angular injector get vs constructor


The Logger and UserService classes serve as tokens for their own class providers. The Car knows nothing about creating an Engine or Tires. Only authorized users should see secret heroes. You could write a giant class to do that: It's not so bad now with only three creation methods. Wouldn't it be nice if you could simply list the things you want to build without and the parent's providers information combine to tell the It needs to know if the user is authorized to see secret heroes. Unlike EvenBetterLogger, you can't inject the UserService into the HeroService. The HERO_DI_CONFIG constant has an interface, AppConfig. The following code tells the injector In this guide's example, a single HeroService instance is shared among the value of logger to null. Now that you know what dependency injection is and appreciate its benefits,

You don't have a class to serve as a token. as a standalone module by other applications and frameworks. You'll take care of the consumer's problem shortly. You can configure the injector with alternative providers that can deliver an object that behaves like a Logger. something like this: The critical point is this: the Car class did not have to change. like HeroesComponent. the HeroesComponent in the same file, You certainly do not want two different NewLogger instances in your app. you'll get a runtime null reference error. While that makes sense for an automobile engine, which is also injected at the application level. Instead, the HeroService constructor takes a boolean flag to control display of secret heroes. the root AppComponent?" Cool! You can register the HeroService with this variable wherever you need it. Angular knows to inject the Applications often define configuration objects with lots of small facts When you can't control the dependencies, a class becomes difficult to test. When you write tests for Car you're at the mercy of its hidden dependencies. One solution to choosing a provider token for non-class dependencies is If you combine the HeroService class with But since the HeroService is only used within the Heroes Non-class dependencies and You can inject the Logger, but you can't inject the boolean isAuthorized.

The chain of creations started with the Logger provider. Angular ships with its own dependency injection framework. They are retrieved by calling injector.get(). In this example, Angular injects the component's own Injector into the component's constructor. You could provide a logger-like object. decoupled from the Car class. It's a coding pattern in which a class receives its dependencies from external Hierarchical Dependency Injection. As soon as you try to test this component or want to get your heroes data from a remote server, What if you want to put a different brand of tires on your Car?

If the app were actually getting data from a remote server, the API would have to be Any of these approaches might be a good choice under the right circumstances. The injector relies on providers to create instances of the services which displays a list of heroes. @Injectable() decorator For more information, see Hierarchical Injectors. Because the HeroService is used only within the HeroesComponent don't register a logger somewhere up the line, the injector will set the Note that the services themselves are not injected into the component. Angular can't find the service if it's not registered with this or any ancestor injector. to return a BetterLogger when something asks for the Logger. interdependent factory methods! A provider provides the concrete, runtime version of a dependency value. So why doesn't HeroesComponent have version of HeroService because it had no injected parameters. (as it should be in the tsconfig.json), 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. On the one hand, a provider in an NgModule is registered in the root injector. Having multiple classes in the same file is confusing and best avoided. where it replaces the previous HeroService registration in the metadata providers array. the very specific classes Engine and Tires. There are many ways to create dependency values just as there are many ways to write a recipe. This situation calls for a factory provider. You do have to configure the injector by registering the providers Too bad.

There is no AppConfig class. Anyone who wants a Car must now Right now each new car gets its own engine. If you Car class inflexible.

The component then asks the injected injector for the services it wants. But what about that poor consumer? The HeroListComponent should get heroes from the injected HeroService. 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. It encourages a careless grab-bag approach such as you see here. It's not Angular's doing. Right now HeroListComponent gets heroes from HEROES, an in-memory collection Sometimes the thing you want to inject is aspan string, function, or object. You can call get() with a second parameter, which is the value to return if the service

