Dependency Injection

Dependency Injection is a powerful pattern for managing code dependencies. In this cookbook we will explore many of the features of Dependency Injection (DI) in Angular.

Table of contents

Application-wide dependencies

External module configuration

@Injectable and nested service dependencies

Limit service scope to a component subtree

Multiple service instances (sandboxing)

Qualify dependency lookup with @Optional and @Host

Inject the component's DOM element

Define dependencies with providers

• The provide object literal
• useValue - the value provider
• useClass - the class provider
• useExisting - the alias provider
• useFactory - the factory provider

Provider token alternatives

• class-interface
• OpaqueToken

Inject into a derived class

Find a parent component by injection

• Find parent with a known component type
• Cannot find a parent by its base class
• Find a parent by its class-interface
• Find a parent in a tree of parents (@SkipSelf)
• A provideParent helper function

Break circularities with a forward class reference (forwardRef)

See the of the code supporting this cookbook.

Application-wide dependencies

Register providers for dependencies used throughout the application in the root application component, AppComponent.

In the following example, we import and register several services (the LoggerService, UserContext, and the UserService) in the @Component metadata providers array.

import { LoggerService }      from './logger.service';
import { UserContextService } from './user-context.service';
import { UserService }        from './user.service';

@Component({
  selector: 'my-app',
  templateUrl: 'app/app.component.html',
  providers: [LoggerService, UserContextService, UserService]
})
export class AppComponent {
/* . . . */
}

All of these services are implemented as classes. Service classes can act as their own providers which is why listing them in the providers array is all the registration we need.

Now that we've registered these services, Angular can inject them into the constructor of any component or service, anywhere in the application.

constructor(logger: LoggerService) {
  logger.logInfo('Creating HeroBiosComponent');
}

constructor(private userService: UserService, private loggerService: LoggerService) {
}

External module configuration

We often register providers in the NgModule rather than in the root application component.

We do this when (a) we expect the service to be injectable everywhere or (b) we must configure another application global service before it starts.

We see an example of the second case here, where we configure the Component Router with a non-default location strategy by listing its provider in the providers list of the AppModule.

providers: [
  { provide: LocationStrategy, useClass: HashLocationStrategy }
]

@Injectable and nested service dependencies

The consumer of an injected service does not know how to create that service. It shouldn't care. It's the dependency injection's job to create and cache that service.

Sometimes a service depends on other services ... which may depend on yet other services. Resolving these nested dependencies in the correct order is also the framework's job. At each step, the consumer of dependencies simply declares what it requires in its constructor and the framework takes over.

For example, we inject both the LoggerService and the UserContext in the AppComponent.

constructor(logger: LoggerService, public userContext: UserContextService) {
  userContext.loadUser(this.userId);
  logger.logInfo('AppComponent initialized');
}

The UserContext in turn has dependencies on both the LoggerService (again) and a UserService that gathers information about a particular user.

@Injectable()
export class UserContextService {
  constructor(private userService: UserService, private loggerService: LoggerService) {
  }
}

When Angular creates anAppComponent, the dependency injection framework creates an instance of the LoggerService and starts to create the UserContextService. The UserContextService needs the LoggerService, which the framework already has, and the UserService, which it has yet to create. The UserService has no dependencies so the dependency injection framework can just new one into existence.

The beauty of dependency injection is that the author of AppComponent didn't care about any of this. The author simply declared what was needed in the constructor (LoggerService and UserContextService) and the framework did the rest.

Once all the dependencies are in place, the AppComponent displays the user information:

@Injectable()

Notice the @Injectable()decorator on the UserContextService class.

@Injectable()
export class UserContextService {
}

That decorator makes it possible for Angular to identify the types of its two dependencies, LoggerService and UserService.

Technically, the @Injectable()decorator is only required for a service class that has its own dependencies. The LoggerService doesn't depend on anything. The logger would work if we omitted @Injectable() and the generated code would be slightly smaller.

But the service would break the moment we gave it a dependency and we'd have to go back and and add @Injectable() to fix it. We add @Injectable() from the start for the sake of consistency and to avoid future pain.

