1 files changed, 59 insertions, 0 deletions
diff --git a/toolkit/components/normandy/lib/EventEmitter.sys.mjs b/toolkit/components/normandy/lib/EventEmitter.sys.mjs
new file mode 100644
index 0000000000..551fc4d6b2
--- /dev/null
+++ b/toolkit/components/normandy/lib/EventEmitter.sys.mjs
@@ -0,0 +1,59 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { LogManager } from "resource://normandy/lib/LogManager.sys.mjs";
+
+const log = LogManager.getLogger("event-emitter");
+
+export var EventEmitter = function () {
+  const listeners = {};
+
+  return {
+    emit(eventName, event) {
+      // Fire events async
+      Promise.resolve().then(() => {
+        if (!(eventName in listeners)) {
+          log.debug(
+            `EventEmitter: Event fired with no listeners: ${eventName}`
+          );
+          return;
+        }
+        // Clone callbacks array to avoid problems with mutation while iterating
+        const callbacks = Array.from(listeners[eventName]);
+        for (const cb of callbacks) {
+          // Clone event so it can't by modified by the handler
+          let eventToPass = event;
+          if (typeof event === "object") {
+            eventToPass = Object.assign({}, event);
+          }
+          cb(eventToPass);
+        }
+      });
+    },
+
+    on(eventName, callback) {
+      if (!(eventName in listeners)) {
+        listeners[eventName] = [];
+      }
+      listeners[eventName].push(callback);
+    },
+
+    off(eventName, callback) {
+      if (eventName in listeners) {
+        const index = listeners[eventName].indexOf(callback);
+        if (index !== -1) {
+          listeners[eventName].splice(index, 1);
+        }
+      }
+    },
+
+    once(eventName, callback) {
+      const inner = event => {
+        callback(event);
+        this.off(eventName, inner);
+      };
+      this.on(eventName, inner);
+    },
+  };
+};