Is it OK to have multiple listener middlewares in the same store?

Question

If I use createListenerMiddleware to create 2 separate middlewares, that handle different responsibilities, is that going to have any significant performance cost over creating a single listener middleware? Are there any undesirable ways they can interact?

One concern is if I dispatch addListener, removeListener or clearAllListeners, are they going to reach both middlewares, or be consumed by the first in the chain?

The alternative is creating a single shared listener middleware in its own module that can be imported to both the other modules to have listeners added, which would likely cover most use-cases but not if I want to pass in different extra arguments or different error handling.

Answer 1

I created the listener middleware :)

You should really only have one listener middleware instance in an app, and I can't really think of a good reason why you would want to have multiple listener middleware instances. And yes, the add/remove listener actions will definitely be handled and stopped by the first listener middleware instance that sees them, with no differentiation.

If the goal is to have separate listener+effect definitions that don't rely on importing the same middleware instance, I'd suggest trying the third file organization approach shown in the docs, where feature files export a callback that gets startListening as a parameter:

• https://redux-toolkit.js.org/api/createListenerMiddleware#organizing-listeners-in-files

// feature1Slice.ts
import type { AppStartListening } from '../../app/listenerMiddleware'

const feature1Slice = createSlice(/* */)
const { action1 } = feature1Slice.actions

export default feature1Slice.reducer

export const addFeature1Listeners = (startListening: AppStartListening) => {
  startListening({
    actionCreator: action1,
    effect: () => {},
  })
}