RecyclerView源码分析

    public static class RecycledViewPool {      private SparseArray<ArrayList<ViewHolder>> mScrap =              new SparseArray<ArrayList<ViewHolder>>();      private SparseIntArray mMaxScrap = new SparseIntArray();      private int mAttachCount = 0;        private static final int DEFAULT_MAX_SCRAP = 5;        public void clear() {          mScrap.clear();      }        public void setMaxRecycledViews(int viewType, int max) {          mMaxScrap.put(viewType, max);          final ArrayList<ViewHolder> scrapHeap = mScrap.get(viewType);          if (scrapHeap != null) {              while (scrapHeap.size() > max) {                  scrapHeap.remove(scrapHeap.size() - 1);              }          }      }        public ViewHolder getRecycledView(int viewType) {          final ArrayList<ViewHolder> scrapHeap = mScrap.get(viewType);          if (scrapHeap != null && !scrapHeap.isEmpty()) {              final int index = scrapHeap.size() - 1;              final ViewHolder scrap = scrapHeap.get(index);              scrapHeap.remove(index);              return scrap;          }          return null;      }        int size() {          int count = 0;          for (int i = 0; i < mScrap.size(); i ++) {              ArrayList<ViewHolder> viewHolders = mScrap.valueAt(i);              if (viewHolders != null) {                  count += viewHolders.size();              }          }          return count;      }        public void putRecycledView(ViewHolder scrap) {          final int viewType = scrap.getItemViewType();          final ArrayList scrapHeap = getScrapHeapForType(viewType);          if (mMaxScrap.get(viewType) <= scrapHeap.size()) {              return;          }          if (DEBUG && scrapHeap.contains(scrap)) {              throw new IllegalArgumentException("this scrap item already exists");          }          scrap.resetInternal();          scrapHeap.add(scrap);      }        void attach(Adapter adapter) {          mAttachCount++;      }        void detach() {          mAttachCount--;      }          /**       * Detaches the old adapter and attaches the new one.       * <p>       * RecycledViewPool will clear its cache if it has only one adapter attached and the new       * adapter uses a different ViewHolder than the oldAdapter.       *       * @param oldAdapter The previous adapter instance. Will be detached.       * @param newAdapter The new adapter instance. Will be attached.       * @param compatibleWithPrevious True if both oldAdapter and newAdapter are using the same       *                               ViewHolder and view types.       */      void onAdapterChanged(Adapter oldAdapter, Adapter newAdapter,              boolean compatibleWithPrevious) {          if (oldAdapter != null) {              detach();          }          if (!compatibleWithPrevious && mAttachCount == 0) {              clear();          }          if (newAdapter != null) {              attach(newAdapter);          }      }        private ArrayList<ViewHolder> getScrapHeapForType(int viewType) {          ArrayList<ViewHolder> scrap = mScrap.get(viewType);          if (scrap == null) {              scrap = new ArrayList<ViewHolder>();              mScrap.put(viewType, scrap);              if (mMaxScrap.indexOfKey(viewType) < 0) {                  mMaxScrap.put(viewType, DEFAULT_MAX_SCRAP);              }          }          return scrap;      }  }

public abstract static class ViewCacheExtension {      abstract public View getViewForPositionAndType(Recycler recycler, int position, int type);  }

获取某个位置需要展示的View，先检查是否有可复用的View，没有则创建新View并返回。具体过程为：

（1）检查 mChangedScrap ，若匹配到则返回相应holder

（2）检查 mAttachedScrap ，若匹配到且holder有效则返回相应holder

（3）检查 mViewCacheExtension ，若匹配到则返回相应holder

（4）检查 mRecyclerPool ，若匹配到则返回相应holder

（5）否则执行 Adapter.createViewHolder() ，新建holder实例

（6）返回holder.itemView

（7）注：以上每步匹配过程都可以匹配position或itemId（如果有stableId）

public static abstract class ViewHolder {      View itemView;//itemView      int mPosition;//位置      int mOldPosition;//上一次的位置      long mItemId;//itemId      int mItemViewType;//itemViewType      int mPreLayoutPosition;      int mFlags;//ViewHolder的状态标志      int mIsRecyclableCount;      Recycler mScrapContainer;//若非空，表明当前ViewHolder对应的itemView可以复用  ｝

关于 ViewHolder ，我最想介绍的是 mFlags 。

FLAG_BOUND ——ViewHolder已经绑定到某个位置，mPosition、mItemId、mItemViewType都有效

FLAG_UPDATE ——ViewHolder绑定的View对应的数据过时需要重新绑定，mPosition、mItemId还是一致的

FLAG_INVALID ——ViewHolder绑定的View对应的数据无效，需要完全重新绑定不同的数据

FLAG_REMOVED ——ViewHolder对应的数据已经从数据集移除

FLAG_NOT_RECYCLABLE ——ViewHolder不能复用

FLAG_RETURNED_FROM_SCRAP ——这个状态的ViewHolder会加到scrap list被复用。

FLAG_CHANGED ——ViewHolder内容发生变化，通常用于表明有ItemAnimator动画

FLAG_IGNORE ——ViewHolder完全由LayoutManager管理，不能复用

FLAG_TMP_DETACHED ——ViewHolder从父RecyclerView临时分离的标志，便于后续移除或添加回来

FLAG_ADAPTER_POSITION_UNKNOWN ——ViewHolder不知道对应的Adapter的位置，直到绑定到一个新位置

FLAG_ADAPTER_FULLUPDATE ——方法 addChangePayload(null) 调用时设置

public static abstract class ItemDecoration {      //itemView绘制之前绘制，通常这部分UI会被itemView盖住      public void onDraw(Canvas c, RecyclerView parent, State state) {          onDraw(c, parent);      }        //itemView绘制之后绘制，这部分UI盖在itemView上面      public void onDrawOver(Canvas c, RecyclerView parent, State state) {          onDrawOver(c, parent);      }        //设置itemView上下左右的间距      public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {          getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),                  parent);      }  }