Merge pull request #25 from adonisjs/feat/listen-if

feat(emitter): add listenIf method

src/emitter.ts

@@ -231,6 +231,32 @@ export class Emitter<EventsList extends Record<string | symbol | number, any>>
     return () => this.off(event, listener)
   }
 
+  /**
+   * Listen for an event depending on a condition
+   */
+  listenIf<Event extends Constructor, ListenerClass extends Constructor>(
+    condition: boolean | (() => boolean),
+    event: Event,
+    listener: Listener<InstanceType<Event>, ListenerClass>
+  ): UnsubscribeFunction
+  listenIf<Name extends keyof EventsList, ListenerClass extends Constructor>(
+    condition: boolean | (() => boolean),
+    event: Name,
+    listener: Listener<EventsList[Name], ListenerClass>
+  ): UnsubscribeFunction
+  listenIf<Event extends AllowedEventTypes>(
+    condition: boolean | (() => boolean),
+    event: Event,
+    listener: Listener<any, Constructor>
+  ): UnsubscribeFunction {
+    if (!condition || (typeof condition === 'function' && !condition())) {
+      return () => {}
+    }
+
+    // @ts-expect-error - TypeScript does not like overloading
+    return this.on(event, listener)
+  }
+
   /**
    * Listen for an event only once
    */

tests/emitter/listen.spec.ts

@@ -35,6 +35,26 @@ test.group('Emitter | listen', () => {
     assert.equal(emitter.eventsListeners.get('new:user')?.size, 1)
   })
 
+  test('listen for an event depending on a condition', async ({ assert }) => {
+    const stack: any[] = []
+
+    const app = new Application(BASE_URL, { environment: 'web' })
+    const emitter = new Emitter(app)
+
+    emitter.listenIf(true, 'new:user', (data) => {
+      stack.push(data)
+    })
+
+    emitter.listenIf(false, 'new:user2', (data) => {
+      stack.push(data)
+    })
+
+    await emitter.emit('new:user', { id: 1 })
+    await emitter.emit('new:user2', { id: 1 })
+    assert.deepEqual(stack, [{ id: 1 }])
+    assert.equal(emitter.eventsListeners.get('new:user')?.size, 1)
+  })
+
   test('do not register multiple listeners when callback is the same', async ({ assert }) => {
     const stack: any[] = []