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Logging

When you instantiate your store, you can optionally pass it a list of actionObservers and a list of stateObservers, which may be used for logging:

var store = Store<AppState>(
  initialState: state,
  actionObservers: [Log.printer(formatter: Log.verySimpleFormatter)],
  stateObservers: [StateLogger()],
);

The ActionObserver is an abstract class with an observe method which you can implement to be notified of any action dispatching:

abstract class ActionObserver<St> {

   void observe(
      ReduxAction<St> action, 
      int dispatchCount, {
      required bool ini,
      }
   );
}

The above observer will actually be called twice, one with ini==true for the INITIAL action observation, and one with ini==false for the END action observation,

In more detail:

  1. The INI action observation means the action was just dispatched and haven't changed anything yet. After that, it may do sync stuff, and it may or may not start async processes, depending on its reducer being sync or async.

  2. The END action observation means the action reducer has just finished returning a new state, thus changing the store state. Only after getting END states you may see store changes.

  3. The state observation is therefore called as soon as possible after the store change has taken place, i.e., right after the END action observation. However, it contains a copy of both the state before the action INI and the state after the action END, in case you need to compare them.

Please note, unless the action reducer is synchronous, getting an END action observation doesn't mean that all the action effects have finished, because the action may have started async processes that may well last into the future. And these processes may later dispatch other actions that will change the store state. However, it does mean that the action can no longer change the state directly.

Meanwhile, the StateObserver is an abstract class which you can implement to be notified of state changes. This observer can also be used for logging, but its main goal is to implement collecting metrics, which is why I'll discuss it here.