Initial computedFrom impl

computedFrom.ts

import { isSignal, signal, Signal } from '@angular/core';
import { toObservable, toSignal } from '@angular/core/rxjs-interop';
import { combineLatest, distinctUntilChanged, from, interval, isObservable, Observable, ObservableInput, of, OperatorFunction } from 'rxjs';


export type ObservableSignalInput<T> = ObservableInput<T> | Signal<T>;

/**
 * So that we can have `fn([Observable<A>, Signal<B>]): Observable<[A, B]>`
 */
type ObservableSignalInputTuple<T> = {
	[K in keyof T]: ObservableSignalInput<T[K]>;
};

export function computedFrom<R, A extends readonly unknown[]>(
	sources: readonly [...ObservableSignalInputTuple<A>],
	operator: OperatorFunction<A, R>
): Signal<R | undefined>;

export function computedFrom<R, A extends readonly unknown[]>(
	sources: readonly [...ObservableSignalInputTuple<A>],
	initialValue: R,
	operator?: OperatorFunction<A, R>
): Signal<R>;

export function computedFrom<R, A extends readonly unknown[]>(
	sources: readonly [...ObservableSignalInputTuple<A>],
	initialValue?: R,
	operator?: OperatorFunction<A, R>
): Signal<R | undefined>;

export function computedFrom<R, A extends readonly unknown[]>(
	sources: readonly [...ObservableSignalInputTuple<A>],
	initialValueOrOperator?: R | OperatorFunction<A, R>,
	operator?: OperatorFunction<A, R>
): Signal<R> {
	const obsSources = sources.map(x => {
		const obs$ = sourceToObservable(x);
		return obs$.pipe(distinctUntilChanged());
	});

	const initialValue =
		typeof initialValueOrOperator === 'function' ? undefined : initialValueOrOperator;

	const pipeOperators =
		typeof initialValueOrOperator === 'function' ? initialValueOrOperator : operator;

	const result$ = combineLatest(obsSources).pipe(
		pipeOperators as unknown as OperatorFunction<any, R>
	);

	return toSignal(result$, { initialValue }) as Signal<R>;
}

function sourceToObservable<T>(source: ObservableSignalInput<T>): Observable<T> {
	if (typeof source === 'function' && isSignal(source)) return toObservable(source);
	if (isObservable(source)) return source;
	return from(source);
}