diff -r 000000000000 -r 6474c204b198 browser/devtools/shared/observable-object.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/shared/observable-object.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,129 @@
+/* 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/. */
+
+/**
+ * ObservableObject
+ *
+ * An observable object is a JSON-like object that throws
+ * events when its direct properties or properties of any
+ * contained objects, are getting accessed or set.
+ *
+ * Inherits from EventEmitter.
+ *
+ * Properties:
+ * ⬩ object: JSON-like object
+ *
+ * Events:
+ * ⬩ "get" / path (array of property names)
+ * ⬩ "set" / path / new value
+ *
+ * Example:
+ *
+ *   let emitter = new ObservableObject({ x: { y: [10] } });
+ *   emitter.on("set", console.log);
+ *   emitter.on("get", console.log);
+ *   let obj = emitter.object;
+ *   obj.x.y[0] = 50;
+ *
+ */
+
+"use strict";
+
+const EventEmitter = require("devtools/toolkit/event-emitter");
+
+function ObservableObject(object = {}) {
+  EventEmitter.decorate(this);
+  let handler = new Handler(this);
+  this.object = new Proxy(object, handler);
+  handler._wrappers.set(this.object, object);
+  handler._paths.set(object, []);
+}
+
+module.exports = ObservableObject;
+
+function isObject(x) {
+  if (typeof x === "object")
+    return x !== null;
+  return typeof x === "function";
+}
+
+function Handler(emitter) {
+  this._emitter = emitter;
+  this._wrappers = new WeakMap();
+  this._values = new WeakMap();
+  this._paths = new WeakMap();
+}
+
+Handler.prototype = {
+  wrap: function(target, key, value) {
+    let path;
+    if (!isObject(value)) {
+      path = this._paths.get(target).concat(key);
+    } else if (this._wrappers.has(value)) {
+      path = this._paths.get(value);
+    } else if (this._paths.has(value)) {
+      path = this._paths.get(value);
+      value = this._values.get(value);
+    } else {
+      path = this._paths.get(target).concat(key);
+      this._paths.set(value, path);
+      let wrapper = new Proxy(value, this);
+      this._wrappers.set(wrapper, value);
+      this._values.set(value, wrapper);
+      value = wrapper;
+    }
+    return [value, path];
+  },
+  unwrap: function(target, key, value) {
+    if (!isObject(value) || !this._wrappers.has(value)) {
+      return [value, this._paths.get(target).concat(key)];
+    }
+    return [this._wrappers.get(value), this._paths.get(target).concat(key)];
+  },
+  get: function(target, key) {
+    let value = target[key];
+    let [wrapped, path] = this.wrap(target, key, value);
+    this._emitter.emit("get", path, value);
+    return wrapped;
+  },
+  set: function(target, key, value) {
+    let [wrapped, path] = this.unwrap(target, key, value);
+    target[key] = value;
+    this._emitter.emit("set", path, value);
+  },
+  getOwnPropertyDescriptor: function(target, key) {
+    let desc = Object.getOwnPropertyDescriptor(target, key);
+    if (desc) {
+      if ("value" in desc) {
+        let [wrapped, path] = this.wrap(target, key, desc.value);
+        desc.value = wrapped;
+        this._emitter.emit("get", path, desc.value);
+      } else {
+        if ("get" in desc) {
+          [desc.get] = this.wrap(target, "get "+key, desc.get);
+        }
+        if ("set" in desc) {
+          [desc.set] = this.wrap(target, "set "+key, desc.set);
+        }
+      }
+    }
+    return desc;
+  },
+  defineProperty: function(target, key, desc) {
+    if ("value" in desc) {
+      let [unwrapped, path] = this.unwrap(target, key, desc.value);
+      desc.value = unwrapped;
+      Object.defineProperty(target, key, desc);
+      this._emitter.emit("set", path, desc.value);
+    } else {
+      if ("get" in desc) {
+        [desc.get] = this.unwrap(target, "get "+key, desc.get);
+      }
+      if ("set" in desc) {
+        [desc.set] = this.unwrap(target, "set "+key, desc.set);
+      }
+      Object.defineProperty(target, key, desc);
+    }
+  }
+};