Platform: createReducer should support intercept default return value

[vthinkxie]

I try to implement an undo-redo app with meta-reducer, in previous ngrx version, we could write a reducer like

const undoRedo = (reducer) => {
  let history = {
    past: [], 
    present: initialState, 
    future: []
  }
  return (state, action) => {
      switch (action.type) {
        case 'UNDO':
          // use first past state as next present ...
          const previous = history.past[0]
          // ... and remove from past
          const newPast = history.past.slice(1)
          history = {
            past: newPast,
            present: previous,
            // push present into future for redo
            future: [history.present, ...history.future]
          }
          return previous
        case 'REDO':
          // use first future state as next present ...
          const next = history.future[0]
          // ... and remove from future
          const newFuture = history.future.slice(1)
          history = {
            // push present into past for undo
            past: [history.present, ...history.past],
            present: next,
            future: newFuture
          }
          return next
        default:
          // derive next state
          const newPresent = reducer(state, action)
          history = {
              // push previous present into past for undo
              past: [history.present, ...history.past],
              present: newPresent,
              future: [] // clear future
          }
          return newPresent
      }
  }
}

and we could not do it with new createReducer api, since there is no way to intercept the default state.

Describe any alternatives/workarounds you're currently using

Other information:

If accepted, I would be willing to submit a PR for this feature

[ ] Yes (Assistance is provided if you need help submitting a pull request)
[x] No

[timdeschryver]

I want to mention that using the switch-case reducers is still a valid way to create your reducers, and in my opinion the preferred way to create meta-reducers.

[vthinkxie]

yes, switch-case is ok, but the switch-case has been removed from the official doc, it would better to add some operators like 'on' to handle this

[timdeschryver]

We discussed this and decided we will not add this feature.
The reason is that we don't have a good use-case for it.
We might re-evaluate later.

[miguel-leon]

Same as with using on(types, reducer) to replace case clauses, there should exist something like onAnythingElse(reducer) to replace the default clause for when you want a reducer to operate in any other type of action not covered. And so, consistently and thoroughly replace usage of the switch statement.

[roddc]

Same as with using on(types, reducer) to replace case clauses, there should exist something like onAnythingElse(reducer) to replace the default clause for when you want a reducer to operate in any other type of action not covered. And so, consistently and thoroughly replace usage of the switch statement.

Hi, do you mean creating a reducer using createReducer and put it in the MetaReducer? Could you give an example?