RecyclerView 缓存机制

//RecyclerView是ViewGroup的子类
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2 {
    ...
    private void initChildrenHelper() {
        mChildHelper = new ChildHelper(new ChildHelper.Callback() {
            ...
            @Override
            public void addView(View child, int index) {
                if (VERBOSE_TRACING) {
                    TraceCompat.beginSection("RV addView");
                }
                //直接调用ViewGroup.addView()
                RecyclerView.this.addView(child, index);
                if (VERBOSE_TRACING) {
                    TraceCompat.endSection();
                }
                dispatchChildAttached(child);
            }
        }
    }
    ...
}

void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,LayoutState layoutState, LayoutChunkResult result) {
        //获得下一个表项
        View view = layoutState.next(recycler);
        ...
        LayoutParams params = (LayoutParams) view.getLayoutParams();
        if (layoutState.mScrapList == null) {
            //将表项插入到列表中
            if (mShouldReverseLayout == (layoutState.mLayoutDirection == LayoutState.LAYOUT_START)) {
                addView(view);
            } else {
                addView(view, 0);
            }
        }
        ...
}

public class LinearLayoutManager extends RecyclerView.LayoutManager implements ItemTouchHelper.ViewDropHandler, RecyclerView.SmoothScroller.ScrollVectorProvider {
    ...
    int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
            RecyclerView.State state, boolean stopOnFocusable) {
        ...
        //recyclerview 剩余空间
        int remainingSpace = layoutState.mAvailable + layoutState.mExtra;
        LayoutChunkResult layoutChunkResult = mLayoutChunkResult;
        //不断填充，直到空间消耗完毕
        while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
            layoutChunkResult.resetInternal();
            if (VERBOSE_TRACING) {
                TraceCompat.beginSection("LLM LayoutChunk");
            }
            //填充一个表项
            layoutChunk(recycler, state, layoutState, layoutChunkResult);
            ...
        }
        ...
    }
}

/**
 * Lay out all relevant child views from the given adapter.
 * 布局所有给定adapter中相关孩子视图
 */
public void onLayoutChildren(Recycler recycler, State state) {
 	Log.e(TAG, "You must override onLayoutChildren(Recycler recycler, State state) ");
}

public class LinearLayoutManager {
    static class LayoutState {       
       /**
         * Gets the view for the next element that we should layout.
         * 获得下一个元素的视图用于布局
         */
        View next(RecyclerView.Recycler recycler) {
            if (mScrapList != null) {
                return nextViewFromScrapList();
            }
            //调用了Recycler.getViewForPosition()
            final View view = recycler.getViewForPosition(mCurrentPosition);
            mCurrentPosition += mItemDirection;
            return view;
        }
    }
}

public class RecyclerView {
    /**
     * A Recycler is responsible for managing scrapped or detached item views for reuse.
     *  Recycler负责管理scrapped和detached表项的复用
     */
    public final class Recycler {
      ...
    }
}

public class RecyclerView {
    public final class Recycler {
       /**
         * Attempts to get the ViewHolder for the given position, either from the Recycler scrap,
         * cache, the RecycledViewPool, or creating it directly.
         * 尝试获得指定位置的ViewHolder，要么从scrap，cache，RecycledViewPool中获取，要么直接重新创建
         */
        @Nullable
        ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                boolean dryRun, long deadlineNs) {
            ...
            boolean fromScrapOrHiddenOrCache = false;
            ViewHolder holder = null;
            //0 从changed scrap集合中获取ViewHolder
            if (mState.isPreLayout()) {
                holder = getChangedScrapViewForPosition(position);
                fromScrapOrHiddenOrCache = holder != null;
            }
            //1. 通过position从attach scrap或一级回收缓存中获取ViewHolder
            if (holder == null) {
                holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
                ...
            }
            
