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angularjs 1.4

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Improve this DocWhat are Scopes?

Scope is an object that refers to the application model. It is an execution context for expressions. Scopes are arranged in hierarchical structure which mimic the DOM structure of the application. Scopes can watch expressions and propagate events.

Scope characteristics

  • Scopes provide APIs ($watch) to observe model mutations.

  • Scopes provide APIs ($apply) to propagate any model changes through the system into the view from outside of the "Angular realm" (controllers, services, Angular event handlers).

  • Scopes can be nested to limit access to the properties of application components while providing access to shared model properties. Nested scopes are either "child scopes" or "isolate scopes". A "child scope" (prototypically) inherits properties from its parent scope. An "isolate scope" does not. See isolated scopes for more information.

  • Scopes provide context against which expressions are evaluated. For example {{username}} expression is meaningless, unless it is evaluated against a specific scope which defines the username property.

Scope as Data-Model

Scope is the glue between application controller and the view. During the template linking phase the directives set up $watch expressions on the scope. The $watch allows the directives to be notified of property changes, which allows the directive to render the updated value to the DOM.

Both controllers and directives have reference to the scope, but not to each other. This arrangement isolates the controller from the directive as well as from the DOM. This is an important point since it makes the controllers view agnostic, which greatly improves the testing story of the applications.

In the above example notice that the MyController assigns World to the username property of the scope. The scope then notifies the input of the assignment, which then renders the input with username pre-filled. This demonstrates how a controller can write data into the scope.

Similarly the controller can assign behavior to scope as seen by the sayHello method, which is invoked when the user clicks on the 'greet' button. The sayHello method can read the username property and create a greeting property. This demonstrates that the properties on scope update automatically when they are bound to HTML input widgets.

Logically the rendering of {{greeting}} involves:

  • retrieval of the scope associated with DOM node where {{greeting}} is defined in template. In this example this is the same scope as the scope which was passed into MyController. (We will discuss scope hierarchies later.)

  • Evaluate the greeting expression against the scope retrieved above, and assign the result to the text of the enclosing DOM element.

You can think of the scope and its properties as the data which is used to render the view. The scope is the single source-of-truth for all things view related.

From a testability point of view, the separation of the controller and the view is desirable, because it allows us to test the behavior without being distracted by the rendering details.

it('should say hello', function() {
  var scopeMock = {};
  var cntl = new MyController(scopeMock);

  // Assert that username is pre-filled
  expect(scopeMock.username).toEqual('World');

  // Assert that we read new username and greet
  scopeMock.username = 'angular';
  scopeMock.sayHello();
  expect(scopeMock.greeting).toEqual('Hello angular!');
});

Scope Hierarchies

Each Angular application has exactly one root scope, but may have several child scopes.

The application can have multiple scopes, because some directives create new child scopes (refer to directive documentation to see which directives create new scopes). When new scopes are created, they are added as children of their parent scope. This creates a tree structure which parallels the DOM where they're attached.

When Angular evaluates {{name}}, it first looks at the scope associated with the given element for the name property. If no such property is found, it searches the parent scope and so on until the root scope is reached. In JavaScript this behavior is known as prototypical inheritance, and child scopes prototypically inherit from their parents.

This example illustrates scopes in application, and prototypical inheritance of properties. The example is followed by a diagram depicting the scope boundaries.