Inject a logger into HeroService in two steps: You're likely to need the same logger service everywhere in your application, Non-class dependencies and The Car class no longer creates an engine or tires. where it replaces the previous HeroService registration in the metadata providers array. At runtime, injectors can read class metadata in the transpiled JavaScript code decoupled from the Car class. When you write tests for Car you're at the mercy of its hidden dependencies. But you must have it now that the service has an injected dependency. Angular creates an application-wide injector for you during the bootstrap process. The get() method throws an error if it can't resolve the requested service. Will a new instance of Engine make an asynchronous call to the server? To see what it can do when building components in Angular, the class type served as its own lookup key.

Given that the service is a HeroService, which it needed to to make the intent clear. to define and use an OpaqueToken. now in the constructor. @Injectable() marks a class as available to an Per the dependency injection pattern, the component must ask for the service in its The following HeroService exposes a getHeroes method that returns Always write @Injectable(), not just @Injectable. A service is nothing more than a class in Angular. sources rather than creating them itself. identify a class as a target for instantiation by an injector. The consumer must update the car creation code to (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. As it happens, you could have omitted @Injectable() from the first What matters is that the injector has a provider to go to when it needs a Logger. You can call get() with a second parameter, which is the value to return if the service Avoid this technique unless you genuinely need it. Here's a revised HeroesComponent that registers the HeroService in its providers array. as when you log in a different user. While any decorator will trigger this effect, mark the service class with the the HeroService must hide secret heroes from normal users. The constructor parameter type, the @Component decorator, The token is the key to the map. Having multiple classes in the same file is confusing and best avoided. You're locked into whatever brand the Tires class creates. in this blog post. register it in the providers array of the application module, AppModule. It encourages a careless grab-bag approach such as you see here. Now you can create a car by passing the engine and tires to the constructor. The second is a provider definition object, Here's why: Injectors are also responsible for instantiating components version of HeroService because it had no injected parameters. An interface is a TypeScript design-time artifact. The Logger class itself is an obvious and natural provider. But what about that poor consumer? If the emitDecoratorMetadata compiler option is true whose implementation is the heroServiceFactory. It needed that provider to create a Logger to inject into a new When you need a Car, you simply ask the injector to get it for you and you're good to go. The injector maintains an internal token-provider map that it references when you really can't build an Angular application without it. If you forget to register the logger, Angular throws an exception when it first looks for the logger: That's Angular telling you that the dependency injector couldn't find the provider for the logger. It can't share an engine with other cars. To illustrate the point, add a new business requirement: They are retrieved by calling injector.get(). that the injector injects into components and other services. Earlier you saw that designing a class for dependency injection makes the class easier to test. Now, if someone extends the Engine class, that is not Car's problem. The Engine constructor parameters weren't even a consideration when you first wrote Car. You call that property within the getHeroes() method when anyone asks for heroes. It is in must acquire services generically and dynamically. service associated with that HeroService class token: This is especially convenient when you consider that most dependency values are provided by classes. They can be object literals such as this one: What if you'd like to make this configuration object available for injection? value of logger to null. Then you register a provider with the useValue option, What if the Car should flash a warning signal when tire pressure is low? You can inject the Logger, but you can't inject the boolean isAuthorized. What's the problem?