            if (holder == null) {
                ...
                final int type = mAdapter.getItemViewType(offsetPosition);
                //2. 通过id在attach scrap集合和一级回收缓存中查找viewHolder
                if (mAdapter.hasStableIds()) {
                    holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),
                            type, dryRun);
                    ...
                }
                //3. 从自定义缓存中获取ViewHolder
                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);
                    ...
                }
                //4.从缓存池中拿ViewHolder
                if (holder == null) { // fallback to pool
                    ...
                    holder = getRecycledViewPool().getRecycledView(type);
                    ...
                }
                //所有缓存都没有命中，只能创建ViewHolder
                if (holder == null) {
                    ...
                    holder = mAdapter.createViewHolder(RecyclerView.this, type);
                    ...
                }
            }

            boolean bound = false;
            if (mState.isPreLayout() && holder.isBound()) {
                // do not update unless we absolutely have to.
                holder.mPreLayoutPosition = position;
            }
            //只有invalid的viewHolder才能绑定视图数据
            else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {
                final int offsetPosition = mAdapterHelper.findPositionOffset(position);
                //获得ViewHolder后，绑定视图数据
                bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
            }
            ...
            return holder;
        }
    }
}

ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                boolean dryRun, long deadlineNs) {
      ...
      if (mState.isPreLayout()) {
          holder = getChangedScrapViewForPosition(position);
          fromScrapOrHiddenOrCache = holder != null;
      }
      ...
}

ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                boolean dryRun, long deadlineNs) {
      ...
      if (holder == null) {
          holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
          //下面一段代码蕴含着一个线索，买个伏笔，先把他略去
          ...
      }
      ...
}

//省略非关键代码
        /**
         * Returns a view for the position either from attach scrap, hidden children, or cache.
         * 从attach scrap，hidden children或者cache中获得指定位置上的一个ViewHolder
         */
        ViewHolder getScrapOrHiddenOrCachedHolderForPosition(int position, boolean dryRun) {
            final int scrapCount = mAttachedScrap.size();
            // Try first for an exact, non-invalid match from scrap.
            //1.在attached scrap中搜索ViewHolder
            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())) {
                    holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
                    return holder;
                }
            }
            //2.从移除屏幕的视图中搜索ViewHolder，找到了之后将他存入scrap回收集合中
            if (!dryRun) {
                View view = mChildHelper.findHiddenNonRemovedView(position);
                if (view != null) {
                    final ViewHolder vh = getChildViewHolderInt(view);
                    mChildHelper.unhide(view);
                    int layoutIndex = mChildHelper.indexOfChild(view);
                    ...
                    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.
            //3.在缓存中搜索ViewHolder
            final int cacheSize = mCachedViews.size();
            for (int i = 0; i < cacheSize; i++) {
                final ViewHolder holder = mCachedViews.get(i);
                //若找到ViewHolder，还需要对ViewHolder的索引进行匹配判断
                if (!holder.isInvalid() && holder.getLayoutPosition() == position) {
                    ...
                    return holder;
                }
            }
            return null;
        }

public final class Recycler {
    final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();
    ...
    final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>();
    ...
    RecycledViewPool mRecyclerPool;
}

ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                boolean dryRun, long deadlineNs) {
      ...
      if (holder == null) {
          holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
          //下面一段代码蕴含这一个线索，买个伏笔，先把他略去
          if (holder != null) {
               //检验ViewHolder有效性
               if (!validateViewHolderForOffsetPosition(holder)) {
                    // recycle holder (and unscrap if relevant) since it can not be used
                    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();
                          }
                          //若不满足有效性检验，则回收ViewHolder
                          recycleViewHolderInternal(holder);
                    }
                    holder = null;
               } else {
                    fromScrapOrHiddenOrCache = true;
               }
          }
      }
      ...
}

ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                boolean dryRun, long deadlineNs) {
      ...
      //只有当Adapter设置了id，才会进行这次查找
      if (mAdapter.hasStableIds()) {
           holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),type, dryRun);
           if (holder != null) {
                // update position
                holder.mPosition = offsetPosition;
                fromScrapOrHiddenOrCache = true;
           }
      }
      ...
}

ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                boolean dryRun, long deadlineNs) {
      ...
      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) {
                //获得view对应的ViewHolder
                holder = getChildViewHolder(view);
                if (holder == null) {
                    throw new IllegalArgumentException("getViewForPositionAndType returned"
                                    + " a view which does not have a ViewHolder"
                                    + exceptionLabel());
                } else if (holder.shouldIgnore()) {
                   throw new IllegalArgumentException("getViewForPositionAndType returned"
                                    + " a view that is ignored. You must call stopIgnoring before"
                                    + " returning this view." + exceptionLabel());
                }
            }
      }
      ...
}