What if the dependency value isn't a class? in this blog post. You can pass in any kind of engine or tires you like, as long as they This logger gets the user from the injected UserService, You could provide a substitute class. On the other hand, a provider registered in an application component is available only on but it's not always the best choice. and use the constructor parameter type information You could write code that explicitly creates an injector if you had to, using a provider object literal with two properties: The first is the token that serves as the key for both locating a dependency value The definition of the dependencies are Keep them happy. Suppose also that the injectable service has no independent access to the source of this information. explicitly: You won't find code like that in the Tour of Heroes or any of the other This is what a dependency injection framework is all about. An Injector is itself an injectable service. That authorization can change during the course of a single application session, read on to see how it is implemented in Angular. The consumer knows nothing about creating a Car. the help of an @Inject decorator: Although the AppConfig interface plays no role in dependency injection, Here's a revised HeroesComponent that registers the HeroService in its providers array. That would break the Car class and it would stay broken until you rewrote it along the lines of Angular has its own dependency injection framework, and What does that dependency depend on? While any decorator will trigger this effect, mark the service class with the for every class with at least one decorator. It is in The HeroService won't have direct access to the user information to decide constructor argument with @Optional(): When using @Optional(), your code must be prepared for a null value. It's difficult to explain, understand, and test. The HeroService requires a Logger, but what if it could get by without This is where the dependency injection framework comes into play. Listing dependencies as constructor parameters may be all you need to test application parts effectively. adding a constructor that takes a Logger parameter. create and inject into a new HeroListComponent. Now you can create a car by passing the engine and tires to the constructor. It doesn't have any dependencies of its own. You're locked into whatever brand the Tires class creates. Now, if someone extends the Engine class, that is not Car's problem. an interface is the preferred dependency lookup key. You can add it if you really want to. It's used so widely that almost everyone just calls it DI. You actually can define the component first with the help of the forwardRef() method as explained defined in another file. What if the Engine class evolves and its constructor requires a parameter? But it's not the only way. What does Engine depend upon? You can't know by inspecting the constructor what this class requires or what it will do. The application will fail mysteriously if you forget the parentheses. If you define the component before the service, Dependencies are singletons within the scope of an injector. that component and all its children. in the NgModule FAQ. You could give it a provider that calls a logger factory function. Framework developers may take this approach when they What if the Car should flash a warning signal when tire pressure is low? If the emitDecoratorMetadata compiler option is true However, Angular DI is a hierarchical injection documentation samples. Unfortunately, that's what you get if you try to alias OldLogger to NewLogger with useClass. In this sample, you need it only in the HeroesComponent, You may not anticipate them even now.

and you know how to ask for an injected object (such as a service) by This Car needs an engine and tires. Providers are the subject of the next section.

Then you register a provider with the useValue option, during each test: You just learned what dependency injection is. create all three parts: the Car, Engine, and Tires. Angular dependency injection is more capable than this guide has described. What's the problem? place to register it. Both Car and consumer simply ask for what they need and the injector delivers. the same mock data as before, but none of its consumers need to know that. separate concern, and, therefore, do not technically require it. Here's why: Injectors are also responsible for instantiating components covered shortly. wireless connection to the manufacturer's service center.

injector for instantiation. HeroService, which it needed to In all previous examples, the dependency value has been a class instance, and

This is actually a shorthand expression for a provider registration Change the Car constructor to a version with DI: See what happened? that create the services the application requires. and its subcomponents, the top-level HeroesComponent is the ideal You need it because Angular requires constructor parameter metadata How can you make Car more robust, flexible, and testable? to make the intent clear.

The deps property is an array of provider tokens.

Car earlier in this guide. The TypeScript interface disappears from the generated JavaScript. you'd like the singleton instance of NewLogger to handle it instead. Imagine the framework had something called an injector.

register it in the providers array of the application module, AppModule. This page covers what DI is, why it's so useful, When you need a Car, you simply ask the injector to get it for you and you're good to go. Is it even possible to create a new Engine in a test environment? You don't have to create an Angular injector. What matters is that the injector has a provider to go to when it needs a Logger. start with a simplified version of the HeroesComponent sources rather than creating them itself. That means that every provider @Injectable(). When the old component logs a message with OldLogger, consider writing the service code in its own file. service associated with that HeroService class token: This is especially convenient when you consider that most dependency values are provided by classes. asked for a dependency. Suppose an old component depends upon an OldLogger class. which you can think of as a recipe for creating the dependency value. But you'll have to start caring because

You need something that takes care of assembling these parts. The injector resolves these tokens and injects the corresponding services into the matching factory function parameters. What if it had a dependency? A service is nothing more than a class in Angular.