You can configure the injector with alternative providers that can deliver an object that behaves like a Logger. Like the EvenBetterLogger, the HeroService needs a fact about the user. which you can think of as a recipe for creating the dependency value. The consumer knows nothing about creating a Car. fix every other use of the HEROES mock data. so put it in the project's app folder and and its subcomponents, the top-level HeroesComponent is the ideal consider writing the service code in its own file. You certainly do not want two different NewLogger instances in your app. In this example, Angular injects the component's own Injector into the component's constructor. You may not anticipate them even now. Angular injector to inject an instance of Angular has its own dependency injection framework, and interdependent factory methods! Suppose an old component depends upon an OldLogger class. the same mock data as before, but none of its consumers need to know that. Notice that you captured the factory provider in an exported variable, heroServiceProvider. that component and all its children. Imagine writing the following code: The Car class creates everything it needs inside its constructor. conform to the general API requirements of an engine or tires. to share services that have been created previously for other consumers. This extra step makes the factory provider reusable. This is where the dependency injection framework comes into play. It doesn't have any dependencies of its own. You'll take care of the consumer's problem shortly. It's used so widely that almost everyone just calls it DI. service locator pattern. Angular ships with its own dependency injection framework. The HeroService won't have direct access to the user information to decide See more useValue examples in the you'll have to change the implementation of heroes and You could provide a logger-like object. You actually can define the component first with the help of the forwardRef() method as explained The @Injectable() decorator above the service class is injector for instantiation. You inject both the Logger and the UserService into the factory provider The consumer of Car has the problem. and you know how to ask for an injected object (such as a service) by you'll get a runtime null reference error. The definition of the engine and tire dependencies are You have no control over the car's hidden dependencies. It's difficult to explain, understand, and test. You can register the HeroService with this variable wherever you need it. during each test: You just learned what dependency injection is. It could acquire services from any ancestor component, not just its own. Of course, this isn't a real service. feature area and nowhere else, it makes sense to register it in Sometimes you need to create the dependent value dynamically, Not every JavaScript class has metadata. You can register various kinds of providers, This situation calls for a factory provider. But what should you use as the token? Here's the AppModule that registers two providers, UserService and an APP_CONFIG provider, for every class with at least one decorator. You need it because Angular requires constructor parameter metadata Unlike EvenBetterLogger, you can't inject the UserService into the HeroService. don't register a logger somewhere up the line, the injector will set the You don't have to create an Angular injector. You don't have a gigantic factory class to maintain. The injector relies on providers to create instances of the services asked for a dependency. under test: The HeroService is very simple. but it's not always the best choice. If you let Angular do its job, you'll enjoy the benefits of automated dependency injection. There are many ways to create dependency values just as there are many ways to write a recipe. To understand why dependency injection is so important, consider an example without it. In this guide's example, a single HeroService instance is shared among the is not found. It's a coding pattern in which a class receives its dependencies from external It remains nothing more than a class until you register it with an Angular injector. Also recall that the parent component (HeroesComponent) asynchronous, perhaps returning a Promise. create and inject into a new HeroListComponent. But since the HeroService is only used within the Heroes having to define which dependency gets injected into what? This stripped down version has only one child, HeroListComponent, Suppose also that the injectable service has no independent access to the source of this information. system, which means that nested injectors can create their own service instances. Change the Car constructor to a version with DI: See what happened? That would break the Car class and it would stay broken until you rewrote it along the lines of This Car lacks the flexibility You'd apply the same constructor injection pattern, Too bad. decorator class (like @Directive and @Pipe, which you learn about later) There is no interface type information left for Angular to find at runtime. and how to use it in an Angular app. That makes Car brittle. in order to inject a Logger. How cool is that? of its dependencies. That authorization can change during the course of a single application session, like HeroesComponent. If you combine the HeroService class with When the old component logs a message with OldLogger,

Aternatively, you can provide and inject the configuration object in an ngModule like AppModule. which makes this object play the logger role. or after navigating to a component with the router. The injector resolves these tokens and injects the corresponding services into the matching factory function parameters. Dependency injection is an important application design pattern. a logger?

place to register it. the compiler adds the metadata to the generated JavaScript You can't know by inspecting the constructor what this class requires or what it will do. Maybe the information changes repeatedly in the course of the browser session. if you can't swap in low-pressure tires during the test? One solution to choosing a provider token for non-class dependencies is This page covers what DI is, why it's so useful, documentation samples. who is authorized and who is not. You can pass in any kind of engine or tires you like, as long as they Consider adding @Injectable() to every service class, even those that don't have dependencies The HERO_DI_CONFIG constant has an interface, AppConfig. It isn't necessary because the For more information, see Hierarchical Injectors. Car earlier in this guide. You could write a giant class to do that: It's not so bad now with only three creation methods. adding a parameter to a constructor. create all three parts: the Car, Engine, and Tires. You must register a service provider with the injector, or it won't know how to create the service. What does that dependency depend on? an interface is the preferred dependency lookup key. Also see "Should I add app-wide providers to the root AppModule or Developers rarely work directly with an injector, but and use the constructor parameter type information

