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Android RecyclerView四级缓存源码层详细分析

作者:lpf_wei

RecyclerView是Android一个更强大的控件,其不仅可以实现和ListView同样的效果,还有优化了ListView中的各种不足。其可以实现数据纵向滚动,也可以实现横向滚动(ListView做不到横向滚动)。接下来讲解RecyclerView的用法

RecyclerView是一个非常重要的控件,是任何一个研发都需要掌握的,这个控件的设计也是非常优秀的,值得我们去学习。RecyclerView的核心就是缓存机制,RecyclerView为了提升效率使用了4级缓存:

1.缓存的使用流程源码分析-滑动入口

当用户在滑动Item的时候会进行ViewHolder的复用,下面来看滑动方法:RecyclerView的onTouchEvent方法case MotionEvent.ACTION_MOVE

@Override
public boolean onTouchEvent(MotionEvent e) {
    if (mLayoutFrozen || mIgnoreMotionEventTillDown) {
        return false;
    }
    if (dispatchOnItemTouch(e)) {
        cancelTouch();
        return true;
    }
    if (mLayout == null) {
        return false;
    }
   ...
    switch (action) {
        case MotionEvent.ACTION_DOWN: 
	...
        case MotionEvent.ACTION_MOVE: {
            ...
            if (mScrollState == SCROLL_STATE_DRAGGING) {
                mLastTouchX = x - mScrollOffset[0];
                mLastTouchY = y - mScrollOffset[1];
                //入口在这里  因为滑动的时候会发生缓存操作  所以一个入口在这里
                if (scrollByInternal(
                        canScrollHorizontally ? dx : 0,
                        canScrollVertically ? dy : 0,
                        vtev)) {
                    getParent().requestDisallowInterceptTouchEvent(true);
                }
                if (mGapWorker != null && (dx != 0 || dy != 0)) {
                    mGapWorker.postFromTraversal(this, dx, dy);
                }
            }
        } break;
        case MotionEvent.ACTION_POINTER_UP: {
            onPointerUp(e);
        } break;
        case MotionEvent.ACTION_UP: 
            ...
    vtev.recycle();
    return true;
}

scrollByInternal 方法就是使用缓存的入口方法

下面来看scrollByInternal方法

boolean scrollByInternal(int x, int y, MotionEvent ev) {
    int unconsumedX = 0, unconsumedY = 0;
    int consumedX = 0, consumedY = 0;
    consumePendingUpdateOperations();
    if (mAdapter != null) {
        eatRequestLayout();
        onEnterLayoutOrScroll();
        Trace.beginSection(TRACE_SCROLL_TAG);
        if (x != 0) {
            consumedX = mLayout.scrollHorizontallyBy(x, mRecycler, mState);
            unconsumedX = x - consumedX;
        }
        if (y != 0) {
            consumedY = mLayout.scrollVerticallyBy(y, mRecycler, mState);
            unconsumedY = y - consumedY;
        }
        Trace.endSection();
        repositionShadowingViews();
        onExitLayoutOrScroll();
        resumeRequestLayout(false);
    }
    ...
    return consumedX != 0 || consumedY != 0;
}

这里区分横向和纵向滑动:scrollHorizontallyBy与scrollVerticallyBy

下面分析纵向滑动的情况scrollVerticallyBy(横向类似):

public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler,
        RecyclerView.State state) {
    if (mOrientation == HORIZONTAL) {
        return 0;
    }
    return scrollBy(dy, recycler, state);
}

这里调用了scrollBy方法,继续往下跟

int scrollBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
    if (getChildCount() == 0 || dy == 0) {
        return 0;
    }
    mLayoutState.mRecycle = true;
    ensureLayoutState();
    final int layoutDirection = dy > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;
    final int absDy = Math.abs(dy);
    updateLayoutState(layoutDirection, absDy, true, state);
    final int consumed = mLayoutState.mScrollingOffset
            + fill(recycler, mLayoutState, state, false);
    if (consumed < 0) {
        if (DEBUG) {
            Log.d(TAG, "Don't have any more elements to scroll");
        }
        return 0;
    }
    final int scrolled = absDy > consumed ? layoutDirection * consumed : dy;
    mOrientationHelper.offsetChildren(-scrolled);
    if (DEBUG) {
        Log.d(TAG, "scroll req: " + dy + " scrolled: " + scrolled);
    }
    mLayoutState.mLastScrollDelta = scrolled;
    return scrolled;
}

这里有个关键方法:fill,当布局或者上下滚动的时候会调用fill方法。

int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
        RecyclerView.State state, boolean stopOnFocusable) { //布局或者上下滚动的时候会调用
    // 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); //回收ViewHolder
    }
    int remainingSpace = layoutState.mAvailable + layoutState.mExtra;
    LayoutChunkResult layoutChunkResult = mLayoutChunkResult;
    while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
        layoutChunkResult.resetInternal();
        layoutChunk(recycler, state, layoutState, layoutChunkResult);  //循环调用   这里是layout的核心
        if (layoutChunkResult.mFinished) {
            break;
        }
      ...
    }
    if (DEBUG) {
        validateChildOrder();
    }
    return start - layoutState.mAvailable;
}

layoutChunk这个方法是使用缓存的入口,recycleByLayoutState这个是进行ViewHolder缓存的入口。

下面来看layoutChunk:

