Added usefull utility methods to Screen and ScreenContext.

chrome/browser/resources/login/screen.js

Index: chrome/browser/resources/login/screen.js
diff --git a/chrome/browser/resources/login/screen.js b/chrome/browser/resources/login/screen.js
index fa03371f6a3d8384c9a25ee55d0c35e505250a34..6bbf1f35489b8f8470faeb82882151e5e8c55289 100644
--- a/chrome/browser/resources/login/screen.js
+++ b/chrome/browser/resources/login/screen.js
@@ -8,11 +8,58 @@
 cr.define('login', function() {
   var Screen = cr.ui.define('div');
 
+  /** @const */ var CALLBACK_BUTTON_CLICKED = 'buttonClicked';
+  /** @const */ var CALLBACK_CONTEXT_CHANGED = 'contextChanged';
+
   function doNothing() {};
 
+  var querySelectorAll = HTMLDivElement.prototype.querySelectorAll;
+
   Screen.prototype = {
     __proto__: HTMLDivElement.prototype,
 
+    /**
+     * Prefix added to sent to Chrome messages' names.
+     */
+    sendPrefix_: '',
+
+    /**
+     * Context used by this screen.
+     */
+    context_: null,
+
+    /**
+     * Dictionary of context observers that are methods of |this| binded to

[Denis Kuznetsov (DE-MUC), 2014/06/11 14:29:17]

s/binded/bound/

[dzhioev (left Google), 2014/06/20 07:24:16]

Done.

+     * |this|.
+     */
+    contextObservers_: {},
+
+    get context() {
+      return this.context_;
+    },
+
+    /**
+     * Sends recent context changes to C++ handler.
+     */
+    commitContextChanges: function() {
+      this.send(CALLBACK_CONTEXT_CHANGED, this.context_.getChangesAndReset());
+    },
+
+    /**
+     * Sends message to Chrome, adding needed prefix to message name. All
+     * arguments after |messageName| are packed into message parameters list.
+     *
+     * @param {string} messageName Name of message without a prefix.
+     * @param {...*} varArgs parameters for message.
+     */
+    send: function(messageName, varArgs) {
+      if (arguments.length == 0)
+        throw Error('Message name is not provided.');
+      var fullMessageName = this.sendPrefix_ + messageName;
+      var payload = Array.prototype.slice.call(arguments, 1);
+      chrome.send(fullMessageName, payload);
+    },
+
     decorate: doNothing,
 
     /**
@@ -28,6 +75,94 @@ cr.define('login', function() {
      * Called for currently active screen when screen size changed.
      */
     onWindowResize: doNothing,
+
+    /**
+     * Does the following things:
+     *  * Creates screen context.
+     *  * Looks for elements having "is" property and adds them as the proprties
+     *    of the screen with name equal to value of "is", i.e. HTML element
+     *    <div is="myDiv"></div> will be stored in this.myDiv.
+     *  * Looks for buttons having "action" properties and adds click handlers
+     *    to them. These handlers send |CALLBACK_BUTTON_CLICKED| messages to
+     *    C++ with "action" property's value as payload.
+     */
+    makeMagic: function() {

[ygorshenin1, 2014/06/11 10:44:00]

Don't see how this method is used in https://codereview.chromium.org/326933004/.
In any case, we need to devise a better name than "makeMagic".

[Denis Kuznetsov (DE-MUC), 2014/06/11 14:29:17]

I agree about naming.
I'm also concerned about using one more non-standard element atttribute here,
especially with such short and non-noun name.
I'd better stick with identifying div's/buttons by id, and having some "wire"
function that would take id, and do the stuff.
While it will require few more lines in screen initialization, everything will
be explicit (bugs caused by implicit behavior are much harder to
detect/trace/fix).

[dzhioev (left Google), 2014/06/18 14:43:39]

It is called in |decorate| method of new screen.
I'll rename it. Note: in the future this method will be called automatically for
every registered screen.

