Dependency Injection 6.0

Dependency injection is an important app design pattern. It’s used so widely that almost everyone just calls it DI.

Angular has its own dependency injection framework, and you really can’t build an Angular app without it.

This page covers what DI is, why it’s useful, and how to use Angular DI.

Run the live example (view source).

Why dependency injection?

To understand why dependency injection is so important, consider an example without it. Imagine writing the following code:

class Car {
  Engine engine;
  Tires tires;
  var description = 'No DI';

  Car() {
    engine = Engine();
    tires = Tires();
  }

  // Method using the engine and tires
  String drive() => '$description car with '
      '${engine.cylinders} cylinders and '
      '${tires.make} tires.';
}

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

This 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? That would break the Car class and it would stay broken until you rewrote it along the lines of engine = Engine(theNewParameter). The Engine constructor parameters weren’t even a consideration when you first wrote Car. You may not anticipate them even now. But you’ll have to start caring because when the definition of Engine changes, the Car class must change. That makes Car brittle.

What if you want to put a different brand of tires on your Car? Too bad. You’re locked into whatever brand the Tires class creates. That makes the Car class 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, surely you can think of other dependencies that should be shared, such as the onboard wireless connection to the manufacturer’s service center. This Car lacks the flexibility to share services that have been created previously for other consumers.

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

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

You have no control over the car’s hidden dependencies. When you can’t control the dependencies, a class becomes difficult to test.

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

That’s super easy. Change the Car constructor to a version with DI:

See what happened? The definition of the dependencies are now in the constructor. The Car class no longer creates an engine or tires. It just consumes them.

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

// Simple car with 4 cylinders and Flintstone tires.
Car(Engine(), Tires())

How cool is that? The definition of the engine and tire dependencies are decoupled from the Car class. You can pass in any kind of engine or tires you 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.

class Engine2 extends Engine {
    Engine2(cylinders) : super.withCylinders(cylinders);
  }

  Car superCar() =>
      // Super car with 12 cylinders and Flintstone tires.
      Car(Engine2(12), Tires())
        ..description = 'Super';

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

class MockEngine extends Engine {
  MockEngine() : super.withCylinders(8);
}

class MockTires extends Tires {
  MockTires() {
    make = 'YokoGoodStone';
  }
}

Car testCar() =>
    // Test car with 8 cylinders and YokoGoodStone tires.
    Car(MockEngine(), MockTires())
      ..description = 'Test';

You just learned what dependency injection is.

It’s a 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. You need something that takes care of assembling these parts.

You could write a giant class to do that:

import 'car.dart';

// BAD pattern!
class CarFactory {
  Car createCar() => Car(createEngine(), createTires())
    ..description = 'Factory';

  Engine createEngine() => Engine();
  Tires createTires() => Tires();
}

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

Wouldn’t it be nice if you could simply list the things you 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. You register some classes with this injector, and it figures out how to create them.

When you need a Car, you simply ask the injector to get it for you and you’re good to go.

car = injector.get(Car);

Everyone wins. The Car knows nothing about creating an Engine or Tires. The consumer knows nothing about creating a Car. You 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.

Angular dependency injection

Angular ships with its own dependency injection framework. You’ll learn Angular dependency injection through a discussion of the sample app that accompanies this page. Run the live example (view source) 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 mockHeroes, an in-memory collection defined in a separate file.

class HeroListComponent {
  final List<Hero> heroes = mockHeroes;
}

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 HeroListComponent and replace every other use of the mockHeroes data.

Create an injectable HeroService

It’s better to hide the details concerning hero data access inside a service class, defined in its own file.

import 'hero.dart';
import 'mock_heroes.dart';

class HeroService {
  List<Hero> getAll() => mockHeroes;
}

The service class exposes a getHeroes() method that returns the same mock data as before.

Of course, this isn’t a real data service. If the service were actually getting data from a remote server, the getHeroes() method signature would be asynchronous. Such a hero service is presented in the tutorial section on Heroes and HTTP. The focus here is on service injection, so a synchronous service will suffice.

Register a service provider

A service is just a class (or a top-level function) 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.

Angular creates most injectors for you as it executes the app, including the app’s root injector. When your app needs a custom root injector, supply it as an argument to the runApp() function.

You must 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.

The most common way to register a provider is with any Angular annotation that has a providers list argument. The most common of these is @Component().

@Component providers

Here’s a revised HeroesComponent that registers the HeroService in its providers list.

import 'package:angular/angular.dart';

import 'hero_list_component.dart';
import 'hero_service.dart';

@Component(
  selector: 'my-heroes',
  template: '''
    <h2>Heroes</h2>
    <hero-list></hero-list>
  ''',
  providers: [ClassProvider(HeroService)],
  directives: [HeroListComponent],
)
class HeroesComponent {}

An instance of the HeroService is now available for injection in this HeroesComponent and all of its child components.

