Generic over states and transitions

[RunDevelopment]

PR for my suggestion under your reddit post.

Type changes:

• Removed all any types and as casts (except for one). This means that everything is now verified by TS.
• Removed the KeysOfTransition helper type. TypeScript can infer all state and transition types, so this helper is no longer necessary.
• Removed BaseStateConfig and BaseConfig. These were purely internal types. I don't know why you added them in the first place. I just used MachineStateConfig and MachineConfig everywhere.
• The type parameter of useStateMachine is no longer constrained. From what I see, this only got in the way of TS inferring the type. I'll re-add it if it's important.
• useStateMachineWithContext is now generic over states and transitions and not over the config type. This allows TS to infer all states and transitions. However, this is a breaking change for TS users.

Behavioral changes:

• reducer: I changed the currentState?.on to currentState.on. Since the state machine is now type-checked, users will get a compilation error if they try to transition into an undefined state. Therefore currentState must be non-nullable.

• Context will no longer default to {}. This was actually a bug of the old implementation. I fixed this bug here because TS would not compile (that's the advantage of not using any and as :) ). Example:

  const [state, send] = useStateMachine<{ foo: { bar: number } }>()({
      initial: 'inactive',
      states: {
          inactive: {
              on: { TOGGLE: 'active' },
          },
          active: {
              on: {
                  TOGGLE: {
                      target: 'inactive',
                      guard(context) {
                          // no type error but this will crash at runtime
                          return context.foo.bar > 4;
                      }
                  }
              },
          },
      },
  });
  
  console.log(state); // { value: 'inactive', nextEvents: ['TOGGLE'] }
  
  send('TOGGLE');
  
  console.log(state); // { value: 'active', nextEvents: ['TOGGLE'] }
  
  send('TOGGLE'); // crash

Feel free to request changes.

You can try out everything here.

[cassiozen]

Wow, @RunDevelopment, this is spectacular; thanks so much for your time!

I don't know why you added them in the first place.

Because I lack knowledge. This is the first project where I use TypeScript beyond the basics...

The type parameter of useStateMachine is no longer constrained. From what I see, this only got in the way of TS inferring the type. I'll re-add it if it's important.

It makes sense now. I don't think we need to re-add.

Therefore currentState must be non-nullable

👍

Context will no longer default to {}

👍

I also noticed you changed the reducer to always accept a type in the action and expose send as a function that dispatches { type: "Transition" ... instead of exposing the useReducer dispatch directly. A clever solution, very obvious, I don't know why I didn't think of that from the beginning 😅.

One change I will do in this case is renaming the internal send to dispatch to be more idiomatic - but I can do that separately, you already contributed with a lot.

Overall, thanks again a lot for your PR - this is as much an improvement for the library as a learning opportunity for me. Please let me know if you think of any other suggestions in the future.

[cassiozen]

Oh, I remembered why I this all that trickery with the send method: I wanted to completely reuse dispatch because it's stable across re-renders. There is the possibility that some user will pass send to child components - if these components use PureComponent or React.memo, an unstable send method will make them useless.

Not a big issue, can be definitely dealt with on a separate occasion.

[RunDevelopment]

Ahh, that's true. The methods we give out might behave the same but they aren't the same object.

I wonder, is there an idiomatic way to do this in the React world or will it be a WeakMap solution?

[cassiozen]

Yes, I can use either useMemo or useCallback.

[cassiozen]

@all-contributors please add @RunDevelopment for code, test and ideas.

[allcontributors[bot]]

@cassiozen

I've put up a pull request to add @RunDevelopment! :tada: