interfaces.ts

// This Tuple type (and tuple() function) is a hack to induce
// TypeScript to infer tuple types rather than array types. (Source:
// https://github.com/microsoft/TypeScript/issues/27179#issuecomment-422606990)
//
// Without it, [123, 'hi', true] will often get the type (string |
// number | boolean)[] instead of [number, string, boolean].
//
export type Tuple = any[] | [any];
export const tuple = <A extends Tuple>(... args: A) => args;

// Type ValidSelector captures TypeScript's notion of a valid object
// property name.
//
export type ValidSelector = string | number | symbol;

export type EventMessage<Selector extends ValidSelector, Args extends any[]> =
    { selector: Selector, args: Args };

export type RequestMessage<Selector extends ValidSelector, Args extends any[], Result extends Exclude<any, void>> =
    { selector: Selector, args: Args, callback: (result: Result) => void };

export type Message<Selector extends ValidSelector, Args extends any[], Result> =
    void extends Result ? EventMessage<Selector, Args> : RequestMessage<Selector, Args, Result>;

// Function message() is needed for similar reasons to tuple() above:
// to help TypeScript infer the correct literal type for the selector
// (as well as the arguments).
//
export const message = <S extends ValidSelector, A extends Tuple, R>(m: Message<S, A, R>) => m;

type MessagesProduct<I, ContextArgs extends any[]> = {
    [K in keyof I]: (I[K] extends (...args: [...ContextArgs, ...infer P]) => infer Q
        ? Message<K, P, Q>
        : never);
};

export type Messages<I, ContextArgs extends any[] = []> = MessagesProduct<I, ContextArgs>[keyof I];

export type Methods<M extends { selector: ValidSelector }, ContextArgs extends any[] = []> = {
    [S in M['selector']]: (
        M extends RequestMessage<S, infer P, infer R>
            ? (void extends R ? never : (...args: [...ContextArgs, ...P]) => R)
            : (M extends EventMessage<S, infer P>
                ? (...args: [...ContextArgs, ...P]) => void
               : never));
};

export function perform<I extends Methods<M, ContextArgs>,
                        S extends ValidSelector,
                        M extends RequestMessage<S, Tuple, any>,
                        ContextArgs extends any[] = []>
    (i: I, m: M, ...ctxt: ContextArgs): (M extends RequestMessage<S, Tuple, infer R> ? R : never);
export function perform<I extends Methods<M, ContextArgs>,
                        S extends ValidSelector,
                        M extends EventMessage<S, Tuple>,
                        ContextArgs extends any[] = []>
    (i: I, m: M, ...ctxt: ContextArgs): void;
export function perform<I extends Methods<M, ContextArgs>,
                        S extends ValidSelector,
                        M extends RequestMessage<S, Tuple, any>,
                        ContextArgs extends any[] = []>
    (i: I, m: M, ...ctxt: ContextArgs): any
{
    const r = i[m.selector](...ctxt, ... m.args);
    m.callback?.(r);
    return r;
}