1 files changed, 214 insertions, 0 deletions
diff --git a/toolkit/modules/EventEmitter.sys.mjs b/toolkit/modules/EventEmitter.sys.mjs
new file mode 100644
index 0000000000..5cc5964829
--- /dev/null
+++ b/toolkit/modules/EventEmitter.sys.mjs
@@ -0,0 +1,214 @@
+/* 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/. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  console: "resource://gre/modules/Console.sys.mjs",
+});
+
+export function EventEmitter() {}
+
+let loggingEnabled = Services.prefs.getBoolPref("toolkit.dump.emit");
+Services.prefs.addObserver("toolkit.dump.emit", {
+  observe: () => {
+    loggingEnabled = Services.prefs.getBoolPref("toolkit.dump.emit");
+  },
+});
+
+/**
+ * Decorate an object with event emitter functionality.
+ *
+ * @param Object objectToDecorate
+ *        Bind all public methods of EventEmitter to
+ *        the objectToDecorate object.
+ */
+EventEmitter.decorate = function(objectToDecorate) {
+  let emitter = new EventEmitter();
+  objectToDecorate.on = emitter.on.bind(emitter);
+  objectToDecorate.off = emitter.off.bind(emitter);
+  objectToDecorate.once = emitter.once.bind(emitter);
+  objectToDecorate.emit = emitter.emit.bind(emitter);
+};
+
+function describeNthCaller(n) {
+  let caller = Components.stack;
+  // Do one extra iteration to skip this function.
+  while (n >= 0) {
+    --n;
+    caller = caller.caller;
+  }
+
+  let func = caller.name;
+  let file = caller.filename;
+  if (file.includes(" -> ")) {
+    file = caller.filename.split(/ -> /)[1];
+  }
+  let path = file + ":" + caller.lineNumber;
+
+  return func + "() -> " + path;
+}
+
+EventEmitter.prototype = {
+  /**
+   * Connect a listener.
+   *
+   * @param string event
+   *        The event name to which we're connecting.
+   * @param function listener
+   *        Called when the event is fired.
+   */
+  on(event, listener) {
+    if (!this._eventEmitterListeners) {
+      this._eventEmitterListeners = new Map();
+    }
+    if (!this._eventEmitterListeners.has(event)) {
+      this._eventEmitterListeners.set(event, []);
+    }
+    this._eventEmitterListeners.get(event).push(listener);
+  },
+
+  /**
+   * Listen for the next time an event is fired.
+   *
+   * @param string event
+   *        The event name to which we're connecting.
+   * @param function listener
+   *        (Optional) Called when the event is fired. Will be called at most
+   *        one time.
+   * @return promise
+   *        A promise which is resolved when the event next happens. The
+   *        resolution value of the promise is the first event argument. If
+   *        you need access to second or subsequent event arguments (it's rare
+   *        that this is needed) then use listener
+   */
+  once(event, listener) {
+    return new Promise(resolve => {
+      let handler = (_, first, ...rest) => {
+        this.off(event, handler);
+        if (listener) {
+          listener(event, first, ...rest);
+        }
+        resolve(first);
+      };
+
+      handler._originalListener = listener;
+      this.on(event, handler);
+    });
+  },
+
+  /**
+   * Remove a previously-registered event listener.  Works for events
+   * registered with either on or once.
+   *
+   * @param string event
+   *        The event name whose listener we're disconnecting.
+   * @param function listener
+   *        The listener to remove.
+   */
+  off(event, listener) {
+    if (!this._eventEmitterListeners) {
+      return;
+    }
+    let listeners = this._eventEmitterListeners.get(event);
+    if (listeners) {
+      this._eventEmitterListeners.set(
+        event,
+        listeners.filter(l => {
+          return l !== listener && l._originalListener !== listener;
+        })
+      );
+    }
+  },
+
+  /**
+   * Emit an event.  All arguments to this method will
+   * be sent to listener functions.
+   */
+  emit(event) {
+    this.logEvent(event, arguments);
+
+    if (
+      !this._eventEmitterListeners ||
+      !this._eventEmitterListeners.has(event)
+    ) {
+      return;
+    }
+
+    let originalListeners = this._eventEmitterListeners.get(event);
+    for (let listener of this._eventEmitterListeners.get(event)) {
+      // If the object was destroyed during event emission, stop
+      // emitting.
+      if (!this._eventEmitterListeners) {
+        break;
+      }
+
+      // If listeners were removed during emission, make sure the
+      // event handler we're going to fire wasn't removed.
+      if (
+        originalListeners === this._eventEmitterListeners.get(event) ||
+        this._eventEmitterListeners.get(event).some(l => l === listener)
+      ) {
+        try {
+          listener.apply(null, arguments);
+        } catch (ex) {
+          // Prevent a bad listener from interfering with the others.
+          let msg = ex + ": " + ex.stack;
+          lazy.console.error(msg);
+          if (loggingEnabled) {
+            dump(msg + "\n");
+          }
+        }
+      }
+    }
+  },
+
+  logEvent(event, args) {
+    if (!loggingEnabled) {
+      return;
+    }
+
+    let description = describeNthCaller(2);
+
+    let argOut = "(";
+    if (args.length === 1) {
+      argOut += event;
+    }
+
+    let out = "EMITTING: ";
+
+    // We need this try / catch to prevent any dead object errors.
+    try {
+      for (let i = 1; i < args.length; i++) {
+        if (i === 1) {
+          argOut = "(" + event + ", ";
+        } else {
+          argOut += ", ";
+        }
+
+        let arg = args[i];
+        argOut += arg;
+
+        if (arg && arg.nodeName) {
+          argOut += " (" + arg.nodeName;
+          if (arg.id) {
+            argOut += "#" + arg.id;
+          }
+          if (arg.className) {
+            argOut += "." + arg.className;
+          }
+          argOut += ")";
+        }
+      }
+    } catch (e) {
+      // Object is dead so the toolbox is most likely shutting down,
+      // do nothing.
+    }
+
+    argOut += ")";
+    out += "emit" + argOut + " from " + description + "\n";
+
+    dump(out);
+  },
+};