@ngdoc overview
@name Developer Guide: About MVC in Angular: Understanding the Controller Component
@description

In angular, a controller is a JavaScript function(type/class) that is used to augment instances of
angular {@link dev_guide.scopes Scope}, excluding the root scope. When you or angular create a new
child scope object via the {@link api/angular.scope.$new scope.$new} API        , there is an
option to pass in a controller as a method argument. This will tell angular to associate the
controller with the new scope and to augment its behavior.

Use controllers to:

- Set up the initial state of a scope object.
- Add behavior to the scope object.

# Setting up the initial state of a scope object

Typically, when you create an application you need to set up an initial state for an angular scope.

Angular applies (in the sense of JavaScript's `Function#apply`) the controller constructor function
to a new angular scope object, which sets up an initial scope state. This means that angular never
creates instances of the controller type (by invoking the `new` operator on the controller
constructor). Constructors are always applied to an existing scope object.

You set up the initial state of a scope by creating model properties. For example:

function GreetingCtrl() {
 this.greeting = 'Hola!';
}

The `GreetingCtrl` controller creates a `greeting` model which can be referred to in a template.

When a controller function is applied to an angular scope object, the `this` of the controller
function becomes the scope of the angular scope object, so any assignment to `this` within the
controller function happens on the angular scope object.

# Adding Behavior to a Scope Object

Behavior on an angular scope object is in the form of scope method properties available to the
template/view. This behavior interacts with and modifies the application model.

As discussed in the {@link dev_guide.mvc.understanding_model Model} section of this guide, any
objects (or primitives) assigned to the scope become model properties. Any functions assigned to
the scope, along with any prototype methods of the controller type, become functions available in
the template/view, and can be invoked via angular expressions and `ng:` event handlers (e.g. {@link
api/angular.directive.ng:click ng:click}). These controller methods are always evaluated within the
context of the angular scope object that the controller function was applied to (which means that
the `this` keyword of any controller method is always bound to the scope that the controller
augments). This is how the second task of adding behavior to the scope is accomplished.


# Using Controllers Correctly

In general, a controller shouldn't try to do too much. It should contain only the business logic
needed for a single view.

The most common way to keep controllers slim is by encapsulating work that doesn't belong to
controllers into services and then using these services in controllers via dependency injection.
This is discussed in the {@link dev_guide.di Dependency Injection} {@link dev_guide.services
Services} sections of this guide.

Do not use controllers for:

- Any kind of DOM manipulation — Controllers should contain only business logic. DOM
manipulation—the presentation logic of an application—is well known for being hard to test.
Putting any presentation logic into controllers significantly affects testability of the business
logic. Angular offers {@link dev_guide.templates.databinding} for automatic DOM manipulation. If
you have to perform your own manual DOM manipulation, encapsulate the presentation logic in {@link
dev_guide.compiler.widgets widgets} and {@link dev_guide.compiler.directives directives}.
- Input formatting — Use {@link dev_guide.forms angular form widgets} instead.
- Output filtering — Use {@link dev_guide.templates.filters angular filters} instead.
- Run stateless or stateful code shared across controllers — Use {@link dev_guide.services angular
services} instead.
- Instantiate or manage the life-cycle of other components (for example, to create service
instances).


# Associating Controllers with Angular Scope Objects

You can associate controllers with scope objects explicitly via the {@link api/angular.scope.$new
scope.$new} api or implicitly via the {@link api/angular.directive.ng:controller ng:controller
directive} or {@link api/angular.service.$route $route service}.


## Controller Constructor and Methods Example

To illustrate how the controller component works in angular, let's create a little app with the
following components:

- A {@link dev_guide.templates template} with two buttons and a simple message
- A model consisting of a string named `spice`
- A controller with two functions that set the value of `spice`

The message in our template contains a binding to the `spice` model, which by default is set to the
string "very". Depending on which button is clicked, the `spice` model is set to `chili` or
`jalapeño`, and the message is automatically updated by data-binding.


