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Angular Dependency Injection

Dependency Injection (DI) is a way to create objects that depend upon other objects. A Dependency Injection system supplies the dependent objects (called the dependencies) when it creates an instance of an object.

The Dependency Injection pattern page describes this general approach. The guide you're reading now explains how Angular's own Dependency Injection system works.

DI by example

You'll learn Angular Dependency Injection through a discussion of the sample app that accompanies this guide. Run the anytime.

Start by reviewing this simplified version of the heroes feature from the The Tour of Heroes.

The HeroesComponent is the top-level heroes component. It's only purpose is to display the HeroListComponent which displays a list of hero names.

This version of the HeroListComponent gets its heroes from the HEROES array, an in-memory collection defined in a separate mock-heroes file.

That may suffice in the early stages of development, but it's far from ideal. As soon as you try to test this component or get heroes from a remote server, you'll have to change the implementation of HerosListComponent and replace every other use of the HEROES mock data.

It's better to hide these details inside a service class, defined in its own file.

Create an injectable HeroService

The Angular CLI can generate a new HeroService class in the src/app/heroes folder with this command.

ng generate service heroes/hero

That command creates the following HeroService skeleton.

Assume for now that the @Injectable decorator is an essential ingredient in every Angular service definition. The rest of the class has been rewritten to expose a getHeroes method that returns the same mock data as before.

Of course, this isn't a real data service. If the app were actually getting data from a remote server, the getHeroes method signature would have to be asynchronous.

That's a defect we can safely ignore in this guide where our focus is on injecting the service into the HeroList component.

{@a injector-config} {@a bootstrap}

Register a service provider

A service is just a class in Angular until you register it with an Angular dependency injector.

An Angular injector is responsible for creating service instances and injecting them into classes like the HeroListComponent.

You rarely create an Angular injector yourself. Angular creates injectors for you as it executes the app, starting with the root injector that it creates during the bootstrap process.

You do have to register providers with an injector before the injector can create that service.

Providers tell the injector how to create the service. Without a provider, the injector would not know that it is responsible for injecting the service nor be able to create the service.

You'll learn much more about providers below. For now it is sufficient to know that they create services and must be registered with an injector.

You can register a provider with any Angular decorator that supports the providers array property.

Many Angular decorators accept metadata with a providers property. The two most important examples are @Component and @NgModule.

{@a register-providers-component}

@Component providers

Here's a revised HeroesComponent that registers the HeroService in its providers array.

{@a register-providers-ngmodule}

@NgModule providers

In the following excerpt, the root AppModule registers two providers in its providers array.

The first entry registers the UserService class (not shown) under the UserService injection token. The second registers a value (HERO_DI_CONFIG) under the APP_CONFIG injection token.

Thanks to these registrations, Angular can inject the UserService or the HERO_DI_CONFIG value into any class that it creates.

You'll learn about injection tokens and provider syntax below.

{@a ngmodule-vs-comp}

@NgModule or @Component?

Should you register a service with an Angular module or with a component? The two choices lead to differences in service scope and service lifetime.

Angular module providers (@NgModule.providers) are registered with the application's root injector. Angular can inject the corresponding services in any class it creates. Once created, a service instance lives for the life of the app and Angular injects this one service instance in every class that needs it.

You're likely to inject the UserService in many places throughout the app and will want to inject the same service instance every time. Providing the UserService with an Angular module is a good choice.

To be precise, Angular module providers are registered with the root injector unless the module is lazy loaded. In this sample, all modules are eagerly loaded when the application starts, so all module providers are registered with the app's root injector.

A component's providers (@Component.providers) are registered with each component instance's own injector.

Angular can only inject the corresponding services in that component instance or one of its descendant component instances. Angular cannot inject the same service instance anywhere else.

Note that a component-provided service may have a limited lifetime. Each new instance of the component gets its own instance of the service and, when the component instance is destroyed, so is that service instance.

In this sample app, the HeroComponent is created when the application starts and is never destroyed so the HeroService created for the HeroComponent also live for the life of the app.

If you want to restrict HeroService access to the HeroComponent and its nested HeroListComponent, providing the HeroService in the HeroComponent may be a good choice.

The scope and lifetime of component-provided services is a consequence of the way Angular creates component instances.

Inject a service

The HeroListComponent should get heroes from the HeroService.

