@Override
protected void onMeasure(int widthSpec, int heightSpec) {
    // 第一種情況：沒有設(shè)置 LayoutManager
    if (mLayout == null) {
        defaultOnMeasure(widthSpec, heightSpec);
        return;
    }
    // 第二種情況：設(shè)置的 LayoutManager 開啟自動測量
    if (mLayout.isAutoMeasureEnabled()) {
        // ...
        mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
        // ...
        mLastAutoMeasureSkippedDueToExact =
                widthMode == MeasureSpec.EXACTLY && heightMode == MeasureSpec.EXACTLY;
        if (mLastAutoMeasureSkippedDueToExact || mAdapter == null) {
            return;
        }
        if (mState.mLayoutStep == State.STEP_START) {
            dispatchLayoutStep1();
        }
        mLayout.setMeasureSpecs(widthSpec, heightSpec);
        mState.mIsMeasuring = true;
        dispatchLayoutStep2();
        // 需要二次測量
        if (mLayout.shouldMeasureTwice()) {
            // ...
            dispatchLayoutStep2();
            mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
        }
        // ...
    } else {  // 第三種情況：設(shè)置的 LayoutManager 沒有開啟自動測量
        // ...
        mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
        // ...
        mState.mInPreLayout = false; // clear
    }
}

public abstract static class LayoutManager{
    // ...
    public void onMeasure(@NonNull Recycler recycler, @NonNull State state, int widthSpec,int heightSpec) {
        mRecyclerView.defaultOnMeasure(widthSpec, heightSpec);
    }
}

void defaultOnMeasure(int widthSpec, int heightSpec) {
    // 通過 LayoutManager.chooseSize 獲取的寬和高的值
    final int width = LayoutManager.chooseSize(widthSpec,
            getPaddingLeft() + getPaddingRight(), // 橫向內(nèi)邊距
            ViewCompat.getMinimumWidth(this)); // 反射獲取是否設(shè)置最小寬度
    final int height = LayoutManager.chooseSize(heightSpec,
            getPaddingTop() + getPaddingBottom(), // 縱向內(nèi)邊距
            ViewCompat.getMinimumHeight(this)); // 反射獲取是否設(shè)置最小寬度
    // 設(shè)置寬高
    setMeasuredDimension(width, height);
}

public static int chooseSize(int spec, int desired, int min) {
    final int mode = View.MeasureSpec.getMode(spec);
    final int size = View.MeasureSpec.getSize(spec);
    switch (mode) {
        case View.MeasureSpec.EXACTLY:
            return size;
        case View.MeasureSpec.AT_MOST:
            return Math.min(size, Math.max(desired, min));
        case View.MeasureSpec.UNSPECIFIED:
        default:
            return Math.max(desired, min);
    }
}

protected void onLayout(boolean changed, int l, int t, int r, int b) {
    TraceCompat.beginSection(TRACE_ON_LAYOUT_TAG);
    dispatchLayout(); //  只有一處方法調(diào)用 分發(fā)布局
    TraceCompat.endSection();
    mFirstLayoutComplete = true;
}
void dispatchLayout() {
    // ...
    // 在 onMeasure 中這個(gè)值被設(shè)置為 true
    mState.mIsMeasuring = false;
    // ...
    if (mState.mLayoutStep == State.STEP_START) {
        dispatchLayoutStep1();
        mLayout.setExactMeasureSpecsFrom(this);
        dispatchLayoutStep2();
    } else if (mAdapterHelper.hasUpdates()
            || needsRemeasureDueToExactSkip
            || mLayout.getWidth() != getWidth()
            || mLayout.getHeight() != getHeight()) {
        mLayout.setExactMeasureSpecsFrom(this);
        dispatchLayoutStep2();
    } else {
        mLayout.setExactMeasureSpecsFrom(this);
    }
    dispatchLayoutStep3(); // mState.mLayoutStep 的值在里面被設(shè)置為 State.STEP_START
}

