1 files changed, 95 insertions, 14 deletions
diff --git a/app/assets/javascripts/helpers/event_hub_factory.js b/app/assets/javascripts/helpers/event_hub_factory.js
index 4d7f7550a94..a9c301e3a93 100644
--- a/app/assets/javascripts/helpers/event_hub_factory.js
+++ b/app/assets/javascripts/helpers/event_hub_factory.js
@@ -1,20 +1,101 @@
-import mitt from 'mitt';
+/**
+ * An event hub with a Vue instance like API
+ *
+ * NOTE: There's an [issue open][4] to eventually remove this when some
+ * coupling in our codebase has been fixed.
+ *
+ * NOTE: This is a derivative work from [mitt][1] v1.2.0 which is licensed by
+ * [MIT License][2] © [Jason Miller][3]
+ *
+ * [1]: https://github.com/developit/mitt
+ * [2]: https://opensource.org/licenses/MIT
+ * [3]: https://jasonformat.com/
+ * [4]: https://gitlab.com/gitlab-org/gitlab/-/issues/223864
+ */
+class EventHub {
+  constructor() {
+    this.$_all = new Map();
+  }
 
-export default () => {
-  const emitter = mitt();
+  dispose() {
+    this.$_all.clear();
+  }
+
+  /**
+   * Register an event handler for the given type.
+   *
+   * @param {string|symbol} type Type of event to listen for
+   * @param {Function} handler Function to call in response to given event
+   */
+  $on(type, handler) {
+    const handlers = this.$_all.get(type);
+    const added = handlers && handlers.push(handler);
+
+    if (!added) {
+      this.$_all.set(type, [handler]);
+    }
+  }
+
+  /**
+   * Remove an event handler or all handlers for the given type.
+   *
+   * @param {string|symbol} type Type of event to unregister `handler`
+   * @param {Function} handler Handler function to remove
+   */
+  $off(type, handler) {
+    const handlers = this.$_all.get(type) || [];
 
-  emitter.once = (event, handler) => {
-    const wrappedHandler = evt => {
-      handler(evt);
-      emitter.off(event, wrappedHandler);
+    const newHandlers = handler ? handlers.filter(x => x !== handler) : [];
+
+    if (newHandlers.length) {
+      this.$_all.set(type, newHandlers);
+    } else {
+      this.$_all.delete(type);
+    }
+  }
+
+  /**
+   * Add an event listener to type but only trigger it once
+   *
+   * @param {string|symbol} type Type of event to listen for
+   * @param {Function} handler Handler function to call in response to event
+   */
+  $once(type, handler) {
+    const wrapHandler = (...args) => {
+      this.$off(type, wrapHandler);
+      handler(...args);
     };
-    emitter.on(event, wrappedHandler);
-  };
+    this.$on(type, wrapHandler);
+  }
 
-  emitter.$on = emitter.on;
-  emitter.$once = emitter.once;
-  emitter.$off = emitter.off;
-  emitter.$emit = emitter.emit;
+  /**
+   * Invoke all handlers for the given type.
+   *
+   * @param {string|symbol} type The event type to invoke
+   * @param {Any} [evt] Any value passed to each handler
+   */
+  $emit(type, ...args) {
+    const handlers = this.$_all.get(type) || [];
 
-  return emitter;
+    handlers.forEach(handler => {
+      handler(...args);
+    });
+  }
+}
+
+/**
+ * Return a Vue like event hub
+ *
+ * - $on
+ * - $off
+ * - $once
+ * - $emit
+ *
+ * Please note, this was once implemented with `mitt`, but since then has been reverted
+ * because of some API issues. https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35074
+ *
+ * We'd like to shy away from using a full fledged Vue instance from this in the future.
+ */
+export default () => {
+  return new EventHub();
 };