Dependency Injection

Dependency injection is an important application design pattern. Angular has its own dependency injection framework, and we really can't build an Angular application without it. It's used so widely that almost everyone just calls it DI.

In this chapter we'll learn what DI is and why we want it. Then we'll learn how to use it in an Angular app.

• Why dependency injection?
• Angular dependency injection
• Injector providers
• Dependency injection tokens
• Summary

Run the .

Why dependency injection?

Let's start with the following code.

Our Car creates everything it needs inside its constructor. What's the problem? The problem is that our Car class is brittle, inflexible, and hard to test.

Our Car needs an engine and tires. Instead of asking for them, the Car constructor instantiates its own copies from the very specific classes Engine and Tires.

What if the Engine class evolves and its constructor requires a parameter? Our Car is broken and stays broken until we rewrite it along the lines of engine = new Engine(theNewParameter). We didn't care about Engine constructor parameters when we first wrote Car. We don't really care about them now. But we'll have to start caring because when the definition of Engine changes, our Car class must change. That makes Car brittle.

What if we want to put a different brand of tires on our Car? Too bad. We're locked into whatever brand the Tires class creates. That makes our Car inflexible.

Right now each new car gets its own engine. It can't share an engine with other cars. While that makes sense for an automobile engine, we can think of other dependencies that should be shared, such as the onboard wireless connection to the manufacturer's service center. Our Car lacks the flexibility to share services that have been created previously for other consumers.

When we write tests for our Car we're at the mercy of its hidden dependencies. Is it even possible to create a new Engine in a test environment? What does Engineitself depend upon? What does that dependency depend on? Will a new instance of Engine make an asynchronous call to the server? We certainly don't want that going on during our tests.

What if our Car should flash a warning signal when tire pressure is low? How do we confirm that it actually does flash a warning if we can't swap in low-pressure tires during the test?

We have no control over the car's hidden dependencies. When we can't control the dependencies, a class becomes difficult to test.

How can we make Car more robust, flexible, and testable?

That's super easy. We change our Car constructor to a version with DI:

See what happened? We moved the definition of the dependencies to the constructor. Our Car class no longer creates an engine or tires. It just consumes them.

Now we create a car by passing the engine and tires to the constructor.

How cool is that? The definition of the engine and tire dependencies are decoupled from the Car class itself. We can pass in any kind of engine or tires we like, as long as they conform to the general API requirements of an engine or tires.

If someone extends the Engine class, that is not Car's problem.

The Car class is much easier to test because we are in complete control of its dependencies. We can pass mocks to the constructor that do exactly what we want them to do during each test:

We just learned what dependency injection is.

It's a coding pattern in which a class receives its dependencies from external sources rather than creating them itself.

Cool! But what about that poor consumer? Anyone who wants a Car must now create all three parts: the Car, Engine, and Tires. The Car class shed its problems at the consumer's expense. We need something that takes care of assembling these parts for us.

We could write a giant class to do that:

It's not so bad now with only three creation methods. But maintaining it will be hairy as the application grows. This factory is going to become a huge spiderweb of interdependent factory methods!

Wouldn't it be nice if we could simply list the things we want to build without having to define which dependency gets injected into what?

This is where the dependency injection framework comes into play. Imagine the framework had something called an injector. We register some classes with this injector, and it figures out how to create them.

When we need a Car, we simply ask the injector to get it for us and we're good to go.

Everyone wins. The Car knows nothing about creating an Engine or Tires. The consumer knows nothing about creating a Car. We don't have a gigantic factory class to maintain. Both Car and consumer simply ask for what they need and the injector delivers.

This is what a dependency injection framework is all about.

Now that we know what dependency injection is and appreciate its benefits, let's see how it is implemented in Angular.

Angular dependency injection

Angular ships with its own dependency injection framework. This framework can also be used as a standalone module by other applications and frameworks.

That sounds nice. What does it do for us when building components in Angular? Let's see, one step at a time.

We'll begin with a simplified version of the HeroesComponent that we built in the The Tour of Heroes.

The HeroesComponent is the root component of the Heroes feature area. It governs all the child components of this area. Our stripped down version has only one child, HeroListComponent, which displays a list of heroes.

Right now HeroListComponent gets heroes from HEROES, an in-memory collection defined in another file. That may suffice in the early stages of development, but it's far from ideal. As soon as we try to test this component or want to get our heroes data from a remote server, we'll have to change the implementation of heroes and fix every other use of the HEROES mock data.

Let's make a service that hides how we get hero data.

Our HeroService exposes a getHeroes method that returns the same mock data as before, but none of its consumers need to know that.

A service is nothing more than a class in Angular. It remains nothing more than a class until we register it with an Angular injector.

Configuring the injector

We don't have to create an Angular injector. Angular creates an application-wide injector for us during the bootstrap process.

We do have to configure the injector by registering the providers that create the services our application requires. We'll explain what providers are later in this chapter.

Before we do, let's see an example of provider registration during bootstrapping:

The injector now knows about our HeroService. An instance of our HeroService will be available for injection across our entire application.

