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。