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;
    }
 	...
    result.mFocusable = view.isFocusable();
}

这个方法里边调用了layoutState的next方法得到一个View,那么关键就是next方法了

View next(RecyclerView.Recycler recycler) {
    if (mScrapList != null) {
        return nextViewFromScrapList();
    }
    final View view = recycler.getViewForPosition(mCurrentPosition);
    mCurrentPosition += mItemDirection;
    return view;
}
public View getViewForPosition(int position) {
            return getViewForPosition(position, false);
        }
View getViewForPosition(int position, boolean dryRun) {
       return tryGetViewHolderForPositionByDeadline(position, dryRun, FOREVER_NS).itemView;
 }

这个方法又调用了recycler.getViewForPosition方法,最终调到了tryGetViewHolderForPositionByDeadline这个方法。

下面来分析tryGetViewHolderForPositionByDeadline这个方法,整个ViewHolder的复用流程都在这里,这里是最核心的位置:

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());
    }
    boolean fromScrapOrHiddenOrCache = false;
    ViewHolder holder = null;
    // 0) If there is a changed scrap, try to find from there
    if (mState.isPreLayout()) { //通过位置从mChangeScrap缓存中获取ViewHolder
        holder = getChangedScrapViewForPosition(position);
        fromScrapOrHiddenOrCache = holder != null;
    }
    // 1) Find by position from scrap/hidden list/cache
    if (holder == null) {//通过position的方式从mAttachScrap或者mCacheViews中获取ViewHolder
        holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
        ...
    }
    if (holder == null) {
        final int offsetPosition = mAdapterHelper.findPositionOffset(position);
        if (offsetPosition < 0 || offsetPosition >= mAdapter.getItemCount()) {
            throw new IndexOutOfBoundsException("Inconsistency detected. Invalid item "
                    + "position " + position + "(offset:" + offsetPosition + ")."
                    + "state:" + mState.getItemCount());
        }
        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);  //通过id的方式从mAttachScrap或者mCacheViews中获取ViewHolder
            if (holder != null) {
                // update position
                holder.mPosition = offsetPosition;
                fromScrapOrHiddenOrCache = true;
            }
        }
        if (holder == null && mViewCacheExtension != null) { //从用户自定义缓存获取ViewHolder
            // We are NOT sending the offsetPosition because LayoutManager does not
            // know it.
            final View view = mViewCacheExtension
                    .getViewForPositionAndType(this, position, type);
            ...
        }
        if (holder == null) { // 从缓存池获取ViewHolder
            if (DEBUG) {
                Log.d(TAG, "tryGetViewHolderForPositionByDeadline("
                        + position + ") fetching from shared pool");
            }
            holder = getRecycledViewPool().getRecycledView(type);
            if (holder != null) {
                holder.resetInternal();
                if (FORCE_INVALIDATE_DISPLAY_LIST) {
                    invalidateDisplayListInt(holder);
                }
            }
        }
        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;
            } //如果还是获取不到ViewHolder,那么就需要通过createViewHolder创建了
            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");
            }
        }
    }
   ...
    boolean bound = false;
    if (mState.isPreLayout() && holder.isBound()) {
        // do not update unless we absolutely have to.
        holder.mPreLayoutPosition = position;
    } else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {
        if (DEBUG && holder.isRemoved()) {
            throw new IllegalStateException("Removed holder should be bound and it should"
                    + " come here only in pre-layout. Holder: " + holder);
        }
        final int offsetPosition = mAdapterHelper.findPositionOffset(position);
        //这里会调用到onBindViewHolder方法进行数据的绑定
        bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
    }
   ...
    return holder;
}

以上就是整个ViewHolder获取过程,首先从缓存池获取,获取不到才会创建,然后进行数据绑定。

2.RecyclerView的缓存流程

在进行layout操作的时候就会进行ViewHolder的缓存操作,将创建好的ViewHolder缓存到缓存池,以便直接使用,下面分析一下ViewHolder是如何缓存到缓存池中的。

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    Trace.beginSection(TRACE_ON_LAYOUT_TAG);
    dispatchLayout();  //这里是摆放的入口
    Trace.endSection();
    mFirstLayoutComplete = true;
}

