@workInProgress
@ngdoc overview
@name Developer Guide: Angular Services: Managing Service Dependencies
@description

Angular allows services to declare other services as dependencies needed for construction of their
instances.

To declare dependencies, you specify them in the factory function signature and via the `$inject`
property, as an array of string identifiers. Optionally the `$inject` property declaration can be
dropped (see "Inferring `$inject`" but note that that is currently an experimental feature).

Here is an example of two services that depend on each other, as well as on other services that are
provided by angular's web framework:

<pre>
/**
* batchLog service allows for messages to be queued in memory and flushed
* to the console.log every 50 seconds.
*
* @param {*} message Message to be logged.
*/
angular.service('batchLog', function($defer, $log) {
  var messageQueue = [];

  function log() {
    if (messageQueue.length) {
      $log('batchLog messages: ', messageQueue);
      messageQueue = [];
    }
    $defer(log, 50000);
  }

  // start periodic checking
  log();

  return function(message) {
    messageQueue.push(message);
  }
}, {$inject: ['$defer', '$log']});
// note how we declared dependency on built-in $defer and $log services above

/**
* routeTemplateMonitor monitors each $route change and logs the current
* template via the batchLog service.
*/
angular.service('routeTemplateMonitor', function($route, batchLog) {
  this.$on('$afterRouteChange', function() {
    batchLog($route.current ? $route.current.template : null);
  });
}, {$inject: ['$route', 'batchLog'], $eager: true});
</pre>

Things to notice in this example:

* The `batchLog` service depends on the built-in {@link api/angular.service.$defer $defer} and
{@link api/angular.service.$log $log} services, and allows messages to be logged into the
`console.log` in batches.
* The `routeTemplateMonitor` service depends on the built-in {@link api/angular.service.$route
$route} service as well as our custom `batchLog` service.
* The `routeTemplateMonitor` service is declared to be eager, so that it is started as soon as the
application starts.
* To underline the need for the eager instantiation of the `routeTemplateMonitor` service, nothing
else in the application depends on this service, and in this particular case the factory function
of this service doesn't return anything at all.
* Both of our services use the factory function signature as well as the `$inject` property to
declare their dependencies. It is important that the order of the string identifiers in the array
associated with the `$inject` property is the same as the order of argument names in the signature
of the factory function. Unless the dependencies are inferred from the function signature, it is
this array with IDs and their order that the injector uses to determine which services and in which
order to inject.


## Related Topics

* {@link dev_guide.services.understanding_services Understanding Angular Services}
* {@link dev_guide.services.creating_services Creating Angular Services}
* {@link dev_guide.services.registering_services Registering Services}
* {@link dev_guide.services.injecting_controllers Injecting Services Into Controllers }
* {@link dev_guide.services.testing_services Testing Angular Services}

## Related API

* {@link api/angular.service Angular Service API}
* {@link api/angular.injector Angular Injector API}
