How does AngularJS implement its two-way data binding mechanism?

Question

AngularJS allows you to implement two-way data binding. However, the interesting part is how it detects model changes? The model is usually a plain object like the code below. We can change the name property of $scope.user but how does AngularJS detect the model changed? Does AngularJS enum all the properties of the $scope object?

angular.module('myApp', [])
      .controller('BusinessCardController', function($scope){
        $scope.user = {
          name: 'Tanay Pant'
        }
      });

<input type="text" ng-model="user.name" placeholder="Full Name" />

Answer 1

There is a digest cycle, where the scope examines all of the $watch expressions and compares them with the previous value. It looks at the object models for changes, if the old value isn't the same as the new value, AngularJS will update the appropriate places, a.k.a dirty checking.

In order for the digest cycle to be execute $apply(fn) has to be run, this is how you enter the Angular world from JavaScript. How does $apply(fn) get called (taken from AngularJs integration with browser):

1. The browser's event-loop waits for an event to arrive. An event is a user interaction, timer event, or network event (response from a server).
2. The event's callback gets executed. This enters the JavaScript context. The callback can modify the DOM structure.
3. Once the callback executes, the browser leaves the JavaScript context and re-renders the view based on DOM changes.

Data Binding

Digest Cycle Explanation

In order to achieve two-way binding, directives register watchers. For a page to be fast and efficient we need to try and reduce all these watchers that we create. So you should be careful when using two-way binding - i.e. only use it when you really needed. Otherwise use one-way:

<h1> {{ ::vm.title }} </h1>

Here it is quite obvious that the title of the page probably won't be changed while the user is on the page - or needs to see the new one if it is changed. So we can use :: to register a one-way binding during the template linking phase.

The main issues I've seen with explosions of watchers are grids with hundreds of rows. If these rows have quite a few columns and in each cell there is two-way data binding, then you're in for a treat. You can sit back and wait like in modem times for the page to load!