RecyclerView 缓存策略

// 1) Find by position from scrap/hidden list/cache
if (holder == null) {
    holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
    if (holder != null) {
        if (!validateViewHolderForOffsetPosition(holder)) {
            // recycle holder (and unscrap if relevant) since it can't 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();
                }
                recycleViewHolderInternal(holder);
            }
            holder = null;
        } else {
            fromScrapOrHiddenOrCache = 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() + exceptionLabel());
    }

    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);
        if (holder != null) {
            // update position
            holder.mPosition = offsetPosition;
            fromScrapOrHiddenOrCache = 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"
                        + exceptionLabel());
            } else if (holder.shouldIgnore()) {
                throw new IllegalArgumentException("getViewForPositionAndType returned"
                        + " a view that is ignored. You must call stopIgnoring before"
                        + " returning this view." + exceptionLabel());
            }
        }
    }
    if (holder == null) { // fallback to pool
        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;
        }
        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");
        }
    }
}

recyclerview 的 getViewForPosition 会先判断holder为不为空，为空就去getScrapViewForPosition 方法获取holder，如果这时候holder还是空，就去执行viewCacheExtension$getViewFromPositionAndType方法，返回view 并且实例化成holder。第三部，如果这时候 holder还是空的话，就从recyclerviewpool调用getRecyclerView里面根据type获取 holder。第四部。如果这时候 holder还是空的话，就去调用adapter的oncreateViewHolder方法。返回holder。经过以上四部，最后holder肯定不是空了。

流程

一开始缓存都是空的，所以会走oncreateviewholder方法来创建viewholder，然后通过onbindviewholder来绑定数据。

假如屏幕有一次只能显示3个item，然后将item0 滑出屏幕，就会将item0加入到长度为空的mcacheviews，同理item1也滑出屏幕，加入mcacheviews。当item2滑出屏幕，而mcacheviews默认长度是2，就会将item0转移到recyclerdviewpool中，item2放到mcacheviews中。然后继续创建item3 item4 item5，（其中item0recyclerdviewpool，item1和item2在mcacheviews）在当要创建item6时，复用了recyclerdviewpool里面的item0了。然后循环复用recyclerdviewpool里面的item。