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 $inject
" but note that that is currently an experimental feature).
Here is an example of two services that depend on each other, as well as on other services that are provided by angular's web framework:
/** * batchLog service allows for messages to be queued in memory and flushed * to the console.log every 50 seconds. * * @param {*} message Message to be logged. */ angular.service('batchLog', function($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) { $route.onChange(function() { batchLog($route.current ? $route.current.template : null); }); }, {$inject: ['$route', 'batchLog'], $eager: true});
Things to notice in this example:
batchLog
service depends on the built-in $defer
and
$log
services, and allows messages to be logged into the
console.log
in batches.routeTemplateMonitor
service depends on the built-in $route
service as well as our custom batchLog
service.routeTemplateMonitor
service is declared to be eager, so that it is started as soon as the
application starts.routeTemplateMonitor
service, nothing
else in the application depends on this service, and in this particular case the factory function
of this service doesn't return anything at all.$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.