redux-watch.ts

import { Store, Unsubscribe } from 'redux';

const defaultEqual = <P>(prev: P, next: P) => prev === next;
const defaultImmediately = true;

interface IWatcherParams<S, V, P> {
    store: Store<S>;
    selector: (state: S, params?: P) => V;
    onChange: (prev: V, next: V) => void;
    props?: P;
    equal?: (prev: V, next: V) => boolean;
    immediately?: boolean;
}

const createWatcher = <S, V, P = undefined>(
    { store, selector, onChange, props, equal = defaultEqual, immediately = defaultImmediately }: IWatcherParams<S, V, P>,
): Unsubscribe => {
    let initial = selector(store.getState(), props);
    
    if (immediately) {
        onChange(initial, initial);
    }

    return store.subscribe(() => {
        const prev = initial;
        const next = selector(store.getState(), props);
        
        if (!equal(prev, next)) {
            initial = next;
            onChange(prev, next);
        }
    });
};

export { createWatcher };