RecyclerView 缓存机制详解

一 前言

RecyclerView据官方的介绍，该控件用于在有限的窗口中展示大量数据集，其实这样功能的控件我们并不陌生，例如：ListView、GridView。RecyclerView可以用来代替传统的ListView,GridView，更加强大和灵活。RecyclerView的使用网上有非常多案例，这里就不多说了，我们今天主要来看看RecyclerView 的缓存机制。

二 缓存机制Recycler详解

Recycler是RecyclerView的一个内部类。我们来看一下它的主要的成员变量。

1. mChangedScrap 表示数据已经改变的ewHolder列表
2. mAttachedScrap 未与RecyclerView分离的ViewHolder列表
3. mCachedViews ViewHolder缓存列表，其大小由mViewCacheMax决定，默认DEFAULT_CACHE_SIZE为2，可动态设置。
4. mViewCacheExtension 开发者可自定义的一层缓存，是虚拟类ViewCacheExtension的一个实例，开发者可实现方法getViewForPositionAndType(Recycler recycler, int position, int type)来实现自己的缓存。
5. mRecyclerPool ViewHolder缓存池，在有限的mCachedViews中如果存不下ViewHolder时，就会把ViewHolder存入RecyclerViewPool中。

我们来看一张RecyclerView 缓存机制的流程图，如下图

贴上源码，如下。我们根据流程图和源码来分析RecyclerView的缓存机制。

        public View getViewForPosition(int position) {
            return getViewForPosition(position, false);
        }

        View getViewForPosition(int position, boolean dryRun) {
            if (position < 0 || position >= mState.getItemCount()) {
                throw new IndexOutOfBoundsException("Invalid item position " + position
                        + "(" + position + "). Item count:" + mState.getItemCount());
            }
            boolean fromScrap = false;
            ViewHolder holder = null;
            // 0) If there is a changed scrap, try to find from there
            if (mState.isPreLayout()) {
                holder = getChangedScrapViewForPosition(position);
                fromScrap = holder != null;
            }
            // 1) Find from scrap by position
            if (holder == null) {
                holder = getScrapViewForPosition(position, INVALID_TYPE, dryRun);
                if (holder != null) {
                    if (!validateViewHolderForOffsetPosition(holder)) {
                        // recycle this scrap
                        if (!dryRun) {
                            // we would like to recycle this but need to make sure it is not used by
                            // animation logic etc.
                            holder.addFlags(ViewHolder.FLAG_INVALID);
                            if (holder.isScrap()) {
                                removeDetachedView(holder.itemView, false);
                                holder.unScrap();
                            } else if (holder.wasReturnedFromScrap()) {
                                holder.clearReturnedFromScrapFlag();
                            }
                            recycleViewHolderInternal(holder);
                        }
                        holder = null;
                    } else {
                        fromScrap = true;
                    }
                }
            }
            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 via stable ids, if exists
                if (mAdapter.hasStableIds()) {
                    holder = getScrapViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);
                    if (holder != null) {
                        // update position
                        holder.mPosition = offsetPosition;
                        fromScrap = true;
                    }
                }
                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);
                    if (view != null) {
                        holder = getChildViewHolder(view);
                        if (holder == null) {
                            throw new IllegalArgumentException("getViewForPositionAndType returned"
                                    + " a view which does not have a ViewHolder");
                        } else if (holder.shouldIgnore()) {
                            throw new IllegalArgumentException("getViewForPositionAndType returned"
                                    + " a view that is ignored. You must call stopIgnoring before"
                                    + " returning this view.");
                        }
                    }
                }
                if (holder == null) { // fallback to recycler
                    // try recycler.
                    // Head to the shared pool.
                    if (DEBUG) {
                        Log.d(TAG, "getViewForPosition(" + position + ") fetching from shared "
                                + "pool");
                    }
                    holder = getRecycledViewPool().getRecycledView(type);
                    if (holder != null) {
                        holder.resetInternal();
                        if (FORCE_INVALIDATE_DISPLAY_LIST) {
                            invalidateDisplayListInt(holder);
                        }
                    }
                }
                if (holder == null) {
                    holder = mAdapter.createViewHolder(RecyclerView.this, type);
                    if (DEBUG) {
                        Log.d(TAG, "getViewForPosition created new ViewHolder");
                    }
                }
            }

            // This is very ugly but the only place we can grab this information
            // before the View is rebound and returned to the LayoutManager for post layout ops.
            // We don't need this in pre-layout since the VH is not updated by the LM.
            if (fromScrap && !mState.isPreLayout() && holder
                    .hasAnyOfTheFlags(ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST)) {
                holder.setFlags(0, ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);
                if (mState.mRunSimpleAnimations) {
                    int changeFlags = ItemAnimator
                            .buildAdapterChangeFlagsForAnimations(holder);
                    changeFlags |= ItemAnimator.FLAG_APPEARED_IN_PRE_LAYOUT;
                    final ItemHolderInfo info = mItemAnimator.recordPreLayoutInformation(mState,
                            holder, changeFlags, holder.getUnmodifiedPayloads());
                    recordAnimationInfoIfBouncedHiddenView(holder, info);
                }
            }

