RecyclerView控件几乎在我们每个项目中都会使用，所以懂Recyclerview源码对于我们来说尤为重要，会让我们在平时开发更容易解决使用Recyclerview遇到的奇怪问题，而且Recyclerview强大功能的实现谁不好奇呢？
RecyclerView的优点：
- 提供了多种LayoutManager，可轻松实现多种样式的布局
- 支持局部刷新
- 已经实现了View的复用，不需要类似if(convertView == null)的实现，而且回收机制更加完善
- 容易实现添加item、删除item的动画效果
- 实现了item间距空控制
LayoutManager
与其他绑定 adapter 展示数据的控件，比如 ListView、GrideView 相比，RecyclerView 允许自定义规则去放置子 view，这个规则的控制者就是 LayoutManager。一个 RecyclerView 如果想展示内容，就必须设置一个 LayoutManager。
RecyclerView默认提供了LinearLayoutManager、GridLayoutManager和StaggeredGridLayoutManager，它们都继承于RecyclerView.LayoutManager。
GridLayoutManager继承于LinearLayoutManager。
setLayoutManager方法入口：
public void setLayoutManager(@Nullable LayoutManager layout) {
if (layout == mLayout) {
return;
}
//停止滚动
stopScroll();
// TODO We should do this switch a dispatchLayout pass and animate children. There is a good
// chance that LayoutManagers will re-use views.
if (mLayout != null) {
// end all running animations
if (mItemAnimator != null) {
mItemAnimator.endAnimations();
}
//移除并回收视图
mLayout.removeAndRecycleAllViews(mRecycler);
//移除废弃视图
mLayout.removeAndRecycleScrapInt(mRecycler);
mRecycler.clear();
if (mIsAttached) {
mLayout.dispatchDetachedFromWindow(this, mRecycler);
}
mLayout.setRecyclerView(null);
mLayout = null;
} else {
mRecycler.clear();
}
// this is just a defensive measure for faulty item animators.
mChildHelper.removeAllViewsUnfiltered();
mLayout = layout;
if (layout != null) {
if (layout.mRecyclerView != null) {
throw new IllegalArgumentException("LayoutManager " + layout
+ " is already attached to a RecyclerView:"
+ layout.mRecyclerView.exceptionLabel());
}
mLayout.setRecyclerView(this);
if (mIsAttached) {
mLayout.dispatchAttachedToWindow(this);
}
}
mRecycler.updateViewCacheSize();
requestLayout();
}
这段代码意思是，当之前设置过 LayoutManager 时，移除之前的视图，并缓存视图在 Recycler 中，将recyclerview设置给layoutmanager，更新缓存的View数量，最后去调用requestLayout()，重新请求 measure、layout、draw。
Recyclerview是ViewGroup，需要去实现放置各个子View的功能。LayoutManager 的作用就是为 RecyclerView 放置子 view，所以我直接去定位 RecyclerView 的 onLayout 和 onMeasure 方法，研究一下 LayoutManager 的一些关键函数的作用。
@Override
protected void onMeasure(int widthSpec, int heightSpec) {
if (mLayout == null) {
defaultOnMeasure(widthSpec, heightSpec);
return;
}
if (mLayout.isAutoMeasureEnabled()) {
final int widthMode = MeasureSpec.getMode(widthSpec);
final int heightMode = MeasureSpec.getMode(heightSpec);
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
final boolean measureSpecModeIsExactly =
widthMode == MeasureSpec.EXACTLY && heightMode == MeasureSpec.EXACTLY;
if (measureSpecModeIsExactly || 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;
dispatchLayoutStep2();
// now we can get the width and height from the children.
mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
// if RecyclerView has non-exact width and height and if there is at least one child
// which also has non-exact width & height, we have to re-measure.
if (mLayout.shouldMeasureTwice()) {
mLayout.setMeasureSpecs(
MeasureSpec.makeMeasureSpec(getMeasuredWidth(), MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(getMeasuredHeight(), MeasureSpec.EXACTLY));
mState.mIsMeasuring = true;
dispatchLayoutStep2();
// now we can get the width and height from the children.
mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
}
} else {
if (mHasFixedSize) {
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
return;
}
// custom onMeasure
if (mAdapterUpdateDuringMeasure) {
startInterceptRequestLayout();
onEnterLayoutOrScroll();
processAdapterUpdatesAndSetAnimationFlags();
onExitLayoutOrScroll();
if (mState.mRunPredictiveAnimations) {
mState.mInPreLayout = true;
} else {
// consume remaining updates to provide a consistent state with the layout pass.
mAdapterHelper.consumeUpdatesInOnePass();
mState.mInPreLayout = false;
}
mAdapterUpdateDuringMeasure = false;
stopInterceptRequestLayout(false);
} else if (mState.mRunPredictiveAnimations) {
setMeasuredDimension(getMeasuredWidth(), getMeasuredHeight());
return;
}
if (mAdapter != null) {
mState.mItemCount = mAdapter.getItemCount();
} else {
mState.mItemCount = 0;
}
startInterceptRequestLayout();
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
stopInterceptRequestLayout(false);
mState.mInPreLayout = false; // clear
}
}
来分析一下 onMeasure 方法，mAutoMeasure 字段用来标记是否使用 RecyclerView 的默认规则进行自动测量，否则就必须在 LayoutManager 中自己实现 onMeasure 来进行测量。
当 RecyclerView 的 MeasureSpec 为 MeasureSpec.EXACTLY时，这个时候可以直接确定 RecyclerView 的宽高，所以 return 退出测量。当 RecyclerView 的宽高为不为 EXACTLY 时，则调用的方法是dispatchLayoutStep1。dispatchLayoutStep1的主要作用是保存有关当前视图的信息。
然后调用dispatchLayoutStep2方法
private void dispatchLayoutStep2() {
startInterceptRequestLayout();
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();
stopInterceptRequestLayout(false);
}
onLayoutChildren 这个函数由 LayoutManager 实现，来规定放置子 view 的算法，寻找锚点填充 view。第二步就是将 mState.mLayoutStep 置为 State.STEP_ANIMATIONS，刚才我们忘记说 mLayoutStep 这个属性了，从它的命名就知道它是来标记 layout 这个过程进行到哪一步了。在 dispatchLayoutStep1 中 mState.mLayoutStep 被置为 State.STEP_LAYOUT。
LayoutManager功能总结：
- 协助 RecyclerView 完成 onMeasure 过程
- 通过 onLayoutChildren 完成对子 view 的布局
- 滚动子视图
- 滚动过程中判断何时添加 view ，何时回收 view，也就是对缓存时机的判断。
Recycler源码
Recycler 的作用就是控制 RecyclerView 缓存的核心类。理解了它的工作过程，就可以弄明白 RecyclerView 如何回收 view，如何复用 view 的。
Recycler属性代码：
public final class Recycler {
final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();
ArrayList<ViewHolder> mChangedScrap = null;
final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>();
private final List<ViewHolder>
mUnmodifiableAttachedScrap = Collections.unmodifiableList(mAttachedScrap);
private int mRequestedCacheMax = DEFAULT_CACHE_SIZE;
int mViewCacheMax = DEFAULT_CACHE_SIZE;
RecycledViewPool mRecyclerPool;
private ViewCacheExtension mViewCacheExtension;
static final int DEFAULT_CACHE_SIZE = 2;
}
RecyclerView 其实可以算作是四级缓存、mAttachedScrap 、mCachedViews 、mViewCacheExtension、mRecyclerPool 这四个对象就是作为每一级缓存的结构的。
在LayoutManager方法里面，onLayoutChildren，该方法主要功能为通过布局算法，检查子代和其他变量，找到锚点坐标和锚点位置，然后填充子View。而在 onLayoutChildren 调用的 fill 方法就是真正开始填充 layout 的方法。
fill方法代码：
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.mExtraFillSpace;
LayoutChunkResult layoutChunkResult = mLayoutChunkResult;
while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
layoutChunkResult.resetInternal();
if (RecyclerView.VERBOSE_TRACING) {
TraceCompat.beginSection("LLM LayoutChunk");
}
layoutChunk(recycler, state, layoutState, layoutChunkResult);
if (RecyclerView.VERBOSE_TRACING) {
TraceCompat.endSection();
}
if (layoutChunkResult.mFinished) {
break;
}
layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;
if (!layoutChunkResult.mIgnoreConsumed || layoutState.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;
}
while 循环中，通过判断 LaytouState 中保存的状态来不断的通过 LayoutChunk 方法填充 view。所以接着再看 LayoutChunk 方法。
layoutChunk方法代码：
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;
}
RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) view.getLayoutParams();
if (layoutState.mScrapList == null) {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addView(view);
} else {
addView(view, 0);
}
} else {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addDisappearingView(view);
} else {
addDisappearingView(view, 0);
}
}
measureChildWithMargins(view, 0, 0);
result.mConsumed = mOrientationHelper.getDecoratedMeasurement(view);
int left, top, right, bottom;
if (mOrientation == VERTICAL) {
if (isLayoutRTL()) {
right = getWidth() - getPaddingRight();
left = right - mOrientationHelper.getDecoratedMeasurementInOther(view);
} else {
left = getPaddingLeft();
right = left + mOrientationHelper.getDecoratedMeasurementInOther(view);
}
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
bottom = layoutState.mOffset;
top = layoutState.mOffset - result.mConsumed;
} else {
top = layoutState.mOffset;
bottom = layoutState.mOffset + result.mConsumed;
}
} else {
top = getPaddingTop();
bottom = top + mOrientationHelper.getDecoratedMeasurementInOther(view);
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
right = layoutState.mOffset;
left = layoutState.mOffset - result.mConsumed;
} else {
left = layoutState.mOffset;
right = layoutState.mOffset + result.mConsumed;
}
}
// We calculate everything with View's bounding box (which includes decor and margins)
// To calculate correct layout position, we subtract margins.
layoutDecoratedWithMargins(view, left, top, right, bottom);
if (DEBUG) {
Log.d(TAG, "laid out child at position " + getPosition(view) + ", with l:"
+ (left + params.leftMargin) + ", t:" + (top + params.topMargin) + ", r:"
+ (right - params.rightMargin) + ", b:" + (bottom - params.bottomMargin));
}
// Consume the available space if the view is not removed OR changed
if (params.isItemRemoved() || params.isItemChanged()) {
result.mIgnoreConsumed = true;
}
result.mFocusable = view.hasFocusable();
}
通过 next 方法取出来 view ，并且通过 addView 添加到 RecyclerView 里面去。
next方法代码：
View next(RecyclerView.Recycler recycler) {
if (mScrapList != null) {
return nextViewFromScrapList();
}
final View view = recycler.getViewForPosition(mCurrentPosition);
mCurrentPosition += mItemDirection;
return view;
}
该方法调用了Recycler类中的getViewForPosition
@NonNull
public View getViewForPosition(int position) {
return getViewForPosition(position, false);
}
View getViewForPosition(int position, boolean dryRun) {
return tryGetViewHolderForPositionByDeadline(position, dryRun, FOREVER_NS).itemView;
}
这长长的tryGetViewHolderForPositionByDeadline方法部分代码
ViewHolder tryGetViewHolderForPositionByDeadline(int position,
boolean dryRun, long deadlineNs) {
if (position < 0 || position >= mState.getItemCount()) {
throw new IndexOutOfBoundsException("Invalid item position " + position
+ "(" + position + "). Item count:" + mState.getItemCount()
+ exceptionLabel());
}
boolean fromScrapOrHiddenOrCache = false;
ViewHolder holder = null;
