android - When do we use the recyclerView.setHasFixedSize?

Question

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.

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.

Answer 1

It's interesting for the RecyclerView to know if its size (width and height dimensions) depends on the adapter content to avoid expensive layout operations. If the RecyclerView knows in advance that its size doesn't depend on the adapter content, then it will skip checking if its size should change every time an item is added or removed from the adapter. This is especially important because inserting an deleting elements can happen very often.

If the size of the RecyclerView (the RecyclerView itself)...

...doesn't depend on the adapter content:

mRecyclerView.setHasFixedSize(true);

...depends on the adapter content:

mRecyclerView.setHasFixedSize(false);

If you check the RecyclerView class you'll see it in more details because as of right now mHasFixedSize isn't used in that many places in that class.

Setting it as true doesn't mean that the RecyclerView size is fixed, just means it won't change because of change in the adapter content. For example the RecyclerView size can change because of a size change on its parent.