Bug 1603035 - Return a function from EventEmitter.on that removes the event listener when called. r=rcaliman.

This will be helpful when consumers don't want to
keep the target around.
A test is added to ensure this work as expected (and
was failing if the returned function does not call
EventEmitter.off).

Differential Revision: https://phabricator.services.mozilla.com/D56685

--HG--
extra : moz-landing-system : lando

devtools/shared/event-emitter.js

@@ -28,6 +28,8 @@ class EventEmitter {
    *    The type of event.
    * @param {Function|Object} listener
    *    The listener that processes the event.
+   * @returns {Function}
+   *    A function that removes the listener when called.
    */
   static on(target, type, listener) {
     if (typeof listener !== "function" && !isEventHandler(listener)) {
@@ -45,6 +47,8 @@ class EventEmitter {
     } else {
       events.set(type, new Set([listener]));
     }
+
+    return () => EventEmitter.off(target, type, listener);
   }
 
   /**
@@ -253,7 +257,7 @@ class EventEmitter {
   }
 
   on(...args) {
-    EventEmitter.on(this, ...args);
+    return EventEmitter.on(this, ...args);
   }
 
   off(...args) {

devtools/shared/tests/unit/test_eventemitter_basic.js

@@ -255,6 +255,21 @@ const TESTS = {
     emitter.emit("c", 1);
     equal(received.length, 3, "the listener was not called after clearEvents");
   },
+
+  testOnReturn() {
+    const emitter = getEventEmitter();
+
+    let called = false;
+    const removeOnTest = emitter.on("test", () => {
+      called = true;
+    });
+
+    equal(typeof removeOnTest, "function", "`on` returns a function");
+    removeOnTest();
+
+    emitter.emit("test");
+    equal(called, false, "event listener wasn't called");
+  },
 };
 
 /**