title: When do we use the recyclerView.setHasFixedSize?
body: <p>here's the thing:
  Anybody know the setHasFixedSize method? some says that it allows for optimizations if all items are of the same size, and in RecyclerView class from android.support.v7.widget, it commented with this: RecyclerView can perform several optimizations if it can know in advance that changes in
  adapter content cannot change the size of the RecyclerView itself. If your use of RecyclerView falls into this category, set this to true.</p>
  
  <p>What's that suppose to mean? Can anyone show me the context of using it or explain about the meaning "this category" above?
  thanks a lot.</p>