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`$injector` is used to retrieve object instances as defined by provider, instantiate types, invoke methods, and load modules.

The following always holds true:

    var $injector = angular.injector();
    expect($injector.get('$injector')).toBe($injector);
    expect($injector.invoke(function($injector) {
      return $injector;
    })).toBe($injector);

# Injection Function Annotation

JavaScript does not have annotations, and annotations are needed for dependency injection. The following are all valid ways of annotating function with injection arguments and are equivalent.

    // inferred (only works if code not minified/obfuscated)
    $injector.invoke(function(serviceA){});

    // annotated
    function explicit(serviceA) {};
    explicit.$inject = ['serviceA'];
    $injector.invoke(explicit);

    // inline
    $injector.invoke(['serviceA', function(serviceA){}]);

## Inference

In JavaScript calling `toString()` on a function returns the function definition. The definition can then be parsed and the function arguments can be extracted. This method of discovering annotations is disallowed when the injector is in strict mode. NOTE: This does not work with minification, and obfuscation tools since these tools change the argument names.

## `$inject` Annotation

By adding an `$inject` property onto a function the injection parameters can be specified.

## Inline

As an array of injection names, where the last item in the array is the function to call.

## Methods

- ###

  `get(name, [caller]);`

  Return an instance of the service.

  #### Parameters

  Param | Type | Details
  --- | --- | ---
  name | string | The name of the instance to retrieve.
  caller (optional) | string | An optional string to provide the origin of the function call for error messages.

  #### Returns

  * | The instance.

- ###

  `invoke(fn, [self], [locals]);`

  Invoke the method and supply the method arguments from the `$injector`.

  #### Parameters

  Param | Type | Details
  --- | --- | ---
  fn | function()Array.<(string|function())> | The injectable function to invoke. Function parameters are injected according to the $inject Annotation rules.
  self (optional) | Object | The `this` for the invoked method.
  locals (optional) | Object | Optional object. If preset then any argument names are read from this object first, before the `$injector` is consulted.

  #### Returns

  * | the value returned by the invoked `fn` function.

- ###

  `has(name);`

  Allows the user to query if the particular service exists.

  #### Parameters

  Param | Type | Details
  --- | --- | ---
  name | string | Name of the service to query.

  #### Returns

  boolean | `true` if injector has given service.

- ###

  `instantiate(Type, [locals]);`

  Create a new instance of JS type. The method takes a constructor function, invokes the new operator, and supplies all of the arguments to the constructor function as specified by the constructor annotation.

  #### Parameters

  Param | Type | Details
  --- | --- | ---
  Type | Function | Annotated constructor function.
  locals (optional) | Object | Optional object. If preset then any argument names are read from this object first, before the `$injector` is consulted.

  #### Returns

  Object | new instance of `Type`.

- ###

  `annotate(fn, [strictDi]);`

  Returns an array of service names which the function is requesting for injection. This API is used by the injector to determine which services need to be injected into the function when the function is invoked. There are three ways in which the function can be annotated with the needed dependencies.

  # Argument names

  The simplest form is to extract the dependencies from the arguments of the function. This is done by converting the function into a string using `toString()` method and extracting the argument names.

      // Given
      function MyController($scope, $route) {
        // ...
      }

      // Then
      expect(injector.annotate(MyController)).toEqual(['$scope', '$route']);

  You can disallow this method by using strict injection mode.

  This method does not work with code minification / obfuscation. For this reason the following annotation strategies are supported.

  # The `$inject` property

  If a function has an `$inject` property and its value is an array of strings, then the strings represent names of services to be injected into the function.

      // Given
      var MyController = function(obfuscatedScope, obfuscatedRoute) {
        // ...
      }
      // Define function dependencies
      MyController['$inject'] = ['$scope', '$route'];

      // Then
      expect(injector.annotate(MyController)).toEqual(['$scope', '$route']);

  # The array notation

  It is often desirable to inline Injected functions and that's when setting the `$inject` property is very inconvenient. In these situations using the array notation to specify the dependencies in a way that survives minification is a better choice:

      // We wish to write this (not minification / obfuscation safe)
      injector.invoke(function($compile, $rootScope) {
        // ...
      });

      // We are forced to write break inlining
      var tmpFn = function(obfuscatedCompile, obfuscatedRootScope) {
        // ...
      };
      tmpFn.$inject = ['$compile', '$rootScope'];
      injector.invoke(tmpFn);

      // To better support inline function the inline annotation is supported
      injector.invoke(['$compile', '$rootScope', function(obfCompile, obfRootScope) {
        // ...
      }]);

      // Therefore
      expect(injector.annotate(
         ['$compile', '$rootScope', function(obfus_$compile, obfus_$rootScope) {}])
       ).toEqual(['$compile', '$rootScope']);

  #### Parameters

  Param | Type | Details
  --- | --- | ---
  fn | function()Array.<(string|function())> | Function for which dependent service names need to be retrieved as described above.
  strictDi (optional) | boolean | Disallow argument name annotation inference. (default: false)

  #### Returns

  Array.<string> | The names of the services which the function requires.