/**
 * ViewCacheExtension is a helper class to provide an additional layer of view caching that can
 * be controlled by the developer.
 * ViewCacheExtension提供了额外的表项缓存层，用户帮助开发者自己控制表项缓存
 * <p>
 * When {@link Recycler#getViewForPosition(int)} is called, Recycler checks attached scrap and
 * first level cache to find a matching View. If it cannot find a suitable View, Recycler will
 * call the {@link #getViewForPositionAndType(Recycler, int, int)} before checking
 * {@link RecycledViewPool}.
 * 当Recycler从attached scrap和first level cache中未能找到匹配的表项时，它会在去RecycledViewPool中查找之前，先尝试从自定义缓存中查找
 * <p>
 */
public abstract static class ViewCacheExtension {

    /**
     * Returns a View that can be binded to the given Adapter position.
     * <p>
     * This method should <b>not</b> create a new View. Instead, it is expected to return
     * an already created View that can be re-used for the given type and position.
     * If the View is marked as ignored, it should first call
     * {@link LayoutManager#stopIgnoringView(View)} before returning the View.
     * <p>
     * RecyclerView will re-bind the returned View to the position if necessary.
     */
    public abstract View getViewForPositionAndType(Recycler recycler, int position, int type);
}

ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                boolean dryRun, long deadlineNs) {
      ...
      if (holder == null) { 
          ...
          //从回收池中获取ViewHolder对象
          holder = getRecycledViewPool().getRecycledView(type);
          if (holder != null) {
               holder.resetInternal();
               if (FORCE_INVALIDATE_DISPLAY_LIST) {
                    invalidateDisplayListInt(holder);
               }
          }
      }
      ...
}

public final class Recycler {
    ...
    RecycledViewPool mRecyclerPool;
    //获得RecycledViewPool实例
    RecycledViewPool getRecycledViewPool() {
          if (mRecyclerPool == null) {
              mRecyclerPool = new RecycledViewPool();
          }
          return mRecyclerPool;
    }
    ...
}
public static class RecycledViewPool {
    ...
    //从回收池中获取ViewHolder对象
    public ViewHolder getRecycledView(int viewType) {
          final ScrapData scrapData = mScrap.get(viewType);
          if (scrapData != null && !scrapData.mScrapHeap.isEmpty()) {
              final ArrayList<ViewHolder> scrapHeap = scrapData.mScrapHeap;
              return scrapHeap.remove(scrapHeap.size() - 1);
          }
          return null;
    }
    ...
}

    public static class RecycledViewPool {
        //同类ViewHolder缓存个数上限
        private static final int DEFAULT_MAX_SCRAP = 5;

        /**
         * Tracks both pooled holders, as well as create/bind timing metadata for the given type.
         * 回收池中存放单个类型ViewHolder的容器
         */
        static class ScrapData {
            //同类ViewHolder存储在ArrayList中
            ArrayList<ViewHolder> mScrapHeap = new ArrayList<>();
            //每种类型的ViewHolder最多存5个
            int mMaxScrap = DEFAULT_MAX_SCRAP;
        }
        //回收池中存放所有类型ViewHolder的容器
        SparseArray<ScrapData> mScrap = new SparseArray<>();
        ...
        //ViewHolder入池 按viewType分类入池，一个类型的ViewType存放在一个ScrapData中
        public void putRecycledView(ViewHolder scrap) {
            final int viewType = scrap.getItemViewType();
            final ArrayList<ViewHolder> scrapHeap = getScrapDataForType(viewType).mScrapHeap;
            //如果超限了，则放弃入池
            if (mScrap.get(viewType).mMaxScrap <= scrapHeap.size()) {
                return;
            }
            if (DEBUG && scrapHeap.contains(scrap)) {
                throw new IllegalArgumentException("this scrap item already exists");
            }
            scrap.resetInternal();
            //回收时，ViewHolder从列表尾部插入
            scrapHeap.add(scrap);
        }
        //从回收池中获取ViewHolder对象
        public ViewHolder getRecycledView(int viewType) {
              final ScrapData scrapData = mScrap.get(viewType);
              if (scrapData != null && !scrapData.mScrapHeap.isEmpty()) {
                  final ArrayList<ViewHolder> scrapHeap = scrapData.mScrapHeap;
                  //复用时，从列表尾部获取ViewHolder（优先复用刚入池的ViewHoler）
                  return scrapHeap.remove(scrapHeap.size() - 1);
              }
              return null;
        }
}