Angular dependency injection is more capable than this guide has described. adding a constructor that takes a Logger parameter. There is no AppConfig class. Cool! (as it should be in the tsconfig.json), Providers are the subject of the next section. that create the services the application requires. What if it had a dependency? You don't have a class to serve as a token. HeroesComponent is already marked with @Component, and this Wouldn't it be nice if you could simply list the things you want to build without That means that every provider What if the Engine class evolves and its constructor requires a parameter? Both Car and consumer simply ask for what they need and the injector delivers. In this sample, you need it only in the HeroesComponent, OldLogger has the same interface as the NewLogger, but for some reason when a component asks for either the new or the old logger. You can tell Angular that the dependency is optional by annotating the Only authorized users should see secret heroes. That's super easy. (scroll up to confirm that fact). parameters and properties simultaneously. Unfortunately, that's what you get if you try to alias OldLogger to NewLogger with useClass. covered shortly. Car class inflexible. But you'll have to start caring because the very specific classes Engine and Tires. Occasionally you'll ask a different class to provide the service. Applications often define configuration objects with lots of small facts Here, the APP_CONFIG service needs to be available all across the application, so it's While that makes sense for an automobile engine, This framework can also be used registered in the AppModule @NgModule providers array. Everyone wins. which is also injected at the application level. Earlier you registered the Logger service in the providers array of the metadata for the AppModule like this: There are many ways to provide something that looks and behaves like a Logger. In all previous examples, the dependency value has been a class instance, and HeroService whenever it creates a new HeroListComponent. the help of an @Inject decorator: Although the AppConfig interface plays no role in dependency injection, The definition looks like this: Register the dependency provider using the OpaqueToken object: Now you can inject the configuration object into any constructor that needs it, with Developers expect one class per file. But why flirt with trouble? It needs to know if the user is authorized to see secret heroes. when the definition of Engine changes, the Car class must change. Anyone who wants a Car must now Imagine the framework had something called an injector. You can pass mocks to the constructor that do exactly what you want them to do When you define a constructor parameter with the HeroService class type, The following code tells the injector OpaqueToken sections. It's not Angular's doing. Instead, the HeroService constructor takes a boolean flag to control display of secret heroes. 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. The technique is an example of the the HeroesComponent. to determine what things to inject. separate concern, The definition of the dependencies are This is actually a shorthand expression for a provider registration That makes the Because the HeroService is used only within the HeroesComponent Sometimes the thing you want to inject is aspan string, function, or object. You know you can register an object with a value provider. registered within an NgModule will be accessible in the entire application. in the NgModule FAQ. Note that the services themselves are not injected into the component. Right now each new car gets its own engine. The dependency injector should inject that singleton instance The constructor now asks for an injected instance of a Logger and stores it in a private property called logger. It governs all the child components of this area. Angular's dependency injection system creates and delivers dependent services "just-in-time". What does Engine depend upon? define the component last. You'd also have to rewrite the way components consume the service. The Car class shed its problems at the consumer's expense. You need something that takes care of assembling these parts. The Car knows nothing about creating an Engine or Tires. You do have to configure the injector by registering the providers You register some classes with this injector, and it figures out how to create them. the HeroListComponent class has an @Component decorator Generally speaking, an injector reports an and let the injector pass them along to the factory function: The useFactory field tells Angular that the provider is a factory function explicitly: You won't find code like that in the Tour of Heroes or any of the other You certainly don't want that going on during tests. the root AppComponent?" The Logger and UserService classes serve as tokens for their own class providers. This guide explains what providers are later. The problem is that the Car class is brittle, inflexible, and hard to test. Listing dependencies as constructor parameters may be all you need to test application parts effectively. It's better to make a service that hides how the app gets hero data. Any of these approaches might be a good choice under the right circumstances. new Car(new Engine2(bigCylinders), new Tires()); new Car(new MockEngine(), new MockTires()); Creating and registering a logger service, Appendix: Working with injectors directly. This is important in general, but not in this example. JavaScript doesn't have interfaces. Avoid the problem altogether by defining components and services in separate files. If you This Car needs an engine and tires. constructor, as discussed earlier. has providers information for HeroService. Sometimes it's easier to provide a ready-made object rather than ask the injector to create it from a class. This is what a dependency injection framework is all about. You saw how to use an injector to create a new wireless connection to the manufacturer's service center. it supports typing of the configuration object within the class. You'll have to take over the creation of new instances of this HeroService with a factory provider. You could create such an injector that from the The Tour of Heroes. This factory is going to become a huge spiderweb of For example, you can create a new HeroListComponent with a mock service that you can manipulate Dependencies are singletons within the scope of an injector. You can add it if you really want to. But it's not the only way. Instead of asking for them, You're forced to spelunk the implementation to discover what it does. based on information you won't have until the last possible moment. The deps property is an array of provider tokens. start with a simplified version of the HeroesComponent nested injectors, in The HeroesComponent is the root component of the Heroes feature area. as a standalone module by other applications and frameworks. using a provider object literal with two properties: The first is the token that serves as the key for both locating a dependency value fact @Injectable() decorators that How do you confirm that it actually does flash a warning You could write code that explicitly creates an injector if you had to, @Injectable() decorator Right now HeroListComponent gets heroes from HEROES, an in-memory collection defined in another file. Angular takes care of creating and calling injectors this.engine = new Engine(theNewParameter). Angular can't find the service if it's not registered with this or any ancestor injector. surely you can think of other dependencies that should be shared, such as the onboard constructor argument with @Optional(): When using @Optional(), your code must be prepared for a null value. What if the dependency value isn't a class? It's a small change: Adding a parameter to the constructor isn't all that's happening here. If the app were actually getting data from a remote server, the API would have to be error when trying to instantiate a class that is not marked as You learned the basics of Angular dependency injection in this page. What if it reported its activities through a logging service? and registering the provider. The solution: alias with the useExisting option. The OldLogger should be an alias for NewLogger. Now that you know what dependency injection is and appreciate its benefits, On the other hand, a provider registered in an application component is available only on If you define the component before the service, The HeroService requires a Logger, but what if it could get by without

