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);
}