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## What 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 isolate application components while providing access to shared model properties. A scope (prototypically) inherits properties from its parent scope.

- 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 DOM. This is an important point since it makes the controllers view agnostic, which greatly improves the testing story of the applications.

### Source

### Demo

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 `{{username}}`, it first looks at the scope associated with the given element for the `username` 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.

### Source

### Demo

Notice that Angular automatically places `ng-scope` class on elements where scopes are attached. The `<style>` definition in this example highlights in red the new scope locations. The child scopes are necessary because the repeater evaluates `{{employee.name}}` expression, but depending on which scope the expression is evaluated it produces different result. Similarly the evaluation of `{{department}}` prototypically inherits from root scope, as it is the only place where the `department` property is defined.

### Retrieving Scopes from the DOM.

Scopes are attached to the DOM as `$scope` data property, and can be retrieved for debugging purposes. (It is unlikely that one would need to retrieve scopes in this way inside the application.) The location where the root scope is attached to the DOM is defined by the location of ``ng-app`` directive. Typically `ng-app` is placed an the `<html>` element, but it can be placed on other elements as well, if, for example, only a portion of the view needs to be controlled by Angular.

To examine the scope in the debugger:

1. right click on the element of interest in your browser and select 'inspect element'. You should see the browser debugger with the element you clicked on highlighted.

2. The debugger allows you to access the currently selected element in the console as `$0` variable.

3. To retrieve the associated scope in console execute: `angular.element($0).scope()`

### Scope Events Propagation

Scopes can propagate events in similar fashion to DOM events. The event can be `broadcasted` to the scope children or `emitted` to scope parents.

### Source

### Demo

### Scope Life Cycle

The normal flow of a browser receiving an event is that it executes a corresponding JavaScript callback. Once the callback completes the browser re-renders the DOM and returns to waiting for more events.

When the browser calls into JavaScript the code executes outside the Angular execution context, which means that Angular is unaware of model modifications. To properly process model modifications the execution has to enter the Angular execution context using the ``$apply`` method. Only model modifications which execute inside the `$apply` method will be properly accounted for by Angular. For example if a directive listens on DOM events, such as ``ng-click`` it must evaluate the expression inside the `$apply` method.

After evaluating the expression, the `$apply` method performs a ``$digest``. In the $digest phase the scope examines all of the `$watch` expressions and compares them with the previous value. This dirty checking is done asynchronously. This means that assignment such as `$scope.username="angular"` will not immediately cause a `$watch` to be notified, instead the `$watch` notification is delayed until the `$digest` phase. This delay is desirable, since it coalesces multiple model updates into one `$watch` notification as well as it guarantees that during the `$watch` notification no other `$watch`es are running. If a `$watch` changes the value of the model, it will force additional `$digest` cycle.

1. Creation

   The `root scope` is created during the application bootstrap by the `$injector`. During template linking, some directives create new child scopes.

2. Watcher registration

   During template linking directives register `watches` on the scope. These watches will be used to propagate model values to the DOM.

3. Model mutation

   For mutations to be properly observed, you should make them only within the `scope.$apply()`. (Angular APIs do this implicitly, so no extra `$apply` call is needed when doing synchronous work in controllers, or asynchronous work with `$http` or `$timeout` services.

4. Mutation observation

   At the end `$apply`, Angular performs a `$digest` cycle on the root scope, which then propagates throughout all child scopes. During the `$digest` cycle, all `$watch`ed expressions or functions are checked for model mutation and if a mutation is detected, the `$watch` listener is called.

5. Scope destruction

   When child scopes are no longer needed, it is the responsibility of the child scope creator to destroy them via `scope.$destroy()` API. This will stop propagation of `$digest` calls into the child scope and allow for memory used by the child scope models to be reclaimed by the garbage collector.

#### Scopes and Directives

During the compilation phase, the compiler matches `directives` against the DOM template. The directives usually fall into one of two categories:

- Observing `directives`, such as double-curly expressions `{{expression}}`, register listeners using the `$watch()` method. This type of directive needs to be notified whenever the expression changes so that it can update the view.

- Listener directives, such as `ng-click`, register a listener with the DOM. When the DOM listener fires, the directive executes the associated expression and updates the view using the `$apply()` method.

When an external event (such as a user action, timer or XHR) is received, the associated expression must be applied to the scope through the `$apply()` method so that all listeners are updated correctly.

#### Directives that Create Scopes

In most cases, `directives` and scopes interact but do not create new instances of scope. However, some directives, such as `ng-controller` and `ng-repeat`, create new child scopes and attach the child scope to the corresponding DOM element. You can retrieve a scope for any DOM element by using an `angular.element(aDomElement).scope()` method call.

#### Controllers and Scopes

Scopes and controllers interact with each other in the following situations:

- Controllers use scopes to expose controller methods to templates (see `ng-controller`).

- Controllers define methods (behavior) that can mutate the model (properties on the scope).

- Controllers may register `watches` on the model. These watches execute immediately after the controller behavior executes.

See the `ng-controller` for more information.

#### Scope `$watch` Performance Considerations

Dirty checking the scope for property changes is a common operation in Angular and for this reason the dirty checking function must be efficient. Care should be taken that the dirty checking function does not do any DOM access, as DOM access is orders of magnitude slower then property access on JavaScript object.