@workInProgress
@ngdoc overview
@name Developer Guide: DI: Understanding DI in Angular
@description


While DI is widely used in statically typed languages such as Java or C++, it has not been widely
used in JavaScript. Angular brings the benefits of DI into JavaScript apps.

In angular, DI is implemented as a subsystem that manages dependencies between services,
controllers, widgets, and filters. The most important of these are {@link api/angular.service
services}.

Services are objects that handle common tasks in web applications. Angular provides several{@link
api/angular.service built-in services}, and you can create your own custom services.

The main job of angular's DI subsystem is to provide services to angular components that depend on
them. The way the DI subsystem provides services is as follows: all services are registered with
angular's {@link api/angular.service service API}, and all components that depend on services
define those dependencies as a property (`$inject`). With this information, the DI subsystem
manages the creation of service objects and the provision of those objects to the components that
need them, at the time they need them. The following illustration steps through the sequence of
events:

<img src="img/guide/di_sequence_final.png">

In the illustration above, the dependency injection sequence proceeds as follows:

1. Service factory functions are registered with angular's service factory repository.
2. `ng:autobind` triggers angular's bootstrap sequence, during which angular compiles the template,
creates the root scope, and creates the dependency injector.
3. The `ng:controller` directive implicitly creates a new child scope, augmented by the application
of the `PhoneListCtrl` controller function.
4. The Injector identifies the `$xhr` service as `PhoneListCtrl` controller's only dependency.
5. The Injector checks if the `$xhr` service has already been instantiated, and if not uses the
factory function from the service factory repository to construct it.
6. DI provides the instance of $xhr service to the PhoneListCtrl controller constructor


## How Scope Relates to DI

The {@link api/angular.injector injector} is responsible for resolving the service dependencies in
the application. It gets created and configured with the creation of a root scope. The injector
caches instances of services, with the services cache bound to the root scope.

Different root scopes have different instances of the injector. While typical angular applications
will only have one root scope (and hence the services will act like application singletons), in
tests it is important to not share singletons across test invocations for isolation reasons. We
achieve the necessary isolation by having each test create its own separate root scope.

<pre>
// create a root scope
var rootScope = angular.scope();
// access the service locator
var myService = rootScope.$service('myService');
</pre>

## Inferring dependencies from the signature of the factory function or constructor

**EXPERIMENTAL FEATURE**: This is an experimental feature. See the important note at the end of
this section for drawbacks.

We resort to `$inject` and our own annotation because there is no way in JavaScript to get a list
of arguments. Or is there? It turns out that calling `.toString()` on a function returns the
function declaration along with the argument names as shown below:

<pre>
function myFn(a,b){}
expect(myFn.toString()).toEqual('function myFn(a,b){}');
</pre>

This means that angular can infer the function names after all and use that information to generate
the `$inject` annotation automatically. Therefore the following two function definitions are
equivalent:

<pre>
// given a user defined service
angular.service('serviceA', ...);

// inject '$window', 'serviceA', curry 'name';
function fnA($window, serviceA, name){};
fnA.$inject = ['$window', 'serviceA'];

// inject '$window', 'serviceA', curry 'name';
function fnB($window, serviceA_, name){};
// implies: fnB.$inject = ['$window', 'serviceA'];
</pre>

If angular does not find a `$inject` annotation on the function, then it calls the `.toString()`
method and tries to infer what should be injected by using function argument names as dependency
identifiers.

**IMPORTANT**
Minifiers/obfuscators change the names of function arguments and will therefore break the `$inject`
inference. For this reason, either explicitly declare the `$inject` or do not use
minifiers/obfuscators. In the future, we may provide a pre-processor which will scan the source
code and insert the `$inject` into the source code so that it can be minified/obfuscated.


## Related Topics

* {@link dev_guide.services Angular Services}

## Related API

* {@link api/angular.service Services API}