// 0) If there is a changed scrap, try to find from there
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrapOrHiddenOrCache = holder != null;
}
}
第一步，从 mChangedScrap 中尝试取出缓存的 ViewHolder ，若不存在，返回空。如果在第一步发现没有缓存的 ViewHolder，则去 mAttachedScrap 中取，当然 mAttachedScrap 中没有怎么办呢，接着去 mHiddenViews 里面去找，如果还没有，继续从 mCachedViews 中取缓存的 ViewHolder ，这一系列操作是缓存层级的第二步。在 getScrapOrCachedViewForId 方法中，根据 id 依次在 mAttachedScrap 、mCachedViews 集合中寻找缓存的 ViewHolder，如果都不存在，则在 ViewCacheExtension 对象中寻找缓存，ViewCacheExtension 这个类需要使用者通过 setViewCacheExtension 方法传入，RecyclerView 自身并不会实现它，一般正常的使用也用不到，这是缓存层级的第三步。如果还没有就去RecycledViewPool类中取SparseArray类型的mScrap。如果还没有那就会调用mAdapter.createViewHolder(RecyclerView.this, type)，来创建一个 ViewHolder 了。从而实现了将ViewHolder 从层层缓存中取出来了。
ItemDecoration
ItemDecoration是RecyclerView内部的抽象类，内部维护了一个ItemDecoration类型的集合。
final ArrayList<ItemDecoration> mItemDecorations = new ArrayList<>();
通过addItemDecoration去添加装饰线：
public void addItemDecoration(@NonNull ItemDecoration decor, int index) {
if (mLayout != null) {
mLayout.assertNotInLayoutOrScroll("Cannot add item decoration during a scroll or"
+ " layout");
}
if (mItemDecorations.isEmpty()) {
setWillNotDraw(false);
}
if (index < 0) {
mItemDecorations.add(decor);
} else {
mItemDecorations.add(index, decor);
}
markItemDecorInsetsDirty();
requestLayout();
}
集合中添加decoration之后，调用了 markItemDecorInsetsDirty方法。
void markItemDecorInsetsDirty() {
final int cachedCount = mCachedViews.size();
for (int i = 0; i < cachedCount; i++) {
final ViewHolder holder = mCachedViews.get(i);
LayoutParams layoutParams = (LayoutParams) holder.itemView.getLayoutParams();
if (layoutParams != null) {
layoutParams.mInsetsDirty = true;
}
}
}
mCachedViews为recyclerview缓存的ViewHolder，我们在 ViewHolder 中取出 itemView，然后获得 LayoutParams，将其 mInsetsDirty 字段一样置为 true。
mInsetsDirty 字段的作用其实是一种优化性能的缓存策略，添加分割线对象时，无论是 RecyclerView 的子 view，还是缓存的 view，都将其置为 true，接着就调用了 requestLayout 方法。
在recyclerview的onDraw方法中，取出mItemDecorations，对存储的ItemDecoration进行绘制。
@Override
public void draw(Canvas c) {
super.draw(c);
final int count = mItemDecorations.size();
for (int i = 0; i < count; i++) {
mItemDecorations.get(i).onDrawOver(c, this, mState);
}
}
最终调用方法getItemDecorInsetsForChild从item中插入间隔。
Rect getItemDecorInsetsForChild(View child) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
if (!lp.mInsetsDirty) {
return lp.mDecorInsets;
}
if (mState.isPreLayout() && (lp.isItemChanged() || lp.isViewInvalid())) {
// changed/invalid items should not be updated until they are rebound.
return lp.mDecorInsets;
}
final Rect insets = lp.mDecorInsets;
insets.set(0, 0, 0, 0);
final int decorCount = mItemDecorations.size();
for (int i = 0; i < decorCount; i++) {
mTempRect.set(0, 0, 0, 0);
mItemDecorations.get(i).getItemOffsets(mTempRect, child, this, mState);
insets.left += mTempRect.left;
insets.top += mTempRect.top;
insets.right += mTempRect.right;
insets.bottom += mTempRect.bottom;
}
lp.mInsetsDirty = false;
return insets;
}
首先 getItemDecorInsetsForChild 方法是在 RecyclerView 进行 measureChild 时调用的。目的就是为了取出 RecyclerView 的 ChildView 中的分割线属性 --- 在 LayoutParams 中缓存的 mDecorInsets 。而 mDecorInsets 就是 Rect 对象， 其保存记录的是所有添加分割线需要的空间累加的总和，由分割线的 getItemOffsets 方法影响。
最后在 measureChild 方法里，将分割线 ItemDecoration 的尺寸加入到 itemView 的 padding 中。
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