HeroesComponent and its HeroListComponent children. Maybe the information changes repeatedly in the course of the browser session. The dependency injector should inject that singleton instance 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. system, which means that nested injectors can create their own service instances. OpaqueToken sections. But what should you use as the token? if you can't swap in low-pressure tires during the test? Also see "Should I add app-wide providers to the root AppModule or The injector maintains an internal token-provider map that it references when What if it reported its activities through a logging service? Avoid the problem altogether by defining components and services in separate files. You can tell Angular that the dependency is optional by annotating the It's better to make a service that hides how the app gets hero data. adding a parameter to a constructor. HeroesComponent is already marked with @Component, and this here's an InjectorComponent that does. The technique is an example of the Everyone wins. It isn't necessary because the in order to inject a Logger. This guide explains what providers are later.

The consumer of Car has the problem. @Injectable() marks a class as available to an That may suffice in the early stages of development, but it's far from ideal. you'll have to change the implementation of heroes and How do you confirm that it actually does flash a warning error when trying to instantiate a class that is not marked as

the compiler adds the metadata to the generated JavaScript You must register a service provider with the injector, or it won't know how to create the service. Earlier you saw that designing a class for dependency injection makes the class easier to test. Angular creates an application-wide injector for you during the bootstrap process. But you must have it now that the service has an injected dependency. The get() method throws an error if it can't resolve the requested service. OldLogger has the same interface as the NewLogger, but for some reason

The Car class is much easier to test now because you are in complete control Imagine writing the following code: The Car class creates everything it needs inside its constructor. You learned the basics of Angular dependency injection in this page. conform to the general API requirements of an engine or tires. An interface is a TypeScript design-time artifact. You could create such an injector surely you can think of other dependencies that should be shared, such as the onboard feature area and nowhere else, it makes sense to register it in

fact @Injectable() decorators that under test: The HeroService is very simple. The Logger class itself is an obvious and natural provider. you really can't build an Angular application without it. Like the EvenBetterLogger, the HeroService needs a fact about the user. registered within an NgModule will be accessible in the entire application. You register some classes with this injector, and it figures out how to create them. The problem is that the Car class is brittle, inflexible, and hard to test. Avoid this technique unless you genuinely need it. You know you can register an object with a value provider. When you define a constructor parameter with the HeroService class type, To see what it can do when building components in Angular, This Car lacks the flexibility This extra step makes the factory provider reusable. You saw how to use an injector to create a new cannot use a TypeScript interface as a token: That seems strange if you're used to dependency injection in strongly typed languages, where How cool is that? Note that the constructor parameter has the type HeroService, and that You can pass mocks to the constructor that do exactly what you want them to do Sometimes you need to create the dependent value dynamically, You'll have to take over the creation of new instances of this HeroService with a factory provider.

To illustrate the point, add a new business requirement: Maybe an EvenBetterLogger could display the user name in the log message.

This framework can also be used At runtime, injectors can read class metadata in the transpiled JavaScript code A factory provider needs a factory function: Although the HeroService has no access to the UserService, the factory function does. It remains nothing more than a class until you register it with an Angular injector. Aternatively, you can provide and inject the configuration object in an ngModule like AppModule. when it creates components for youwhether through HTML markup, as in , whose implementation is the heroServiceFactory. Generally speaking, an injector reports an The TypeScript compiler discards metadata by default. Angular's dependency injection system creates and delivers dependent services "just-in-time". as when you log in a different user. As it happens, you could have omitted @Injectable() from the first has providers information for HeroService. The Car class shed its problems at the consumer's expense. the HeroesComponent. (scroll up to confirm that fact). Consider adding @Injectable() to every service class, even those that don't have dependencies and how to use it in an Angular app. which makes this object play the logger role. This factory is going to become a huge spiderweb of that the injector injects into components and other services. Dependency injection is an important application design pattern. new Car(new Engine2(bigCylinders), new Tires()); new Car(new MockEngine(), new MockTires()); Creating and registering a logger service, Appendix: Working with injectors directly. 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 fix every other use of the HEROES mock data.

a logger? Given that the service is a

Always write @Injectable(), not just @Injectable. Developers expect one class per file. Developers rarely work directly with an injector, but The Engine constructor parameters weren't even a consideration when you first wrote Car. service locator pattern. Per the dependency injection pattern, the component must ask for the service in its Inject a logger into HeroService in two steps: You're likely to need the same logger service everywhere in your application, See more useValue examples in the It could acquire services from any ancestor component, not just its own. That makes Car brittle. It's a small change: Adding a parameter to the constructor isn't all that's happening here.

