Create a version of `handleActions` which supports async actions too

[johanneslumpe]

We want to be able to support handlers in the form of:

{
    begin: (state, action) => {},
    end: {
      next: (state, action) => {},
      throw: (state, action) => {},
    }
  }

In order to do so, we have to inspect themeta field of the action and switch on the async field there. We also have to make sure that the handlers receive all arguments the original action receives, so that we can do optimistic updates easily.

In case an async handler receives a non-async action it should just handle it the same way it would handle the end status of an async action.

[acdlite]

I have a branch https://github.com/acdlite/redux-actions/tree/sequence that solves this pretty well. I'll update the docs soon and get it merged in.

[kpaxqin]

@johanneslumpe @acdlite
Here is my idea

Example here:

const handler = (state, action) => newState

const reducer = createReducer()
  .when([ACTION_FOO, ACTION_BAR], handlerForBothActions) // share handler for multi actions
  .when('BAZ', handler)    // optimistic update here if you need
  .done(handler)               // handle 'BAZ_COMPLETED'
  .failed(errorHandler)      // handle 'BAZ_FAILED'
  .build(initValue); 

[dthree]

I don't know why async isn't even mentioned in the docs. Making at least one async call in any real web app seems unavoidable. Am I missing something? Currently looking at this library to handle it.