RecyclerView的部分优化

1.在 onCreateViewHolder 里面写点击事件
这个很好理解，onBindViewHolder会多次调用

2.LinearLayoutManager.setInitialPrefetchItemCount
注意，此方法LinearLayoutManager专属

    public void setInitialPrefetchItemCount(int itemCount) {
        mInitialItemPrefetchCount = itemCount;
    }

mInitialItemPrefetchCount 在这里用到

    @Override
    public void collectInitialPrefetchPositions(int adapterItemCount,
            LayoutPrefetchRegistry layoutPrefetchRegistry) {
        final int direction = fromEnd
                ? LayoutState.ITEM_DIRECTION_HEAD
                : LayoutState.ITEM_DIRECTION_TAIL;
        int targetPos = anchorPos;
        for (int i = 0; i < mInitialItemPrefetchCount; i++) {
            if (targetPos >= 0 && targetPos < adapterItemCount) {
                layoutPrefetchRegistry.addPosition(targetPos, 0);
            } else {
                break; // no more to prefetch
            }
            targetPos += direction;
        }
    }

说白了就是预取操作。collectInitialPrefetchPositions定义当此LayoutManager的RecyclerView嵌套在另一个RecyclerView中时应预取多少内部项目。也就是说只有RecyclerView嵌套才会生效，否则无效

3.setHasFixedSize
setHasFixedSize 会给 mHasFixedSize 赋值，看一下用到的地方

// 第一处
    @Override
    protected void onMeasure(int widthSpec, int heightSpec) {
        if (mLayout == null) {
            defaultOnMeasure(widthSpec, heightSpec);
            return;
        }
        if (mLayout.mAutoMeasure) {
            ......
        } else {
            if (mHasFixedSize) {
                mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
                return;
            }
            // custom onMeasure
            if (mAdapterUpdateDuringMeasure) {
                eatRequestLayout();
                onEnterLayoutOrScroll();
                processAdapterUpdatesAndSetAnimationFlags();
                onExitLayoutOrScroll();

                if (mState.mRunPredictiveAnimations) {
                    mState.mInPreLayout = true;
                } else {
                    // consume remaining updates to provide a consistent state with the layout pass.
                    mAdapterHelper.consumeUpdatesInOnePass();
                    mState.mInPreLayout = false;
                }
                mAdapterUpdateDuringMeasure = false;
                resumeRequestLayout(false);
            }

            if (mAdapter != null) {
                mState.mItemCount = mAdapter.getItemCount();
            } else {
                mState.mItemCount = 0;
            }
            eatRequestLayout();
            mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
            resumeRequestLayout(false);
            mState.mInPreLayout = false; // clear
        }
    }
// 第二处
    private class RecyclerViewDataObserver extends AdapterDataObserver {

        void triggerUpdateProcessor() {
            if (POST_UPDATES_ON_ANIMATION && mHasFixedSize && mIsAttached) {
                RecyclerView.this.postOnAnimation(mUpdateChildViewsRunnable);
            } else {
                mAdapterUpdateDuringMeasure = true;
                requestLayout();
            }
        }
    }

mLayout.mAutoMeasure 默认是false，当然也可以自己设置。当mHasFixedSize是true的时候只会执行onMeasure。另外，每次notifyDataSetChanged的时候只会mUpdateChildViews而不用整个requestLayout。当然，有个前提，adapter的数据变化不会导致RecycleView的大小变化 。
总结，当RecycleView大小固定的时候，setHasFixedSize 为true可以使每次onMeasure和notifyDataSet的时候不用去request整个layout

4.多个RecycleView共用RecycledViewPool
RecycledViewPool 其实并不陌生，就是缓存机制的最后一个pool。如果这里还取不到view就重新create。具体我也写过，参考 RecyclerView的缓存机制和内存优化