One aspect of Angular that I love is it’s dependency injection. Contrary to some criticisms I’ve read, I find it is extremely flexible and powerful enough to address the demands of enterprise line of business apps. I discussed the general benefits of DI in Dependency Injection Explained and specifically Angular’s implementation using Providers, Services, and Factories and even aspect-oriented interception/decoration.
In this post, I address some other features common to advanced Inversion of Control containers, namely lazy-loading, lifetime management, and deferred creation/resolution.
Lazy-Loading
Lazy-loading simply refers to the late instantiation of objects when you need them. Many dependency injection systems will build-up a component the first time it is recognized as a dependency, but in some cases you may not want to instantiate the component until later in the application lifetime. In Angular, the perfect example is when you are setting up a behavior in the configuration pass that references components that haven’t been created yet.
Let’s assume you want to intercept the built-in $log service so that it stores entries on the $rootScope. I don’t recommend this but it works for a simple, contrived example. To intercept, you reference $provide in the configuration pass and then call the decorator method. If you try to reference the $rootScope directly, you’ll get an exception because of circular dependencies. The solution is to lazy-load the $rootScope instead, using the $injector.
The following code will only load the $rootScope the first time it’s needed.
$provide.decorator('$log', ['$delegate', '$injector',
function ($delegate, $injector) {
var log = $delegate.log.bind($delegate);
$delegate.log = function (msg) {
var rs = $injector.get('$rootScope');
if (rs.logs === undefined) {
rs.logs = [];
}
rs.logs.push(msg);
log(msg);
};
return $delegate;
}]);
Subsequent calls will always get the same singleton instance of $rootScope. Here is the working fiddle. I’ve heard this (erroneous) criticism before (that Angular only supports singletons) … not true. The methods on the $injector are what you use to manage the lifetime of your components.
Lifetime Management
Lifetime management relates to how you handle instances of components. By default, when you inject an Angular dependency, the dependency injection will create a single copy and reuse that copy throughout your app. In most circumstances this is exactly the behavior you want. There are some solutions that may require multiple instances of the same component. Consider for a moment a counter service:
function Counter($log) {
$log.log('Counter created.');
} angular.extend(Counter.prototype, {
count: 0,
increment: function () {
this.count += 1;
return this.count;
}
}); Counter.$inject = ['$log']; app.service('counter', Counter);
Your app may need to keep track of different counters. When you inject the service, you always get the same counter. Is this an Angular limitation?
Not exactly. Again, using the $injector service you can instantiate a new copy any time you like. This code uses two separate counters:
app.run(['$rootScope', 'counter', '$injector',
function (rs, c, i) {
rs.count = c.count;
rs.update = c.increment;
rs.update2 = function () {
var c = i.instantiate(Counter);
rs.count2 = c.count;
rs.update2 = function () {
c.increment();
rs.count2 = c.count;
};
};
}]);
You can see each count is tracked in a separate instance in the working fiddle. If you know you are going to generate new instances often, you can register the service like this:
app.factory('counterFactory', ['$injector',
function (i) {
return {
getCounter: function () {
return i.instantiate(Counter);
}
};
}]);
Then it’s simple to grab a new instance as needed, and you can reference your factory component instead of the $injector:
app.run(['$rootScope', 'counterFactory',
function (rs, cf) {
var c1 = cf.getCounter(),
c2 = cf.getCounter();
rs.count = c1.count;
rs.update = c1.increment;
rs.count2 = c2.count;
rs.update2 = function () {
rs.count2 = c2.increment();
};
}]);
You can check out this version by running the full fiddle here. As you can see, it is entirely possible to manage the lifetime of your components using Angular’s built-in dependency injection. But what about deferred resolution – i.e. those components you may introduce after Angular is already configured but still need to be wired up with their own dependencies?
Deferred Resolution
You’ve already seen one way you can defer the resolution of dependencies in Angular. When you want to wire something up you can call instantiate on the $injector service and it will resolve dependencies using either parameter sniffing, by looking for a static $inject property, or by inspecting an array you pass in. In other words, this is perfectly valid:
$injector.instantiate(['dependency', Constructor]);
You can also invoke a function decorated with an array as well. If you have a function that depends on the $log service, you can invoke it at run-time with the dependency resolved like this:
var myFunc = ['$log', function ($log) {
$log.log('This dependency wired at runtime.');
}]; $injector.invoke(myFunc);
You can check out the working fiddle here (open your console to verify what happens when you click the button).
Summary
In summary, Angular’s dependency injection provides many advanced features you would expect and often require for a line of business application. The shortcut methods for factories, services, and providers sometimes confuse Angular developers into believing these are the only options available. The magic really happens on the $injector service where you can grab your singleton instance, build up a new component or dynamically invoke a function with dependencies.
As one final note, the injector is available to your client code even outside of Angular. To see an example of JavaScript code that is wired up outside of Angular yet uses the injector to grab the $log service, click here. Why is ‘ng’ passed in the array for the function? This is the core Angular module that is added implicitly when you wire up your own modules but must be explicitly included when you directly create your own instance of the injector.
