Adding optional callback argument to setState

[ngavalas]

We eventually want to batch calls to setState for performance reasons, and this no longer guarantees immediate, synchronous execution of these calls to setState. As such, this commit adds an optional callback parameter so that actions can be taken once the call actually runs.

@phunt says we shouldn't make the synchronous operation guarantee and @jordow agrees.

[jordwalke]

👍

[leebyron]

this should probably be flowed through as a 2nd arg to this.replaceState, another function accessible by client code and where the ultimate work of generalizing this will go.

[petehunt]

What if we put these in ReactCompositeComponent._performComponentUpdate() instead? That way we won't have to copy-paste it everywhere, and it'll be closer to where the batching logic will eventually live.

[petehunt]

A simple test case here would be great too, since it's unlikely this will have many callsites in prod code for a while it may break and go unnoticed without one.

[ngavalas]

Will add unit tests.

About putting it _performComponentUpdate(), that's where I originally was trying to put it, but I see a small problem with replaceState in particular. Perhaps I'm misunderstanding the component lifecycle (strongly possible), but it appears that once in a while, the "new" state is just dumped into _pendingState, in which case _performComponentUpdate() isn't called in that particular call to replaceState.

[yungsters]

De-dupe this with the preceding comment paragraph? Could just delete the first sentence and smash them together.