            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);
                holder.mOwnerRecyclerView = RecyclerView.this;
                mAdapter.bindViewHolder(holder, offsetPosition);
                attachAccessibilityDelegate(holder.itemView);
                bound = true;
                if (mState.isPreLayout()) {
                    holder.mPreLayoutPosition = position;
                }
            }

            final ViewGroup.LayoutParams lp = holder.itemView.getLayoutParams();
            final LayoutParams rvLayoutParams;
            if (lp == null) {
                rvLayoutParams = (LayoutParams) generateDefaultLayoutParams();
                holder.itemView.setLayoutParams(rvLayoutParams);
            } else if (!checkLayoutParams(lp)) {
                rvLayoutParams = (LayoutParams) generateLayoutParams(lp);
                holder.itemView.setLayoutParams(rvLayoutParams);
            } else {
                rvLayoutParams = (LayoutParams) lp;
            }
            rvLayoutParams.mViewHolder = holder;
            rvLayoutParams.mPendingInvalidate = fromScrap && bound;
            return holder.itemView;
        }

主流程 1
我们来看主流程源码的第14行

  holder = getChangedScrapViewForPosition(position);

我们通过position匹配 mChangedScrap 获取holder缓存。
getChangedScrapViewForPosition(position)方法内部通过2种方法获取holder缓存。第一种通过mChangedScrap匹配
position获取holder缓存。第二种通过mChangedScrap匹配id获取holder缓存。源码如下。

        ViewHolder getChangedScrapViewForPosition(int position) {
            // If pre-layout, check the changed scrap for an exact match.
            final int changedScrapSize;
            if (mChangedScrap == null || (changedScrapSize = mChangedScrap.size()) == 0) {
                return null;
            }
            // 第一种 通过 position来查找
            for (int i = 0; i < changedScrapSize; i++) {
                final ViewHolder holder = mChangedScrap.get(i);
                if (!holder.wasReturnedFromScrap() && holder.getLayoutPosition() == position) {
                    holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
                    return holder;
                }
            }
            //第二种 通过 id来查找
            if (mAdapter.hasStableIds()) {
                final int offsetPosition = mAdapterHelper.findPositionOffset(position);
                if (offsetPosition > 0 && offsetPosition < mAdapter.getItemCount()) {
                    final long id = mAdapter.getItemId(offsetPosition);
                    for (int i = 0; i < changedScrapSize; i++) {
                        final ViewHolder holder = mChangedScrap.get(i);
                        if (!holder.wasReturnedFromScrap() && holder.getItemId() == id) {
                            holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
                            return holder;
                        }
                    }
                }
            }
            return null;
        }

主流程 2
我们看一下主流程第19行代码。

holder = getScrapViewForPosition(position, INVALID_TYPE, dryRun);

通过position查找废弃的holder，我们来看一下getScrapViewForPosition方法内部实现，主要通过3种方法获取holder缓存。
第一种从mAttachedScrap中通过匹配position获取holder缓存。
第二种通过ChildHelper找到隐藏但是没有被移除的View，通过getChildViewHolderInt(view)方法获取holder缓存。
第三种从mCachedViews中通过匹配position获取holder缓存。
getScrapViewForPosition源码如下

  ViewHolder getScrapViewForPosition(int position, int type, boolean dryRun) {
            final int scrapCount = mAttachedScrap.size();

            // 第一种从mAttachedScrap中通过匹配position获取holder缓存。
            for (int i = 0; i < scrapCount; i++) {
                final ViewHolder holder = mAttachedScrap.get(i);
                if (!holder.wasReturnedFromScrap() && holder.getLayoutPosition() == position
                        && !holder.isInvalid() && (mState.mInPreLayout || !holder.isRemoved())) {
                    if (type != INVALID_TYPE && holder.getItemViewType() != type) {
                        Log.e(TAG, "Scrap view for position " + position + " isn't dirty but has" +
                                " wrong view type! (found " + holder.getItemViewType() +
                                " but expected " + type + ")");
                        break;
                    }
                    holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
                    return holder;
                }
            }
            //通过ChildHelper找到隐藏但是没有被移除的View，通过getChildViewHolderInt(view)方法获取holder缓存。
            if (!dryRun) {
                View view = mChildHelper.findHiddenNonRemovedView(position, type);
                if (view != null) {
                    // This View is good to be used. We just need to unhide, detach and move to the
                    // scrap list.
                    final ViewHolder vh = getChildViewHolderInt(view);
                    mChildHelper.unhide(view);
                    int layoutIndex = mChildHelper.indexOfChild(view);
                    if (layoutIndex == RecyclerView.NO_POSITION) {
                        throw new IllegalStateException("layout index should not be -1 after "
                                + "unhiding a view:" + vh);
                    }
                    mChildHelper.detachViewFromParent(layoutIndex);
                    scrapView(view);
                    vh.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP
                            | ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);
                    return vh;
                }
            }