The component shouldn't create the HeroService with new. It should ask for the HeroService to be injected.

You can tell Angular to inject a dependency in the component's constructor by specifying a constructor parameter with the dependency type. Here's the HeroListComponent constructor, asking for the HeroService to be injected.

Of course, the HeroListComponent should do something with the injected HeroService. Here's the revised component, making use of the injected service, side-by-side with the previous version for comparison.

Notice that the HeroListComponent doesn't know where the HeroService comes from. You know that it comes from the parent HeroesComponent. But if you decided instead to provide the HeroService in the AppModule, the HeroListComponent wouldn't change at all. The only thing that matters is that the HeroService is provided in some parent injector.

{@a singleton-services}

Singleton services

Services are singletons within the scope of an injector. There is at most one instance of a service in a given injector.

There is only one root injector and the UserService is registered with that injector. Therefore, there can be just one UserService instance in the entire app and every class that injects UserService get this service instance.

However, Angular DI is a hierarchical injection system, which means that nested injectors can create their own service instances. Angular creates nested injectors all the time.

{@a component-child-injectors}

Component child injectors

For example, when Angular creates a new instance of a component that has @Component.providers, it also creates a new child injector for that instance.

Component injectors are independent of each other and each of them creates its own instances of the component-provided services.

When Angular destroys one of these component instance, it also destroys the component's injector and that injector's service instances.

Thanks to injector inheritance, you can still inject application-wide services into these components. A component's injector is a child of its parent component's injector, and a descendent of its parent's parent's injector, and so on all the way back to the application's root injector. Angular can inject a service provided by any injector in that lineage.

For example, Angular could inject a HeroListComponent with both the HeroService provided in HeroComponent and the UserService provided in AppModule.

{@a testing-the-component}

Testing the component

Earlier you saw that designing a class for dependency injection makes the class easier to test. Listing dependencies as constructor parameters may be all you need to test application parts effectively.

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

Learn more in the Testing guide.

{@a service-needs-service}

When the service needs a service

The 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? You'd apply the same constructor injection pattern, adding a constructor that takes a Logger parameter.

Here is the revised HeroService that injects the Logger, side-by-side with the previous service for comparison.

The constructor asks for an injected instance of a Logger and stores it in a private field called logger. The getHeroes() method logs a message when asked to fetch heroes.

{@a logger-service}

The dependent Logger service

The sample app's Logger service is quite simple:

If the app didn't provide this Logger, Angular would throw an exception when it looked for a Logger to inject into the HeroService.

ERROR Error: No provider for Logger!

Because a singleton logger service is useful everywhere, it's provided in the root AppModule.

{@a injectable}

@Injectable()

The @Injectable() decorator identifies a service class that might require injected dependencies.

The HeroService must be annotated with @Injectable() because it requires an injected Logger.

Always write @Injectable() with parentheses, not just @Injectable.

When Angular creates a class whose constructor has parameters, it looks for type and injection metadata about those parameters so that it can inject the right service.

If Angular can't find that parameter information, it throws an error.

Angular can only find the parameter information if the class has a decorator of some kind. While any decorator will do, the @Injectable() decorator is the standard decorator for service classes.

The decorator requirement is imposed by TypeScript.

TypeScript normally discards parameter type information when it transpiles the code to JavaScript. It preserves this information if the class has a decorator and the emitDecoratorMetadata compiler option is set true in TypeScript's tsconfig.json configuration file, .

The CLI configures tsconfig.json with emitDecoratorMetadata: true It's your job to put @Injectable() on your service classes.

The Logger service is annotated with @Injectable() decorator too, although it has no constructor and no dependencies.

In fact, every Angular service class in this app is annotated with the @Injectable() decorator, whether or not it has a constructor and dependencies. @Injectable() is a required coding style for services.

{@a providers}

Providers

A service 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, directives, pipes, and other services.

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

The next few sections explain the many ways you can specify a provider.

Almost all of the accompanying code snippets are extracts from the sample app's providers.component.ts file.

The class as its own provider

There are many ways to provide something that looks and behaves like a Logger. The Logger class itself is an obvious and natural provider.

But it's not the only way.

You can configure the injector with alternative providers that can deliver an object that behaves like a Logger. You could provide a substitute class. You could provide a logger-like object. You 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.