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);
    }
    // ...
}

public void onDraw(Canvas c) {
    super.onDraw(c);
    // draw 方法中調(diào)用了 ItemDecoration 的 onDrawOver
    // onDraw 方法又調(diào)用了 ItemDecoration 的 onDraw 
    final int count = mItemDecorations.size();
    for (int i = 0; i < count; i++) {
        mItemDecorations.get(i).onDraw(c, this, mState);
    }
}

private void dispatchLayoutStep1() {
    // 確認(rèn)布局步驟 在 dispatchLayoutStep3 會設(shè)置為 STEP_START
    // 并且 onLayout 調(diào)用 dispatchLayoutStep1 之前也進(jìn)行了判斷
    mState.assertLayoutStep(State.STEP_START);
    // 獲取剩余的滾動距離（橫豎向）
    fillRemainingScrollValues(mState);
    // onMeasure 中標(biāo)記為 true 這里再置為 false
    mState.mIsMeasuring = false;
    startInterceptRequestLayout();
    // ViewInfoStore 用于保存動畫相關(guān)信息
    // 清除保存的信息 
    mViewInfoStore.clear();
    // 標(biāo)記進(jìn)入布局或者滾動狀態(tài) 內(nèi)部是int類型進(jìn)行++操作
    onEnterLayoutOrScroll();
    // 適配器更新和動畫預(yù)處理 設(shè)置 mState.mRunSimpleAnimations 和 mState.mRunPredictiveAnimations 的值
    processAdapterUpdatesAndSetAnimationFlags();
    // 保存焦點(diǎn)信息
    saveFocusInfo();
    // 一些信息保存
    mState.mTrackOldChangeHolders = mState.mRunSimpleAnimations && mItemsChanged;
    mItemsAddedOrRemoved = mItemsChanged = false;
    // 預(yù)布局標(biāo)志 和 mRunPredictiveAnimation 有關(guān) 這里先記住 后面會解釋什么是預(yù)布局
    mState.mInPreLayout = mState.mRunPredictiveAnimations;
    mState.mItemCount = mAdapter.getItemCount();
    findMinMaxChildLayoutPositions(mMinMaxLayoutPositions);
    // 下面兩個(gè) if 都是動畫預(yù)處理 保存信息等等
    // mRunSimpleAnimations 可以理解為 需要執(zhí)行動畫
    if (mState.mRunSimpleAnimations) {
        // ...
        int count = mChildHelper.getChildCount();
        for (int i = 0; i < count; ++i) {
            // ...
            // 保存執(zhí)行動畫所需的信息 （預(yù)布局時(shí)的信息）
            mViewInfoStore.addToPreLayout(holder, animationInfo);
            // ...
        }
    }
    // mRunPredictiveAnimations 可以理解為 需要執(zhí)行動畫的情況下需要進(jìn)行預(yù)布局
    // 換而言之需要拿到動畫執(zhí)行前后的各種信息（坐標(biāo)等等）
    if (mState.mRunPredictiveAnimations) {
        // ...
        // 這里如果需要預(yù)布局就調(diào)用 LayoutManager 的 onLayoutChildren 開始布局
        // 注意 mState.mInPreLayout = mRunPredictiveAnimations
        // 當(dāng) mRunPredictiveAnimations 為 ture 時(shí) mInPreLayout 同樣為 true
        mLayout.onLayoutChildren(mRecycler, mState);
        // ...
    } else {
        clearOldPositions();
    }
    onExitLayoutOrScroll();
    // 和 startInterceptRequestLayout 成對使用 貌似是防止多次 requestLayout
    stopInterceptRequestLayout(false);
    // 標(biāo)記 STEP_START 完成 可以執(zhí)行 dispatchLayoutStep2
    mState.mLayoutStep = State.STEP_LAYOUT;
}

private void dispatchLayoutStep2() {
    startInterceptRequestLayout(); // 和 stopInterceptRequestLayout 成對出現(xiàn)
    onEnterLayoutOrScroll(); // 上面已經(jīng)說過了
    // 判斷 State ，在 dispatchLayoutStep1 已經(jīng)標(biāo)記為 STEP_LAYOUT
    mState.assertLayoutStep(State.STEP_LAYOUT | State.STEP_ANIMATIONS);
    mAdapterHelper.consumeUpdatesInOnePass();
    mState.mItemCount = mAdapter.getItemCount();
    mState.mDeletedInvisibleItemCountSincePreviousLayout = 0;
    if (mPendingSavedState != null && mAdapter.canRestoreState()) {
        if (mPendingSavedState.mLayoutState != null) {
            mLayout.onRestoreInstanceState(mPendingSavedState.mLayoutState);
        }
        mPendingSavedState = null;
    }
    // 預(yù)布局標(biāo)記為 fasle
    mState.mInPreLayout = false;
    // 開始布局 這里是真正的測量和布局items
    mLayout.onLayoutChildren(mRecycler, mState);
    mState.mStructureChanged = false;
    mState.mRunSimpleAnimations = mState.mRunSimpleAnimations && mItemAnimator != null;
    // 標(biāo)記 STEP_ANIMATIONS
    mState.mLayoutStep = State.STEP_ANIMATIONS;
    onExitLayoutOrScroll();
    stopInterceptRequestLayout(false); // 和 startInterceptRequestLayout 成對出現(xiàn)
}

