 1; i >= 0; i--) {
mObservers.get(i).onItemRangeMoved(fromPosition, toPosition, 1);
}
}
}

Adapter内部持有AdapterDataObservable对象。

当我们为RecyclerView设置Adapter时，会向Adapter注册一个观察者RecyclerViewDataObserver。

adapter.registerAdapterDataObserver(mObserver);

当数据变更时，调用notify**方法时，Adapter内部的被观察者会遍历通知已经注册的观察者的对应方法，这时界面就会响应变更。

3、多种布局类型展示

基类LayoutManager

子类有LinearLayoutManager、GridLayoutManager、StaggeredGridLayoutManager

测量入口：RecyclerView的onMeasure方法

//mAutoMeasure代表是否是由已经存在的LayoutManager去执行测绘工作。
//当使用自定义的LayoutManager时，需要设置此值为false
if (mLayout.mAutoMeasure) {
final int widthMode = MeasureSpec.getMode(widthSpec);
final int heightMode = MeasureSpec.getMode(heightSpec);
//如果RecyclerView的测量模式为EXACTLY(准确模式)
//则不必依赖子视图的尺寸来确定RecyclerView本身的高度
//子视图的测量工作会延迟到onLayout方法中
final boolean skipMeasure = widthMode == MeasureSpec.EXACTLY
&& heightMode == MeasureSpec.EXACTLY;
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
if (skipMeasure || mAdapter == null) {
return;
}
if (mState.mLayoutStep == State.STEP_START) {
dispatchLayoutStep1();
}
// set dimensions in 2nd step. Pre-layout should happen with old dimensions for
// consistency
mLayout.setMeasureSpecs(widthSpec, heightSpec);
mState.mIsMeasuring = true;
//此方法会真正执行LayoutManager的测量布局操作
dispatchLayoutStep2();
.....
.....

/**
* The second layout step where we do the actual layout of the views for the final state.
* This step might be run multiple times if necessary (e.g. measure).
*/
private void dispatchLayoutStep2() {
...
...
//mLayout即RecyclerView设置的LayoutManager
mLayout.onLayoutChildren(mRecycler, mState);
...
...
}

//LayoutManager的这个方法是一个空实现，具体布局操作由子类决定
//当自定义LayoutManager时，这是必须要实现的一个方法
public void onLayoutChildren(Recycler recycler, State state) {
Log.e(TAG, "You must override onLayoutChildren(Recycler recycler, State state) ");
}

//这里以LinearLayoutManager为例，说明布局过程
//通过注释可以很好的理解测量和布局算法流程
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
// layout algorithm:
// 1) by checking children and other variables, find an anchor coordinate and an anchor
//  item position.
// 2) fill towards start, stacking from bottom
// 3) fill towards end, stacking from top
// 4) scroll to fulfill requirements like stack from bottom.
...
...
//第一步，通过检查子视图和其他变量，找出锚点坐标和item位置锚点
//第二步，从底部向起始位置填充
//第三步，从顶部向末尾位置填充
//第四步，滚动要满足要求的位置，例如当从底部填充时
...
...
//当锚点计算完成后，就开始填充view
//LayoutManager的三个子类都有这个填充方法，但是不同的
//的布局类型，此方法会有不同的实现
fill(recycler, mLayoutState, state, false);

4、Item变更动画

5、其他

notifyDataSetChanged发生了什么

1、为页面中存在的ViewHolder和Recycler中mCachesView中缓存的ViewHolder添加一个标志：ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN

2、检查有没有等待执行的更新操作（add/remove/update/move）。如果没有的话，并且当前没有处于布局状态，并且没有设置禁止LayoutFrozen，调用requestLayout()方法重新布局。

3、如果有等待执行的更新操作怎么办？

不允许刷新

notifyItemInserted发生了什么

1、RecyclerView把Item的插入、删除、移动、变更抽象成了一个个命令UpdateOp，并在AdapterHelper中维护了一个命令池。当插入一条Item时，向更新的命令队列中添加一个“ADD”命令。

mPendingUpdates.add(obtainUpdateOp(UpdateOp.ADD, positionStart, itemCount, null));

2、然后会触发更新处理器，来处理新的命令。

3、RecyclerView会启动一个Runnable来处理等待执行的命令。

/**
* Note: this Runnable is only ever posted if:
* 1) We've been through first layout
* 2) We know we have a fixed size (mHasFixedSize)
* 3) We're attached
*/
private final Runnable mUpdateChildViewsRunnable = new Runnable() {
public void run() {
if (!mFirstLayoutComplete || isLayoutRequested()) {
// a layout request will happen, we should not do layout here.
return;
}
if (mLayoutFrozen) {
mLayoutRequestEaten = true;
return; //we'll process updates when ice age ends.
}
consumePendingUpdateOperations();
}
};

但是我们在单步调试的时候，发现条件isLayoutRequested()为true，所以这个Runnable什么都没干，直接返回了。

？？？？

既然isLayoutRequested()为true，说明接下来会重新布局，可能执行更新的地方放在了layout中。

我们在dispatchLayout()方法中的第二步dispatchLayoutStep1();发现了processAdapterUpdatesAndSetAnimationFlags()方法；

该方法主要做了两件事：第一，根据操作类型和数量更新Adapter中Item的位置（例，插入一个新的Item，原列表中Item的Position要依次加一）。第二，设置RecyclerView的状态mState.mRunSimpleAnimations和mState.mRunPredictiveAnimations为true。