{@a provide}

The provide object literal

Here's the class-provider syntax again.

This is actually a shorthand expression for a provider registration using a provider object literal with two properties:

The provide property holds the token that serves as the key for both locating a dependency value and registering the provider.

The second property is always a provider definition object, which you can think of as a recipe for creating the dependency value. There are many ways to create dependency values just as there are many ways to write a recipe.

{@a class-provider}

Alternative class providers

Occasionally you'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.

{@a class-provider-dependencies}

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 is also injected at the application level.

Configure it like BetterLogger.

{@a aliased-class-providers}

Aliased class providers

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

When the old component logs a message with OldLogger, you'd like 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.

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

The solution: alias with the useExisting option.

{@a value-provider}

Value providers

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

Then you 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 InjectionToken sections.

{@a factory-provider}

Factory providers

Sometimes you need to create the dependent value dynamically, based on information you 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.

To illustrate the point, add 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 you log in a different user.

Unlike EvenBetterLogger, you 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.

You can inject the Logger, but you can't inject the boolean isAuthorized. You'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, the factory function does.

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

The useFactory field tells Angular that the provider is a factory function whose implementation is the heroServiceFactory.

The deps property is an array of provider tokens. The Logger and UserService classes serve as tokens for their own class providers. The injector resolves these tokens and injects the corresponding services into the matching factory function parameters.

Notice that you captured the factory provider in an exported variable, heroServiceProvider. This extra step makes the factory provider reusable. You can register the HeroService with this variable wherever you need it.

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

{@a token}

Dependency injection tokens

When you register a provider with an injector, you 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 you get a HeroService directly from the injector by supplying the HeroService type as the token:

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

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

{@a non-class-dependencies}

Non-class dependencies

What if the dependency value isn't a class? Sometimes the thing you want to inject is a string, function, or object.

Applications often define configuration objects with lots of small facts (like the title of the application or the address of a web API endpoint) but these configuration objects aren't always instances of a class. They can be object literals such as this one:

What if you'd like to make this configuration object available for injection? You know you can register an object with a value provider.

But what should you use as the token? You don't have a class to serve as a token. There is no AppConfig class.

TypeScript interfaces aren't valid tokens

The HERO_DI_CONFIG constant conforms to the AppConfig interface. Unfortunately, you cannot use a TypeScript interface as a token:

That seems strange if you're used to dependency injection in strongly typed languages, where an interface is the preferred dependency lookup key.

It's not Angular's doing. An interface is a TypeScript design-time artifact. JavaScript doesn't have interfaces. The TypeScript interface disappears from the generated JavaScript. There is no interface type information left for Angular to find at runtime.

{@a injection-token}

InjectionToken

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

The type parameter, while optional, conveys the dependency's type to developers and tooling. The token description is another developer aid.

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

Although the AppConfig interface plays no role in dependency injection, it supports typing of the configuration object within the class.

Alternatively, you can provide and inject the configuration object in an ngModule like AppModule.

{@a optional}

Optional dependencies

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

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

Summary

You learned the basics of Angular dependency injection in this page. You can register various kinds of providers, and you 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 this guide has described. You can learn more about its advanced features, beginning with its support for nested injectors, in Hierarchical Dependency Injection.

{@a explicit-injector}

Appendix: Working with injectors directly

Developers 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 in ngOnInit().

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. You can call get() with a second parameter, which is the value to return if the service is not found. Angular can't find the service if it's not registered with this or any ancestor injector.

The technique is an example of the service locator pattern.

Avoid this technique unless you genuinely need it. It encourages a careless grab-bag approach such as you see here. It's difficult to explain, understand, and test. You can't know by inspecting the constructor what this class requires or what it will do. It could acquire services from any ancestor component, not just its own. You're forced to spelunk the implementation to discover what it does.

Framework developers may take this approach when they must acquire services generically and dynamically.

{@a one-class-per-file}

Appendix: one class per file

Having multiple classes in the same file is confusing and best avoided. Developers expect one class per file. Keep them happy.

If you combine the HeroService class with the HeroesComponent in the same file, define the component last. If you define the component before the service, you'll get a runtime null reference error.

You actually can define the component first with the help of the forwardRef() method as explained in this blog post.

But it's best to avoid the problem altogether by defining components and services in separate files.

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