下面是dispatchLayout:

void dispatchLayout() {
    if (mAdapter == null) {
        Log.e(TAG, "No adapter attached; skipping layout");
        // leave the state in START
        return;
    }
    if (mLayout == null) {
        Log.e(TAG, "No layout manager attached; skipping layout");
        // leave the state in START
        return;
    }
    mState.mIsMeasuring = false;
    if (mState.mLayoutStep == State.STEP_START) {
        dispatchLayoutStep1();
        mLayout.setExactMeasureSpecsFrom(this);
        dispatchLayoutStep2();
    } else if (mAdapterHelper.hasUpdates() || mLayout.getWidth() != getWidth()
            || mLayout.getHeight() != getHeight()) {
        // First 2 steps are done in onMeasure but looks like we have to run again due to
        // changed size.
        mLayout.setExactMeasureSpecsFrom(this);
        dispatchLayoutStep2();
    } else {
        // always make sure we sync them (to ensure mode is exact)
        mLayout.setExactMeasureSpecsFrom(this);
    }
    dispatchLayoutStep3();
}

下面来看dispatchLayoutStep2:

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);
}

这个方法中会调用onLayoutChildren方法,这个方法是缓存的核心所在。

public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
    ...
    onAnchorReady(recycler, state, mAnchorInfo, firstLayoutDirection);
    detachAndScrapAttachedViews(recycler);   //分离并废弃附加视图
    ...
}

这个方法内容较多,做了省略。detachAndScrapAttachedViews这个方法会将ViewHolder缓存到缓存池中。

public void detachAndScrapAttachedViews(Recycler recycler) {
    final int childCount = getChildCount();
    for (int i = childCount - 1; i >= 0; i--) {
        final View v = getChildAt(i);
        scrapOrRecycleView(recycler, i, v);
    }
}

调到了scrapOrRecycleView方法

private void scrapOrRecycleView(Recycler recycler, int index, View view) {
    final ViewHolder viewHolder = getChildViewHolderInt(view);
    if (viewHolder.shouldIgnore()) {
        if (DEBUG) {
            Log.d(TAG, "ignoring view " + viewHolder);
        }
        return;
    }
    if (viewHolder.isInvalid() && !viewHolder.isRemoved()
            && !mRecyclerView.mAdapter.hasStableIds()) {
        removeViewAt(index);
        recycler.recycleViewHolderInternal(viewHolder);  //这是一个收集的情况
    } else {
        detachViewAt(index);
        recycler.scrapView(view);    //这是一个情况
        mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
    }
}

下面来分析recycleViewHolderInternal:

void recycleViewHolderInternal(ViewHolder holder) { //主要处理CacheViews 和RecyclerPool 的缓存
    ...
    if (forceRecycle || holder.isRecyclable()) {
        if (mViewCacheMax > 0
                && !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID  
                        | ViewHolder.FLAG_REMOVED
                        | ViewHolder.FLAG_UPDATE
                        | ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)) {
            // Retire oldest cached view
            int cachedViewSize = mCachedViews.size();
            if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
                recycleCachedViewAt(0);
                cachedViewSize--;
            }
            int targetCacheIndex = cachedViewSize;
            if (ALLOW_THREAD_GAP_WORK
                    && cachedViewSize > 0
                    && !mPrefetchRegistry.lastPrefetchIncludedPosition(holder.mPosition)) {
                // when adding the view, skip past most recently prefetched views
                int cacheIndex = cachedViewSize - 1;
                while (cacheIndex >= 0) {
                    int cachedPos = mCachedViews.get(cacheIndex).mPosition;
                    if (!mPrefetchRegistry.lastPrefetchIncludedPosition(cachedPos)) {
                        break;
                    }
                    cacheIndex--;
                }
                targetCacheIndex = cacheIndex + 1;
            }
            mCachedViews.add(targetCacheIndex, holder); //这里是加入到mCachedViews中
            cached = true;
        }
        if (!cached) { //这里是加入到RecycledViewPool缓存池中
            addViewHolderToRecycledViewPool(holder, true);
            recycled = true;
        }
    }
    ...
}

下面是scrapView部分:

void scrapView(View view) {
    final ViewHolder holder = getChildViewHolderInt(view);
    if (holder.hasAnyOfTheFlags(ViewHolder.FLAG_REMOVED | ViewHolder.FLAG_INVALID)
            || !holder.isUpdated() || canReuseUpdatedViewHolder(holder)) {
        if (holder.isInvalid() && !holder.isRemoved() && !mAdapter.hasStableIds()) {
            throw new IllegalArgumentException("Called scrap view with an invalid view."
                    + " Invalid views cannot be reused from scrap, they should rebound from"
                    + " recycler pool.");
        }
        holder.setScrapContainer(this, false);
        mAttachedScrap.add(holder);
    } else {
        if (mChangedScrap == null) {
            mChangedScrap = new ArrayList<ViewHolder>();
        }
        holder.setScrapContainer(this, true);
        mChangedScrap.add(holder);
    }
}

根据不同的情况会将ViewHolder缓存到mAttachedScrap或者mChangedScrap中

3.RecyclerView缓存总结

RecyclerView 缓存的是ViewHolder

RecyclerView采用了四级缓存:缓存的分类是根据功能区分

ViewHolder的创建流程

到此这篇关于Android RecyclerView四级缓存源码层详细分析的文章就介绍到这了,更多相关Android RecyclerView内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!

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