You call that property within the getHeroes() method when anyone asks for heroes. 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. to determine what things to inject. the HeroService must hide secret heroes from normal users.

This example leverages TypeScript's constructor syntax for declaring The constructor now asks for an injected instance of a Logger and stores it in a private property called logger. to share services that have been created previously for other consumers. You'd apply the same constructor injection pattern, You have no control over the car's hidden dependencies. It just consumes them. To understand why dependency injection is so important, consider an example without it. The definition of the engine and tire dependencies are If you let Angular do its job, you'll enjoy the benefits of automated dependency injection. Occasionally you'll ask a different class to provide the service. registered in the AppModule @NgModule providers array. Not every JavaScript class has metadata. You can either register a provider within an NgModule or in application components. and registering the provider. Here, the APP_CONFIG service needs to be available all across the application, so it's so put it in the project's app folder and

You can register various kinds of providers, It needed that provider to create a Logger to inject into a new it supports typing of the configuration object within the class. The Car class no longer creates an engine or tires. the class type served as its own lookup key. Also recall that the parent component (HeroesComponent) @Injectable()? The HeroesComponent is the root component of the Heroes feature area. You don't have a gigantic factory class to maintain. decorator class (like @Directive and @Pipe, which you learn about later) But why flirt with trouble? But maintaining it will be hairy as the application grows. constructor, as discussed earlier. now in the constructor. identify a class as a target for instantiation by an injector. The following HeroService exposes a getHeroes method that returns Unfortunately, you The constructor parameter type, the @Component decorator, having to define which dependency gets injected into what? asynchronous, perhaps returning a Promise. nested injectors, in

That's super easy. or after navigating to a component with the router. is not found. Of course, this isn't a real service. when the definition of Engine changes, the Car class must change. They can be object literals such as this one: What if you'd like to make this configuration object available for injection? The consumer must update the car creation code to The @Injectable() decorator above the service class is For example, you can create a new HeroListComponent with a mock service that you can manipulate based on information you won't have until the last possible moment. You inject both the Logger and the UserService into the factory provider is a subtype of @Injectable(). define the component last. You're forced to spelunk the implementation to discover what it does. (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. This stripped down version has only one child, HeroListComponent,

the HeroListComponent class has an @Component decorator Angular takes care of creating and calling injectors that from the The Tour of Heroes.

The solution: alias with the useExisting option. You certainly don't want that going on during tests. parameters and properties simultaneously. That makes the Instead of asking for them, who is authorized and who is not. The OldLogger should be an alias for NewLogger.

The second is a provider definition object,

you can't update the old component to use it. It governs all the child components of this area. Will a new instance of Engine make an asynchronous call to the server? Here is the revision compared to the original. JavaScript doesn't have interfaces. You'd also have to rewrite the way components consume the service. to define and use an OpaqueToken. Angular injector to inject an instance of

This is important in general, but not in this example. Notice that you captured the factory provider in an exported variable, heroServiceProvider. There is no interface type information left for Angular to find at runtime. It can't share an engine with other cars. The token is the key to the map. and let the injector pass them along to the factory function: The useFactory field tells Angular that the provider is a factory function Here's the AppModule that registers two providers, UserService and an APP_CONFIG provider,

this.engine = new Engine(theNewParameter). must acquire services generically and dynamically. You can learn more about its advanced features, beginning with its support for

of its dependencies. HeroService whenever it creates a new HeroListComponent. Sometimes it's easier to provide a ready-made object rather than ask the injector to create it from a class. when a component asks for either the new or the old logger. in its providers array. the Car constructor instantiates its own copies from

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