Of course we can't help wondering about that comment telling us not to do it this way. It will work. It's just not a best practice. The bootstrap provider option is intended for configuring and overriding Angular's own preregistered services, such as its routing support.

The preferred approach is to register application providers in application components. Because the HeroService is used within the Heroes feature area — and nowhere else — the ideal place to register it is in the top-level HeroesComponent.

Registering providers in a component

Here's a revised HeroesComponent that registers the HeroService.

Look at the providers part of the @Component annotation. An instance of the HeroService is now available for injection in this HeroesComponent and all of its child components.

The HeroesComponent itself doesn't happen to need the HeroService. But its child HeroListComponent does, so we head there next.

Preparing the HeroListComponent for injection

The HeroListComponent should get heroes from the injected HeroService. Per the dependency injection pattern, the component must ask for the service in its constructor, as we explained earlier. It's a small change:

Implicit injector creation

When we introduced the idea of an injector above, we showed how to use it to create a new Car. Here we also show how such an injector would be explicitly created:

We won't find code like that in the Tour of Heroes or any of our other samples. We could write code that explicitly creates an injector if we had to, but we rarely do. Angular takes care of creating and calling injectors when it creates components for us — whether through HTML markup, as in <hero-list></hero-list>, or after navigating to a component with the router. If we let Angular do its job, we'll enjoy the benefits of automated dependency injection.

Singleton services

Dependencies are singletons within the scope of an injector. In our example, a single HeroService instance is shared among the HeroesComponent and its HeroListComponent children.

However, Angular DI is an hierarchical injection system, which means that nested injectors can create their own service instances. Learn more about that in the Hierarchical Injectors chapter.

Testing the component

We emphasized earlier that designing a class for dependency injection makes the class easier to test. Listing dependencies as constructor parameters may be all we need to test application parts effectively.

For example, we can create a new HeroListComponent with a mock service that we can manipulate under test:

When the service needs a service

Our HeroService is very simple. It doesn't have any dependencies of its own.

What if it had a dependency? What if it reported its activities through a logging service? We'd apply the same constructor injection pattern, adding a constructor that takes a Logger parameter.

Here is the revision compared to the original.

The constructor now asks for an injected instance of a Logger and stores it in a private property called _logger. We call that property within our getHeroes method when anyone asks for heroes.

Why @Injectable()?

@Injectable() marks a class as available to an injector for instantiation. Generally speaking, an injector will report an error when trying to instantiate a class that is not marked as @Injectable().

Injectors are also responsible for instantiating components like HeroesComponent. Why haven't we marked HeroesComponent as @Injectable()?

We can add it if we really want to. It isn't necessary because the HeroesComponent is already marked with @Component, and this annotation class (like @Directive and @Pipe, which we'll learn about later) is a subtype of Injectable. It is in fact Injectable annotations that identify a class as a target for instantiation by an injector.

Creating and registering a logger service

We're injecting a logger into our HeroService in two steps:

1. Create the logger service.
2. Register it with the application.

Our logger service is quite simple:

We're likely to need the same logger service everywhere in our application, so we put it in the project's lib folder, and we register it in the providers list of our application component, AppComponent.

If we forget to register the logger, Angular throws an exception when it first looks for the logger:

That's Angular telling us that the dependency injector couldn't find the provider for the logger. It needed that provider to create a Logger to inject into a new HeroService, which it needed to create and inject into a new HeroListComponent.

The chain of creations started with the Logger provider. Providers are the subject of our next section.

Injector providers

A provider provides the concrete, runtime version of a dependency value. The injector relies on providers to create instances of the services that the injector injects into components and other services.

We must register a service provider with the injector, or it won't know how to create the service.

Earlier we registered the Logger service in the providers list of the metadata for the AppModule like this:

There are many ways to provide something that implements Logger. The Logger class itself is an obvious and natural provider. But it's not the only way.

We can configure the injector with alternative providers that can deliver a Logger. We could provide a substitute class. We could give it a provider that calls a logger factory function. Any of these approaches might be a good choice under the right circumstances.

What matters is that the injector has a provider to go to when it needs a Logger.

The Provider class

We wrote the providers list like this:

This is actually a shorthand expression for a provider registration that creates a new instance of the Provider class:

We supply two arguments (or more) to the Provider constructor.

The first is the token that serves as the key for both locating a dependency value and registering the provider.

The second is a named parameter, such as useClass, which we can think of as a recipe for creating the dependency value. There are many ways to create dependency values ... and many ways to write a recipe.

Alternative class providers

Occasionally we'll ask a different class to provide the service. The following code tells the injector to return a BetterLogger when something asks for the Logger.

Class provider with dependencies

Maybe an EvenBetterLogger could display the user name in the log message. This logger gets the user from the injected UserService, which happens also to be injected at the application level.

Configure it like we did BetterLogger.

Aliased class providers

Suppose an old component depends upon an OldLogger class. OldLogger has the same interface as the NewLogger, but for some reason we can't update the old component to use it.

