Adds a listener function. The listener will be returned when using applyPatch.

The callback to be invoked after patching.

An optional function that filters what mutations have to be applied.

A function that unsubscribes.

import { createStore } from 'dop'

const store = createStore({ users: 0 })
const unsubscribe = store.subscribe(patch => {})
store.listeners.size // 1
unsubscribe()
store.listeners.size // 0