Calling event listener remover function on EventsOnce after it has fired causes TypeError

[dreamscached]

Wails 2.9.1

Title basically explains the issue, which occurs here:

wails/v2/internal/frontend/runtime/desktop/events.js

Lines 203 to 212 in 5cd0cac

	function listenerOff(listener) {
	const eventName = listener.eventName;
	// Remove local listener
	eventListeners[eventName] = eventListeners[eventName].filter(l => l !== listener);
	// Clean up if there are no event listeners left
	if (eventListeners[eventName].length === 0) {
	removeListener(eventName);
	}
	}

This happens whenever a one-time event handler (registered with EventsOnce) is removed by remover function (namely, from frontend code) after it has already fired (and therefore its listener is now removed.)

Consider the following example:

// Code in JS frontend

const eventRemover = EventsOnce("myEvent", () => console.log("Event fired.");
window.removeEvent = () => eventRemover();
window.emitEvent = () => EventsEmit("emitMyEvent");

// Code in Go backend

func (a *App) onStartup() {
    runtime.EventsOn(a.Context, "emitMyEvent", func(optionalData ...interface{}) {
        runtime.EventsEmit(a.Context, "myEvent")
    })
}

Now, when myEvent fires and removes itself, its remover function will now cause a TypeError:

This does not happen when the remover is called before event is fired:

This can be fixed pretty easily, by making a remover no-op if the listener can't be found:

 function listenerOff(listener) {
   const eventName = listener.eventName;
+  const listeners = eventListeners[eventName];
+  if (listeners === undefined) return;
  
   eventListeners[eventName] = eventListeners[eventName].filter((l) => l !== listener);
   if (eventListeners[eventName].length === 0) {
       removeListener(eventName);
   }
 }

[leaanthony]

Good catch. Feel free to open a PR 👍