private void dispatchLayoutStep3() {
    // 判斷狀態(tài)是否為 STEP_ANIMATIONS
    mState.assertLayoutStep(State.STEP_ANIMATIONS);
    // 和 stopInterceptRequestLayout 成對出現(xiàn)
    startInterceptRequestLayout(); 
    // 和 onExitLayoutOrScroll 成對出現(xiàn)
    onEnterLayoutOrScroll();
    // 標(biāo)記為 STEP_START， 在步驟 1 中會判斷是否為 STEP_START
    mState.mLayoutStep = State.STEP_START;
    if (mState.mRunSimpleAnimations) {
        for (int i = mChildHelper.getChildCount() - 1; i >= 0; i--) {
            ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));
            // ...
            // 保存動畫信息
            // 布局完成后的坐標(biāo)等等
            mViewInfoStore.addToPostLayout(holder, animationInfo);
            // ...
            }
        }
        // 執(zhí)行 item 動畫
        mViewInfoStore.process(mViewInfoProcessCallback);
    }
    // 清理 mAttachedScrap 和 mChangedScraop 緩存
    mLayout.removeAndRecycleScrapInt(mRecycler);
    // item 數(shù)量
    mState.mPreviousLayoutItemCount = mState.mItemCount;
    // 相關(guān)標(biāo)記設(shè)為初始值
    mDataSetHasChangedAfterLayout = false;
    mDispatchItemsChangedEvent = false;
    mState.mRunSimpleAnimations = false;
    mState.mRunPredictiveAnimations = false;
    mLayout.mRequestedSimpleAnimations = false;
    // ...
    // 布局完成回調(diào)
    mLayout.onLayoutCompleted(mState);
    onExitLayoutOrScroll();
    stopInterceptRequestLayout(false);
    // 清理
    mViewInfoStore.clear();
    // ...
}

public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state){
    // ...
    // 
    detachAndScrapAttachedViews(recycler);
    // ...
    // fill 在第一篇博客中提到了 填充布局 算是回收復(fù)用的入口
    fill(recycler, mLayoutState, state, false);
}
public void detachAndScrapAttachedViews(@NonNull Recycler recycler) {
    // 從這里可以看出將所有的可見的 item 都回收到了 mAttachedScrap 或者 mChangedScrap 中
    final int childCount = getChildCount();
    for (int i = childCount - 1; i >= 0; i--) {
        final View v = getChildAt(i);
        scrapOrRecycleView(recycler, i, v);
    }
}

int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
        RecyclerView.State state, boolean stopOnFocusable) {
    //...
    // 可用空間
    int remainingSpace = layoutState.mAvailable + layoutState.mExtraFillSpace;
    // 當(dāng) remainingSpace > 0 會繼續(xù)循環(huán)
    while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
        // ...
        // 布局
        layoutChunk(recycler, state, layoutState, layoutChunkResult);
        // 計(jì)算可用空間
        // 注意這里的判斷條件
        if (!layoutChunkResult.mIgnoreConsumed || layoutState.mScrapList != null
                || !state.isPreLayout()) {
            layoutState.mAvailable -= layoutChunkResult.mConsumed;
            remainingSpace -= layoutChunkResult.mConsumed;
        }
    }
}

void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
        LayoutState layoutState, LayoutChunkResult result) {
    // ...
    RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) view.getLayoutParams();
    // ...
    // 源碼最后部分有這么一處判斷
    // 如果 viewholder 被標(biāo)記為了移除或者改變 mIgnoreConsumed 設(shè)為 true
    if (params.isItemRemoved() || params.isItemChanged()) {
        result.mIgnoreConsumed = true;
    }
    result.mFocusable = view.hasFocusable();
}