## A Spicy Controller Example

<pre>
<body ng:controller="SpicyCtrl">
 <button ng:click="chiliSpicy()">Chili</button>
 <button ng:click="jalapenoSpicy()">Jalapeño</button>
 <p>The food is {{spice}} spicy!</p>
</body>

function SpicyCtrl() {
 this.spice = 'very';
 this.chiliSpicy = function() {
   this.spice = 'chili';
 }
}

SpicyCtrl.prototype.jalapenoSpicy = function() {
 this.spice = 'jalapeño';
}
</pre>

Things to notice in the example above:

- The `ng:controller` directive is used to (implicitly) create a scope for our template, and the
scope is augmented (managed) by the `SpicyCtrl` controller.
- `SpicyCtrl` is just a plain JavaScript function. As an (optional) naming convention the name
starts with capital letter and ends with "Ctrl" or "Controller".
- The JavaScript keyword `this` in the `SpicyCtrl` function is bound to the scope that the
controller augments.
- Assigning a property to `this` creates or updates the model.
- Controller methods can be created through direct assignment to scope (the `chiliSpicy` method) or
as prototype methods of the controller constructor function(the `jalapenoSpicy` method)
- Both controller methods are available in the template (for the `body` element and and its
children).

Controller methods can also take arguments, as demonstrated in the following variation of the
previous example.

## Controller Method Arguments Example

<pre>
<body ng:controller="SpicyCtrl">
 <input ng:model="customSpice" value="wasabi">
 <button ng:click="spicy('chili')">Chili</button>
 <button ng:click="spicy(customSpice)">Custom spice</button>
 <p>The food is {{spice}} spicy!</p>
</body>

function SpicyCtrl() {
 this.spice = 'very';
 this.spicy = function(spice) {
   this.spice = spice;
 }
}
</pre>

Notice that the `SpicyCtrl` controller now defines just one method called `spicy`, which takes one
argument called `spice`. The template then refers to this controller method and passes in a string
constant `'chili'` in the binding for the first button and a model property `spice` (bound to an
input box) in the second button.


## Controller Inheritance Example

Controller inheritance in angular is based on {@link api/angular.scope Scope} inheritance. Let's
have a look at an example:

<pre>
<body ng:controller="MainCtrl">
 <p>Good {{timeOfDay}}, {{name}}!</p>
 <div ng:controller="ChildCtrl">
   <p>Good {{timeOfDay}}, {{name}}!</p>
   <p ng:controller="BabyCtrl">Good {{timeOfDay}}, {{name}}!</p>
</body>

function MainCtrl() {
 this.timeOfDay = 'morning';
 this.name = 'Nikki';
}

function ChildCtrl() {
 this.name = 'Mattie';
}

function BabyCtrl() {
 this.timeOfDay = 'evening';
 this.name = 'Gingerbreak Baby';
}
</pre>

Notice how we nested three `ng:controller` directives in our template. This template construct will
result in 4 scopes being created for our view:

- The root scope
- The `MainCtrl` scope, which contains `timeOfDay` and `name` models
- The `ChildCtrl` scope, which shadows the `name` model from the previous scope and inherits the
`timeOfDay` model
- The `BabyCtrl` scope, which shadows both the `timeOfDay` model defined in `MainCtrl` and `name`
model defined in the ChildCtrl

Inheritance works between controllers in the same way as it does with models. So in our previous
examples, all of the models could be replaced with controller methods that return string values.

Note: Standard prototypical inheritance between two controllers doesn't work as one might expect,
because as we mentioned earlier, controllers are not instantiated directly by angular, but rather
are applied to the scope object.


## Testing Controllers

The way to test a controller depends upon how complicated the controller is.

- If your controller doesn't use DI or scope methods — create the controller with the `new`
operator and test away. For example:

Controller Function:
<pre>
function myController() {
   this.spices = [{"name":"pasilla", "spiciness":"mild"},
                  {"name":"jalapeno", "spiceiness":"hot hot hot!"},
                  {"name":"habanero", "spiceness":"LAVA HOT!!"}];

   this.spice = "habanero";
}
</pre>

Controller Test:
<pre>
describe('myController function', function() {

  describe('myController', function() {
    var ctrl;

    beforeEach(function() {
      ctrl = new myController();
    });

    it('should create "spices" model with 3 spices', function() {
      expect(ctrl.spices.length).toBe(3);
    });

    it('should set the default value of spice', function() {
      expect(ctrl.spice).toBe('habanero');
    });
  });
});
</pre>

- If your controller does use DI or scope methods — create a root scope, then create the controller
in the root scope with `scope.$new(MyController)`. Test the controller using `$eval`, if necessary.
- If you need to test a nested controller that depends on its parent's state — create a root scope,
create a parent scope, create a child scope, and test the controller using $eval if necessary.


## Related Topics

* {@link dev_guide.mvc About MVC in Angular}
* {@link dev_guide.mvc.understanding_model Understanding the Model Component}
* {@link dev_guide.mvc.understanding_view Understanding the View Component}


