Data loading

gistfile1.txt

We have many different types of data being loaded. Right now, we have Board and List<TrelloAction>, so we'll focus on those.

There are 2 possible data sources, the database and service.

For both of those, there are 3 possible states: NOT_LOADING, LOADING, and LOADED. Let's call this a LoadState enum.

This can be represented by a class TObjectLoadingState. It has three fields: LoadState mDbLoadState, LoadState mServiceLoadState, and TObject mData or List<TObject> mData.

BoardActivityData::State contains a TObjectLoadingState for each of the pieces of data being loaded.

public static class State {
  final TObjectLoadingState<Board> mBoardState;
  final TObjectListLoadingState<List<TrelloAction>> mActionsState;
  ...
  public TObjectListLoadingState getActionsState() {
    return mActionsState;
  }
  ...
  public List<TrelloAction> getActions() {
    return getActionsState().getData();
  }
  ...
  public State withActions(List<TrelloAction> actions, LoadState dbLoadState, LoadState serviceLoadState) {
    return new State(mBoardState, new TObjectListLoadingState(actions, dbLoadState, serviceLoadState);
  }
}


Benefits:
- Extendable. Every piece of data that we load can fit this pattern.
- Explicit. Gives a clear indication of where the data was loaded from and what state it currently is in (e.g. we can wait until it's loaded from the service, we can cancel a service load, etc)
- Cruft free. Moves loading state out of the board activity so we don't end up with 50 loading state objects.

Cons:
- GC churn.
- Verboseness