The Car class is much easier to test now because you are in complete control You can either register a provider within an NgModule or in application components.

@Injectable(). Note that the constructor parameter has the type HeroService, and that The TypeScript interface disappears from the generated JavaScript. is a subtype of @Injectable(). Keep them happy. The component then asks the injected injector for the services it wants. When you can't control the dependencies, a class becomes difficult to test. when it creates components for youwhether through HTML markup, as in , That may suffice in the early stages of development, but it's far from ideal. The application will fail mysteriously if you forget the parentheses. in its providers array.

What if you want to put a different brand of tires on your Car? HeroesComponent and its HeroListComponent children. you'd like the singleton instance of NewLogger to handle it instead. the HeroesComponent in the same file, Is it even possible to create a new Engine in a test environment? Here is the revision compared to the original. It just consumes them. You could provide a substitute class. But maintaining it will be hairy as the application grows. the Car constructor instantiates its own copies from something like this: The critical point is this: the Car class did not have to change.

and the parent's providers information combine to tell the read on to see how it is implemented in Angular. Here you see the new and the old implementation side-by-side: When you register a provider with an injector, you associate that provider with a dependency injection token. So why doesn't HeroesComponent have You can learn more about its advanced features, beginning with its support for On the one hand, a provider in an NgModule is registered in the root injector. A provider provides the concrete, runtime version of a dependency value. Framework developers may take this approach when they You could give it a provider that calls a logger factory function. Hierarchical Dependency Injection. you can't update the old component to use it. Unfortunately, you A factory provider needs a factory function: Although the HeroService has no access to the UserService, the factory function does.
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