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 app starts and is never destroyed so the HeroService created for the HeroComponent also lives for the life of the app.

Root injector providers

You can also register providers in the app’s root injector, which you pass as an argument to the runApp() function. For example, the app from the tutorial (part 5) injects providers from the routerProvidersHash list:

import 'package:ngdart/angular.dart';
import 'package:ngrouter/ngrouter.dart';
import 'package:angular_tour_of_heroes/app_component.template.dart' as ng;

import 'main.template.dart' as self;

@GenerateInjector(
  routerProvidersHash, // You can use routerProviders in production
)
final InjectorFactory rootInjector = self.rootInjector$Injector;

void main() {
  runApp(ng.AppComponentNgFactory, createInjector: rootInjector);
}

Use root injector provisioning for app-wide services declared external to the app package. Registering app specific services like HeroService is discouraged:

@GenerateInjector([
  // DON'T register app-local services here; this is for illustration purposes only
  ClassProvider(HeroService),
])
final InjectorFactory rootInjector = self.rootInjector$Injector;

void main() {
  runApp(ng.AppComponentNgFactory, createInjector: rootInjector);
}

The preferred approach is to register app services in app components. Because the HeroService is used within the Heroes feature set, and nowhere else, the ideal place to register it is in HeroesComponent.

Inject a service

The HeroListComponent should get heroes from the HeroService, and 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 annotated with the dependency’s type. Here’s the HeroListComponent constructor, asking for the HeroService to be injected.

HeroListComponent(HeroService heroService)

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. The only thing that matters is that the HeroService is provided in some parent injector.

Singleton services

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

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.

Component child injectors

For example, when Angular creates an 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 holds its own instances of the component-provided services.

When Angular disposes of a component instance, it also discards the component’s injector and that injector’s service instances.

Thanks to injector inheritance, you can still inject app-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 app’s root injector. Angular can inject a service provided by any injector in that lineage.

Testing a 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 app parts effectively.

For example, the tutorial (part 5) has a HeroListComponent test that uses a mock router provisioned through the root injector:

import 'package:angular_tour_of_heroes/src/hero_list_component.template.dart'
    as ng;
// ···
import 'heroes_test.template.dart' as self;
// ···
@GenerateInjector([
  ClassProvider(HeroService),
  ClassProvider(Router, useClass: MockRouter),
])
final InjectorFactory rootInjector = self.rootInjector$Injector;

void main() {
  final testBed = NgTestBed<HeroListComponent>(
    ng.HeroListComponentNgFactory,
    rootInjector: rootInjector,
  );
  // ···
}

Learn more in Component Testing: Services.

When a 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 a Logger, side-by-side with the previous service for comparison.

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

Logger service

The sample app’s Logger service is quite simple:

/// Logger that keeps only the last log entry.
class Logger {
  String _log = '';
  String get id => 'Logger';

  void fine(String msg) => _log = msg;

  @override
  String toString() => '[$id] $_log';
}

If the app doesn’t provide Logger, Angular will throw an exception when it looks for a Logger to inject into the HeroService.

  EXCEPTION: No provider for Logger! (HeroListComponent -> HeroService -> Logger)

Because a singleton logger service is useful everywhere in the app, it’s registered in AppComponent:

providers: [ClassProvider(Logger)],

Providers

A service provider provides a concrete, runtime instance associated with a dependency token. 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 the injector won’t know how to create the service.

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

Class providers

There are many ways to provide something that implements Logger. The most common way is to use ClassProvider:

providers: [ClassProvider(Logger)],

But it’s not the only way. You can configure the injector with alternative providers that can deliver a Logger. You can provide a substitute class. You can 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.

Use-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.

ClassProvider(Logger, useClass: BetterLogger),

Provider for a class with dependencies

Maybe an EvenBetterLogger could display the user name in log messages.

class EvenBetterLogger extends Logger {
  final UserService _userService;

  EvenBetterLogger(this._userService);

  String get id => 'EvenBetterLogger';
  String toString() => super.toString() + ' (user:${_userService.user.name})';
}

This logger gets the user from the injected UserService, which is also listed in the app component’s providers list:

ClassProvider(UserService),
ClassProvider(Logger, useClass: EvenBetterLogger),

Existing 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 useClass:

ClassProvider(NewLogger),
ClassProvider(OldLogger, useClass: NewLogger),

To ensure that the same NewLogger instance is provided for both OldLogger and NewLogger, use ExistingProvider:

ClassProvider(NewLogger),
ExistingProvider(OldLogger, NewLogger),

Value providers

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

class SilentLogger implements Logger {
  const SilentLogger();
  String get id => 'SilentLogger';
  @override
  void fine(String msg) {}
  @override
  String toString() => '';
}

const silentLogger = SilentLogger();

Then you register the object using ValueProvider:

ValueProvider(Logger, silentLogger),

For more examples of ValueProvider, see OpaqueToken.