[dzhioev (left Google), 2014/06/18 14:43:39]

* The main reason why I don't want use "id"s is that "id"s should be unique.
This restriction makes us to use long identifiers, like
|id="locally-managed-user-creation-some-label"|. For custom attribute we don't
have such restriction so we can use short name, like |is="someLabel_"| and don't
afraid of name conflicts with other screens.
* Using "id" weakens encapsulation, because it is easy to write something like
|$("element-from-another-screen").hidden = true| in any place outside screen.
* The main purpose of Screen and ScreenContext is to get rid of boilerplate
code, and I think this explicit initialization is a good place to start.
Moreover I think new method is less error prone.
* IMHO, the "is" name perfectly describes purpose of this attribute, and I'm
glad that it is so short. Can you explain what is so bad about short names?

[Denis Kuznetsov (DE-MUC), 2014/06/19 19:16:36]

First, attribute names are usually nouns. "Is" is a verb.
Second, it can be mistaken for some abbreviation. 

[dzhioev (left Google), 2014/06/20 07:24:16]

Renamed to "alias"

+      this.context_ = new login.ScreenContext(this);
+      var elements = this.querySelectorAll('[is]');
+      for (var i = 0; i < elements.length; ++i)
+        this[elements[i].getAttribute('is')] = elements[i];
+      var buttons = this.querySelectorAll('button[action]');
+      var self = this;
+      for (var i = 0; i < buttons.length; ++i) {
+        buttons[i].addEventListener('click', function(e) {
+            var action = this.getAttribute('action');
+            self.send(CALLBACK_BUTTON_CLICKED, action);
+            e.stopPropagation();
+          });
+      }
+    },
+
+    /**
+     * Starts observation of property with |key| of the context attached to
+     * current screen. This method differs from "login.ScreenContext" in that
+     * it automatically detects if observer is method of |this| and make
+     * all needed actions to make it work correctly. So it's no need for client
+     * to bind methods to |this| and keep resulting callback for unobserve:
+     *
+     *   this.observeContext('key', this.onKeyChanged_);
+     *   ...
+     *   this.unobserveContext('key', this.onKeyChanged_);
+     */
+    observeContext: function(key, observer) {

[Denis Kuznetsov (DE-MUC), 2014/06/11 14:29:17]

subscribeForKeyChange / watchKeyChange?

[dzhioev (left Google), 2014/06/18 14:43:39]

See comment to ScreenContext.observe

+      var realObserver = observer;
+      var propertyName = this.getPropertyNameOf_(observer);
+      if (propertyName) {
+        if (!this.contextObservers_.hasOwnProperty(propertyName))
+          this.contextObservers_[propertyName] = observer.bind(this);
+        realObserver = this.contextObservers_[propertyName];
+      }
+      this.context.observe(key, realObserver);
+    },
+
+    /**
+     * Removes |observer| from the list of context observers. Supports not only
+     * regular functions but also screen methods (see comment to
+     * |observeContext|).
+     */
+    unobserveContext: function(observer) {
+      var realObserver = observer;
+      var propertyName = this.getPropertyNameOf_(observer);
+      if (propertyName) {
+        if (!this.contextObservers_.hasOwnProperty(propertyName))
+          return;
+        realObserver = this.contextObservers_[propertyName];
+        delete this.contextObservers_[propertyName];
+      }
+      this.context.unobserve(realObserver);
+    },
+
+    /**
+     * Calls standart |querySelectorAll| method and returns its result converted
+     * to Array.
+     */
+    querySelectorAll: function(selector) {
+      var list = querySelectorAll.call(this, selector);
+      return Array.prototype.slice.call(list);
+    },
+
+    /**
+     * Called when context changes are recieved from C++.
+     */
+    contextChanged_: function(diff) {

[ygorshenin1, 2014/06/11 10:44:00]

nit: add @private annotation.

[dzhioev (left Google), 2014/06/18 14:43:39]

Done.

+      this.context.applyChanges(diff);
+    },
+
+    getPropertyNameOf_: function(value) {

[ygorshenin1, 2014/06/11 10:44:00]

nit: add comment to this function + @private annotation.

[dzhioev (left Google), 2014/06/18 14:43:39]

Done.

+      for (var key in this)
+        if (this[key] === value)
+          return key;
+      return '';
+    }
   };
 
   return {
@@ -87,6 +222,10 @@ cr.define('login', function() {
             })(propertyName);
           }
         });
+        constructor.contextChanged = function() {
+          var screen = $(id);
+          screen.contextChanged_.apply(screen, arguments);
+        }
         constructor.prototype.name = function() { return id; };
 
         constructor.register = function() {