michael@0: /* This Source Code Form is subject to the terms of the Mozilla Public
michael@0:  * License, v. 2.0. If a copy of the MPL was not distributed with this
michael@0:  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
michael@0: 
michael@0: "use strict";
michael@0: 
michael@0: module.metadata = {
michael@0:   "stability": "experimental"
michael@0: };
michael@0: 
michael@0: 
michael@0: const { Cc, Ci, Cr } = require("chrome");
michael@0: const { Class } = require("./heritage");
michael@0: const { isWeak } = require("./reference");
michael@0: const method = require("../../method/core");
michael@0: 
michael@0: const { addObserver, removeObserver } = Cc['@mozilla.org/observer-service;1'].
michael@0:                                           getService(Ci.nsIObserverService);
michael@0: 
michael@0: 
michael@0: // This is a method that will be invoked when notification observer
michael@0: // subscribed to occurs.
michael@0: const observe = method("observer/observe");
michael@0: exports.observe = observe;
michael@0: 
michael@0: // Method to subscribe to the observer notification.
michael@0: const subscribe = method("observe/subscribe");
michael@0: exports.subscribe = subscribe;
michael@0: 
michael@0: 
michael@0: // Method to unsubscribe from the observer notifications.
michael@0: const unsubscribe = method("observer/unsubscribe");
michael@0: exports.unsubscribe = unsubscribe;
michael@0: 
michael@0: 
michael@0: // This is wrapper class that takes a `delegate` and produces
michael@0: // instance of `nsIObserver` which will delegate to a given
michael@0: // object when observer notification occurs.
michael@0: const ObserverDelegee = Class({
michael@0:   initialize: function(delegate) {
michael@0:     this.delegate = delegate;
michael@0:   },
michael@0:   QueryInterface: function(iid) {
michael@0:     const isObserver = iid.equals(Ci.nsIObserver);
michael@0:     const isWeakReference = iid.equals(Ci.nsISupportsWeakReference);
michael@0: 
michael@0:     if (!isObserver && !isWeakReference)
michael@0:       throw Cr.NS_ERROR_NO_INTERFACE;
michael@0: 
michael@0:     return this;
michael@0:   },
michael@0:   observe: function(subject, topic, data) {
michael@0:     observe(this.delegate, subject, topic, data);
michael@0:   }
michael@0: });
michael@0: 
michael@0: 
michael@0: // Class that can be either mixed in or inherited from in
michael@0: // order to subscribe / unsubscribe for observer notifications.
michael@0: const Observer = Class({});
michael@0: exports.Observer = Observer;
michael@0: 
michael@0: // Weak maps that associates instance of `ObserverDelegee` with
michael@0: // an actual observer. It ensures that `ObserverDelegee` instance
michael@0: // won't be GC-ed until given `observer` is.
michael@0: const subscribers = new WeakMap();
michael@0: 
michael@0: // Implementation of `subscribe` for `Observer` type just registers
michael@0: // observer for an observer service. If `isWeak(observer)` is `true`
michael@0: // observer service won't hold strong reference to a given `observer`.
michael@0: subscribe.define(Observer, (observer, topic) => {
michael@0:   if (!subscribers.has(observer)) {
michael@0:     const delegee = new ObserverDelegee(observer);
michael@0:     subscribers.set(observer, delegee);
michael@0:     addObserver(delegee, topic, isWeak(observer));
michael@0:   }
michael@0: });
michael@0: 
michael@0: // Unsubscribes `observer` from observer notifications for the
michael@0: // given `topic`.
michael@0: unsubscribe.define(Observer, (observer, topic) => {
michael@0:   const delegee = subscribers.get(observer);
michael@0:   if (delegee) {
michael@0:     subscribers.delete(observer);
michael@0:     removeObserver(delegee, topic);
michael@0:   }
michael@0: });