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 during 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 app 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.

final Logger _logger;
final bool _isAuthorized;

HeroService(this._logger, this._isAuthorized);

List<Hero> getAll() {
  var auth = _isAuthorized ? 'authorized' : 'unauthorized';
  _logger.fine('Getting heroes for $auth user.');
  return mockHeroes.where((hero) => _isAuthorized || !hero.isSecret).toList();
}

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:

HeroService heroServiceFactory(Logger logger, UserService userService) =>
    HeroService(logger, userService.user.isAuthorized);

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:

const heroServiceProvider = FactoryProvider(HeroService, heroServiceFactory);

Notice that you captured the factory provider in a constant, heroServiceProvider. This extra step makes the factory provider reusable. You can register the HeroService with this constant 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 list. Here you see the new and the old implementation side-by-side:

Tokens

When you register a provider with an injector, you associate that provider with a dependency injection token. The injector maintains an internal map from tokens to providers that it references when asked for a dependency.

Class types

In all previous examples, the token has been a class type and the provided value an instance of that type. For example, you get a HeroService directly from the injector by supplying the HeroService type as the token:

heroService = _injector.get(HeroService);

Similarly, when you define a constructor parameter of type HeroService, Angular knows to inject a HeroService instance:

HeroListComponent(HeroService heroService)

OpaqueToken

Sometimes the thing you want to inject is a string, list, map, or even a function. For example, what if you want to inject the app title?

const appTitle = 'Dependency Injection';

You know that a value provider is appropriate in this case, but what can you use as the token? You could use String, but that won’t work if your app depends on several such injected strings.

One solution is to define and use an OpaqueToken:

import 'package:angular/angular.dart';

const appTitleToken = OpaqueToken<String>('app.title');

The generic type argument, while optional, conveys the dependency’s type to developers and tooling (not to be confused with the OpaqueToken constructor argument type, which is always String). The OpaqueToken argument token description is a developer aid.

Register the dependency provider using the OpaqueToken object:

ValueProvider.forToken(appTitleToken, appTitle)

Now you can inject the title into any constructor that needs it, with the help of an @Inject() annotation:

AppComponent(@Inject(appTitleToken) this.title);

Alternatively you can directly use the OpaqueToken constant as an annotation:

AppComponent(@appTitleToken this.title);

You can inject values other than strings. For example, apps sometimes have configuration objects with lots of simple properties captured as a Map:

const appConfigMap = {
  'apiEndpoint': 'api.heroes.com',
  'title': 'Dependency Injection',
  // ...
};

const appConfigMapToken = OpaqueToken<Map>('app.config');

Custom configuration class

As an alternative to injecting a Map for an app configuration object, consider defining a custom app configuration class:

class AppConfig {
  String apiEndpoint;
  String title;
}

AppConfig appConfigFactory() => AppConfig()
  ..apiEndpoint = 'api.heroes.com'
  ..title = 'Dependency Injection';

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

If you use cascades, the configuration object can’t be declared const, so you can’t use a value provider, but you can use a factory provider.

FactoryProvider(AppConfig, appConfigFactory),

You might use the app config like this:

AppComponent(AppConfig config, this._userService) : title = config.title;

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():

HeroService(@Optional() Logger logger) {
  logger?.fine('Hello');
}

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 page has described. You can learn more about its advanced features, beginning with its support for nested injectors, in Hierarchical Dependency Injection.

Appendix: Working with injectors directly

Developers rarely work directly with an injector, but here’s an InjectorComponent that does.

@Component(
  selector: 'my-injectors',
  template: '''
      <h2>Other Injections</h2>
      <div id="car">{{car.drive()}}</div>
      <div id="hero">{{hero.name}}</div>
      <div id="rodent">{{rodent}}</div>''',
  providers: [
    ClassProvider(Car),
    ClassProvider(Engine),
    ClassProvider(Tires),
    heroServiceProvider,
    ClassProvider(Logger),
  ],
)
class InjectorComponent implements OnInit {
  final Injector _injector;
  Car car;
  HeroService heroService;
  Hero hero;

  InjectorComponent(this._injector);

  @override
  void ngOnInit() {
    car = _injector.get(Car);
    heroService = _injector.get(HeroService);
    hero = heroService.getAll()[0];
  }

  String get rodent =>
      _injector.get(ROUS, "R.O.U.S.'s? I don't think they exist!");
}

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.