Add store.resetState for reverting to initial state value

[tgriesser]

I have a situation where I'd like to reset the entire app state to the original value (after ActionTypes.INIT is run), but couldn't find a clean way to write it as a plugin without replacing the majority of createStore.

I can also imagine situations where you'd want to replace the entire currentState with a new value, for example if you wanted to re-hydrate the entire app state from a different serialized session.

So I wanted to see if this resetState or an alternate more generic replaceState seemed like a good enough use case to add to the core API. The replaceState would look similar to this PR, but would pass in the state value and then run the init dispatch:

/**
 * Replaces the state tree with a new value and runs "INIT"
 *
 * @returns {void}
 */
function replaceState(newState) {
  currentState = newState;
  dispatch({ type: ActionTypes.INIT });
}


// When a store is created, an "INIT" action is dispatched so that every
// reducer returns their initial state. This effectively populates
// the initial state tree.
replaceState(initialState);

[alkhe]

We do this in fluxette.

[gaearon]

I don't think there is a need for this in core.

Store API must be as simple as possible so it's easy to write extensions like Redux DevTools.
Even one method is a huge addition.

See how to implement this in userland:

function makeHydratable(reducer, hydrateActionType) {
  return function (state, action) {
    switch (action.type) {
    case hydrateActionType:
      return reducer(action.state, action);
    default:
      return reducer(state, action);
    } 
  }
}

Usage:

const HYDRATE_STATE = 'HYDRATE_STATE';
const reducer = combineReducers(myReducers);
const hydratableReducer = makeHydratable(reducer, HYDRATE_STATE);
const store = createStore(hydratableReducer, initialState);

// later
store.dispatch({
  type: HYDRATE_STATE,
  state: initialState
});

[tgriesser]

I assume you'd need to also dispatch({type: ActionTypes.INIT}) to re-init any of the reducers which aren't covered in the hydrate state?

[gaearon]

Right!

[gaearon]

Oh wait, you wouldn't need to. Just pass action.state to root reducer.

[gaearon]

Here's the fixed version:

function makeHydratable(reducer, hydrateActionType) {
  return function (state, action) {
    switch (action.type) {
    case hydrateActionType:
      return reducer(action.state, action);
    default:
      return reducer(state, action);
    } 
  }
}

[tgriesser]

Awesome, so obvious now!

[vicentedealencar]

@gaearon I don't see a reason to call the reducer when the action is hydrateActionType, I don`t think any reducer should handle this action. I understand that the reducers would all ignore it and return state as is, so why not short-circuit the reducer returning action.state?

I am probably missing something, but I have it implemented this way..

[gaearon]

I understand that the reducers would all ignore it and return state as is, so why not short-circuit the reducer returning action.state?

The author wanted to be able to specify non-full state, and have reducers return initial state in this case: #658 (comment).

[catamphetamine]

@tgriesser Why would you do this:

function replaceState(newState) {
  currentState = newState;
  dispatch({ type: ActionTypes.INIT });
}

Instead of this:

function resetState() {
  currentState = {};
  dispatch({ type: ActionTypes.INIT });
}

What's the point of currentState = newState when it's gonna be overridden with the initial reducer state leaves anyway, so the new state is just gonna be the combined initial state.
Perhaps to store some non-reduceable data in the state tree but I don't see a motivation for that.