ViewHolder tryGetViewHolderForPositionByDeadline(int position,boolean dryRun, long deadlineNs) {
            ...
            //所有缓存都没有命中，只能创建ViewHolder
            if (holder == null) {
                ...
                holder = mAdapter.createViewHolder(RecyclerView.this, type);
                ...
            }
            ...
            boolean bound = false;
            if (mState.isPreLayout() && holder.isBound()) {
                // do not update unless we absolutely have to.
                holder.mPreLayoutPosition = position;
            }
            //如果表项没有绑定过数据 或 表项需要更新 或 表项无效 且表项没有被移除时绑定表项数据
            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
                            + exceptionLabel());
                }
                final int offsetPosition = mAdapterHelper.findPositionOffset(position);
                //为表项绑定数据
                bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
            }
}

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

       // Try first for an exact, non-invalid match from scrap.
       //1.从attached scrap回收集合中
       for (int i = 0; i < scrapCount; i++) {
           final ViewHolder holder = mAttachedScrap.get(i);
           //只有当holder是有效时才返回
           if (!holder.wasReturnedFromScrap() && holder.getLayoutPosition() == position
                   && !holder.isInvalid() && (mState.mInPreLayout || !holder.isRemoved())) {
               holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
               return holder;
           }
       }
}

ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                boolean dryRun, long deadlineNs) {
      ...
      if (holder == null) { 
          ...
          //从回收池中获取ViewHolder对象
          holder = getRecycledViewPool().getRecycledView(type);
          if (holder != null) {
               //重置ViewHolder
               holder.resetInternal();
               if (FORCE_INVALIDATE_DISPLAY_LIST) {
                    invalidateDisplayListInt(holder);
               }
          }
      }
      ...
      //如果表项没有绑定过数据 或 表项需要更新 或 表项无效 且表项没有被移除时绑定表项数据
      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
                            + exceptionLabel());
          }
          final int offsetPosition = mAdapterHelper.findPositionOffset(position);
          //为表项绑定数据
          bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
      }
      ...
}

public abstract static class ViewHolder {
        /**
         * This ViewHolder has been bound to a position; mPosition, mItemId and mItemViewType
         * are all valid.
         * 绑定标志位
         */
        static final int FLAG_BOUND = 1 << 0;
        /**
         * This ViewHolder’s data is invalid. The identity implied by mPosition and mItemId
         * are not to be trusted and may no longer match the item view type.
         * This ViewHolder must be fully rebound to different data.
         * 无效标志位
         */
        static final int FLAG_INVALID = 1 << 2;
        //判断ViewHolder是否无效
        boolean isInvalid() {
            //将当前ViewHolder对象的flag和无效标志位做位与操作
            return (mFlags & FLAG_INVALID) != 0;
        }
        //判断ViewHolder是否被绑定
        boolean isBound() {
            //将当前ViewHolder对象的flag和绑定标志位做位与操作
            return (mFlags & FLAG_BOUND) != 0;
        }
        /**
         * 将ViewHolder重置
         */
        void resetInternal() {
            //将ViewHolder的flag置0
            mFlags = 0;
            mPosition = NO_POSITION;
            mOldPosition = NO_POSITION;
            mItemId = NO_ID;
            mPreLayoutPosition = NO_POSITION;
            mIsRecyclableCount = 0;
            mShadowedHolder = null;
            mShadowingHolder = null;
            clearPayload();
            mWasImportantForAccessibilityBeforeHidden = ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_AUTO;
            mPendingAccessibilityState = PENDING_ACCESSIBILITY_STATE_NOT_SET;
            clearNestedRecyclerViewIfNotNested(this);
        }
}