RecyclerView setHasFixedSize(true)的意义

    /**     * RecyclerView can perform several optimizations if it can know in advance that RecyclerView's     * size is not affected by the adapter contents. RecyclerView can still change its size based     * on other factors (e.g. its parent's size) but this size calculation cannot depend on the     * size of its children or contents of its adapter (except the number of items in the adapter).     * <p>     * If your use of RecyclerView falls into this category, set this to {@code true}. It will allow     * RecyclerView to avoid invalidating the whole layout when its adapter contents change.     *     * @param hasFixedSize true if adapter changes cannot affect the size of the RecyclerView.     */    public void setHasFixedSize(boolean hasFixedSize) {        mHasFixedSize = hasFixedSize;    }

注释说当知道Adapter内Item的改变不会影响RecyclerView宽高的时候，可以设置为true让RecyclerView避免重新计算大小。

设置为true,再调用notifyDataSetChanged()，发现大小重新计算了，看来理解出现错误了。还是再看一下哪些地方用到这个mHasFixedSize吧。

首先是onMeasure里用到，这个和自定义LayoutManager相关，先不管它。

剩下的就是triggerUpdateProcessor()方法了：

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

看一下triggerUpdateProcessor方法被哪些调用

onItemRangeChanged(),

onItemRangeInserted(),

onItemRangeRemoved(),

onItemRangeMoved()

这样看就很明白了，当调用Adapter的增删改插方法，最后就会根据mHasFixedSize这个值来判断需要不需要requestLayout()；

再来看一下notifyDataSetChanged()执行的代码，最后是调用了onChanged，调用了requestLayout()，会去重新测量宽高。

        @Override        public void onChanged() {            assertNotInLayoutOrScroll(null);            mState.mStructureChanged = true;            setDataSetChangedAfterLayout();            if (!mAdapterHelper.hasPendingUpdates()) {                requestLayout();            }        }

总结：当我们确定Item的改变不会影响RecyclerView的宽高的时候可以设置setHasFixedSize(true)，并通过Adapter的增删改插方法去刷新RecyclerView，而不是通过notifyDataSetChanged()。（其实可以直接设置为true，当需要改变宽高的时候就用notifyDataSetChanged()去整体刷新一下）

        public final void notifyItemChanged(int position) {            mObservable.notifyItemRangeChanged(position, 1);        }        public final void notifyItemChanged(int position, Object payload) {            mObservable.notifyItemRangeChanged(position, 1, payload);        }        public final void notifyItemRangeChanged(int positionStart, int itemCount) {            mObservable.notifyItemRangeChanged(positionStart, itemCount);        }        public final void notifyItemRangeChanged(int positionStart, int itemCount, Object payload) {            mObservable.notifyItemRangeChanged(positionStart, itemCount, payload);        }                public final void notifyItemInserted(int position) {            mObservable.notifyItemRangeInserted(position, 1);        }                public final void notifyItemMoved(int fromPosition, int toPosition) {            mObservable.notifyItemMoved(fromPosition, toPosition);        }        public final void notifyItemRangeInserted(int positionStart, int itemCount) {            mObservable.notifyItemRangeInserted(positionStart, itemCount);        }        public final void notifyItemRemoved(int position) {            mObservable.notifyItemRangeRemoved(position, 1);        }        public final void notifyItemRangeRemoved(int positionStart, int itemCount) {            mObservable.notifyItemRangeRemoved(positionStart, itemCount);        }