            // Search in our first-level recycled view cache.
            //第三种从mCachedViews中通过匹配position获取holder缓存。
            final int cacheSize = mCachedViews.size();
            for (int i = 0; i < cacheSize; i++) {
                final ViewHolder holder = mCachedViews.get(i);
                // invalid view holders may be in cache if adapter has stable ids as they can be
                // retrieved via getScrapViewForId
                if (!holder.isInvalid() && holder.getLayoutPosition() == position) {
                    if (!dryRun) {
                        mCachedViews.remove(i);
                    }
                    if (DEBUG) {
                        Log.d(TAG, "getScrapViewForPosition(" + position + ", " + type +
                                ") found match in cache: " + holder);
                    }
                    return holder;
                }
            }
            return null;
        }

主流程 3
我们看一下主流程第52行代码。

 holder = getScrapViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);

通过id获取holder缓存，getScrapViewForId方法内部主要通过2种方法获取holder缓存。
第一种从mAttachedScrap中通过匹配id获取holder缓存。
第二种从mCachedViews中通过匹配id获取holder缓存。
getScrapViewForId方法源码如下。

        ViewHolder getScrapViewForId(long id, int type, boolean dryRun) {
            //第一种从mAttachedScrap中通过匹配id获取holder缓存。
            // Look in our attached views first
            final int count = mAttachedScrap.size();
            for (int i = count - 1; i >= 0; i--) {
                final ViewHolder holder = mAttachedScrap.get(i);
                if (holder.getItemId() == id && !holder.wasReturnedFromScrap()) {
                    if (type == holder.getItemViewType()) {
                        holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
                        if (holder.isRemoved()) {
                            // this might be valid in two cases:
                            // > item is removed but we are in pre-layout pass
                            // >> do nothing. return as is. make sure we don't rebind
                            // > item is removed then added to another position and we are in
                            // post layout.
                            // >> remove removed and invalid flags, add update flag to rebind
                            // because item was invisible to us and we don't know what happened in
                            // between.
                            if (!mState.isPreLayout()) {
                                holder.setFlags(ViewHolder.FLAG_UPDATE, ViewHolder.FLAG_UPDATE |
                                        ViewHolder.FLAG_INVALID | ViewHolder.FLAG_REMOVED);
                            }
                        }
                        return holder;
                    } else if (!dryRun) {
                        // if we are running animations, it is actually better to keep it in scrap
                        // but this would force layout manager to lay it out which would be bad.
                        // Recycle this scrap. Type mismatch.
                        mAttachedScrap.remove(i);
                        removeDetachedView(holder.itemView, false);
                        quickRecycleScrapView(holder.itemView);
                    }
                }
            }
            //第二种从mCachedViews中通过匹配id获取holder缓存。
            // Search the first-level cache
            final int cacheSize = mCachedViews.size();
            for (int i = cacheSize - 1; i >= 0; i--) {
                final ViewHolder holder = mCachedViews.get(i);
                if (holder.getItemId() == id) {
                    if (type == holder.getItemViewType()) {
                        if (!dryRun) {
                            mCachedViews.remove(i);
                        }
                        return holder;
                    } else if (!dryRun) {
                        recycleCachedViewAt(i);
                    }
                }
            }
            return null;
        }

主流程 4
我们看一下主流程第62行代码。
通过mViewCacheExtension.getViewForPositionAndType获取view，通过getChildViewHolder(view)获取holder缓存。源码如下

                final View view = mViewCacheExtension
                        .getViewForPositionAndType(this, position, type);
                if (view != null) {
                    holder = getChildViewHolder(view);
                    if (holder == null) {
                        throw new IllegalArgumentException("getViewForPositionAndType returned"
                                + " a view which does not have a ViewHolder");
                    } else if (holder.shouldIgnore()) {
                        throw new IllegalArgumentException("getViewForPositionAndType returned"
                                + " a view that is ignored. You must call stopIgnoring before"
                                + " returning this view.");
                    }
                }

主流程 5
我们看一下主流程第83行代码。
holder = getRecycledViewPool().getRecycledView(type);
通过RecyclerView 的ViewHolder缓存池获取holder。
通过holder.resetInternal();方法将holder复位，为后续重新绑定做好准备。

主流程 6
我们看一下主流程第92行代码。
holder = mAdapter.createViewHolder(RecyclerView.this, type);创建新的holder

主流程 7
我们看一下主流程第119行代码。
if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid())
判断是否要重新绑定ViewHolder。

主流程就是这样了。

三 总结

经过上面的分析，我们可以看出RecyclerView 缓存机制（Recycler）大致可以分为5级。
第一级 通过mChangedScrap匹配 position或者id获取holder缓存。
第二级 从mAttachedScrap中通过匹配position获取holder缓存，或者通过ChildHelper找到隐藏但是没有被移除的View，通过getChildViewHolderInt(view)方法获取holder缓存，或者
从mCachedViews中通过匹配position获取holder缓存。
第三级 从mAttachedScrap中通过匹配id获取holder缓存，或者
从mCachedViews中通过匹配id获取holder缓存。
第四级 从ViewCacheExtension获取holder缓存。
第五级 通过RecyclerView 的ViewHolder缓存池获取holder。

最后有什么理解不对的地方请大家多多指教。谢谢。