When the old component logs a message with OldLogger, we want the singleton instance of NewLogger to handle it instead.

The dependency injector should inject that singleton instance when a component asks for either the new or the old logger. The OldLogger should be an alias for NewLogger.

We certainly do not want two different NewLogger instances in our app. Unfortunately, that's what we get if we try to alias OldLogger to NewLogger with useClass.

The solution: alias with the useExisting option.

Value providers

Sometimes it's easier to provide a ready-made object rather than ask the injector to create it from a class.

Then we register a provider with the useValue option, which makes this object play the logger role.

See more useValue examples in the Non-class dependencies and OpaqueToken sections.

Factory providers

Sometimes we need to create the dependent value dynamically, based on information we won't have until the last possible moment. Maybe the information changes repeatedly in the course of the browser session.

Suppose also that the injectable service has no independent access to the source of this information.

This situation calls for a factory provider.

Let's illustrate by adding a new business requirement: the HeroService must hide secret heroes from normal users. Only authorized users should see secret heroes.

Like the EvenBetterLogger, the HeroService needs a fact about the user. It needs to know if the user is authorized to see secret heroes. That authorization can change during the course of a single application session, as when we log in a different user.

Unlike EvenBetterLogger, we can't inject the UserService into the HeroService. The HeroService won't have direct access to the user information to decide who is authorized and who is not.

Instead the HeroService constructor takes a boolean flag to control display of secret heroes.

We can inject the Logger, but we can't inject the boolean isAuthorized. We'll have to take over the creation of new instances of this HeroService with a factory provider.

A factory provider needs a factory function:

Although the HeroService has no access to the UserService, our factory function does.

We inject both the Logger and the UserService into the factory provider and let the injector pass them along to the factory function:

Notice that we captured the factory provider in a constant, heroServiceProvider. This extra step makes the factory provider reusable. We can register our HeroService with this constant wherever we need it.

In our sample, we need it only in the HeroesComponent, where it replaces the previous HeroService registration in the metadata providers list. Here we see the new and the old implementation side-by-side:

Dependency injection tokens

When we register a provider with an injector, we associate that provider with a dependency injection token. The injector maintains an internal token-provider map that it references when asked for a dependency. The token is the key to the map.

In all previous examples, the dependency value has been a class instance, and the class type served as its own lookup key. Here we get a HeroService directly from the injector by supplying the HeroService type as the token:

We have similar good fortune when we write a constructor that requires an injected class-based dependency. We define a constructor parameter with the HeroService class type, and Angular knows to inject the service associated with that HeroService class token:

This is especially convenient when we consider that most dependency values are provided by classes.

Non-class dependencies

What if the dependency value isn't a class? Sometimes the thing we want to inject is a string, list, map, or maybe a function.

Applications often define configuration objects with lots of small facts (like the title of the application or the address of a web API endpoint). They can be Map literals such as this one:

We'd like to make this configuration object available for injection. We know we can register an object with a value provider.

But what should we use as the token? While we could use Map, we should not because (like String) Map is too general. Our app might depend on several maps, each for a different purpose.

OpaqueToken

One solution to choosing a provider token for non-class dependencies is to define and use an OpaqueToken. The definition looks like this:

We register the dependency provider using the OpaqueToken object:

Now we can inject the configuration object into any constructor that needs it, with the help of an @Inject annotation:

As an alternative to using a configuration Map, we can define a custom configuration class:

Defining a configuration class has a few benefits. One key benefit is strong static checking: we'll be warned early if we misspell a property name or assign it a value of the wrong type. The Dart cascade notation (..) provides a convenient means of initializing a configuration object.

If we use cascades, the configuration object can't be declared const and we can't use a value provider. A solution is to use a factory provider. We illustrate this next. We also show how to provide and inject the configuration object in our top-level AppComponent:

Optional dependencies

Our HeroService requires a Logger, but what if it could get by without a logger? We can tell Angular that the dependency is optional by annotating the constructor argument with @Optional():

When using @Optional(), our code must be prepared for a null value. If we don't register a logger somewhere up the line, the injector will set the value of logger to null.

Summary

We learned the basics of Angular dependency injection in this chapter. We can register various kinds of providers, and we know how to ask for an injected object (such as a service) by adding a parameter to a constructor.

Angular dependency injection is more capable than we've described. We can learn more about its advanced features, beginning with its support for nested injectors, in the Hierarchical Dependency Injection chapter.

Appendix: Working with injectors directly

We rarely work directly with an injector, but here's an InjectorComponent that does.

An Injector is itself an injectable service.

In this example, Angular injects the component's own Injector into the component's constructor. The component then asks the injected injector for the services it wants.

Note that the services themselves are not injected into the component. They are retrieved by calling injector.get.

The get method throws an error if it can't resolve the requested service. We can call get with a second parameter (the value to return if the service is not found) instead, which we do in one case to retrieve a service (ROUS) that isn't registered with this or any ancestor injector.