Public interface for custom state

[ShacharHarshuv]

We probably already have that function in, but we need to think about it we want to expose it and how. We also need to think about whether we want to expose a "writable" version and also options for how these states propagate from child / parent.

Custom State

We can extend the behavior of the form controls with custom state using the function createFormState. Many of the form states described before use it behind the scenes.

function createFormState<T, Args extends any[]>(
  descr: string,
  fallback: T,
  createInitialState: (form: Form<any>, ...args: Args) => T),
): {
  create: (...args: Args) => StateFactory, 
  get: (form: Form<any>) => T, // returns the fallback if doesn't exist
  set: (form: Form<any>, state: T) => void,
}

Here's an example of how you can use it:

// display-name.ts

export const {
  create: displayName, 
  get: getDisplayName,
  set: setDisplayName
} = createFormState(
  "Display Name",
  "",
  (form: Form<any>, displayName: string) => {
    return displayName;
  }
);

// my-form.ts

import { displayName, getDisplayName } from './my-custom-disabled-state';

const myForm = form({
  name: "John",
  age: 42,
}, displayName('Person'));

const displayNamed = getDisplayName(myForm);