Limit service scope to a component subtree

All injected service dependencies are singletons meaning that, for a given dependency injector ("injector"), there is only one instance of service.

But an Angular application has multiple dependency injectors, arranged in a tree hierarchy that parallels the component tree. So a particular service can be provided (and created) at any component level and multiple times if provided in multiple components.

By default, a service dependency provided in one component is visible to all of its child components and Angular injects the same service instance into all child components that ask for that service.

Accordingly, dependencies provided in the root AppComponent can be injected into any component anywhere in the application.

That isn't always desirable. Sometimes we want to restrict service availability to a particular region of the application.

We can limit the scope of an injected service to a branch of the application hierarchy by providing that service at the sub-root component for that branch. Here we provide the HeroService to the HeroesBaseComponent by listing it in the providers array:

@Component({
  selector: 'unsorted-heroes',
  template: `<div *ngFor="let hero of heroes">{{hero.name}}</div>`,
  providers: [HeroService]
})
export class HeroesBaseComponent implements OnInit {
  constructor(private heroService: HeroService) { }
}

When Angular creates the HeroesBaseComponent, it also creates a new instance of HeroService that is visible only to the component and its children (if any).

We could also provide the HeroService to a different component elsewhere in the application. That would result in a different instance of the service, living in a different injector.

Multiple service instances (sandboxing)

Sometimes we want multiple instances of a service at the same level of the component hierarchy.

A good example is a service that holds state for its companion component instance. We need a separate instance of the service for each component. Each service has its own work-state, isolated from the service-and-state of a different component. We call this sandboxing because each service and component instance has its own sandbox to play in.

Imagine a HeroBiosComponent that presents three instances of the HeroBioComponent.

@Component({
  selector: 'hero-bios',
  template: `
    <hero-bio [heroId]="1"></hero-bio>
    <hero-bio [heroId]="2"></hero-bio>
    <hero-bio [heroId]="3"></hero-bio>`,
  providers: [HeroService]
})
export class HeroBiosComponent {
}

Each HeroBioComponent can edit a single hero's biography. A HeroBioComponent relies on a HeroCacheService to fetch, cache, and perform other persistence operations on that hero.

@Injectable()
export class HeroCacheService {
  hero: Hero;
  constructor(private heroService: HeroService) {}

  fetchCachedHero(id: number) {
    if (!this.hero) {
      this.hero = this.heroService.getHeroById(id);
    }
    return this.hero;
  }
}

Clearly the three instances of the HeroBioComponent can't share the same HeroCacheService. They'd be competing with each other to determine which hero to cache.

Each HeroBioComponent gets its own HeroCacheService instance by listing the HeroCacheService in its metadata providers array.

@Component({
  selector: 'hero-bio',
  template: `
    <h4>{{hero.name}}</h4>
    <ng-content></ng-content>
    <textarea cols="25" [(ngModel)]="hero.description"></textarea>`,
  providers: [HeroCacheService]
})

export class HeroBioComponent implements OnInit  {
  @Input() heroId: number;

  constructor(private heroCache: HeroCacheService) { }

  ngOnInit() { this.heroCache.fetchCachedHero(this.heroId); }

  get hero() { return this.heroCache.hero; }
}

The parent HeroBiosComponent binds a value to the heroId. The ngOnInit pass that id to the service which fetches and caches the hero. The getter for the hero property pulls the cached hero from the service. And the template displays this data-bound property.

Find this example in live code and confirm that the three HeroBioComponent instances have their own cached hero data.

Qualify dependency lookup with @Optional and @Host

We learned that dependencies can be registered at any level in the component hierarchy.

When a component requests a dependency, Angular starts with that component's injector and walks up the injector tree until it finds the first suitable provider. Angular throws an error if it can't find the dependency during that walk.

We want this behavior most of the time. But sometimes we need to limit the search and/or accommodate a missing dependency. We can modify Angular's search behavior with the @Host and @Optional qualifying decorators, used individually or together.

The @Optional decorator tells Angular to continue when it can't find the dependency. Angular sets the injection parameter to null instead.

The @Host decorator stops the upward search at the host component.

The host component is typically the component requesting the dependency. But when this component is projected into a parent component, that parent component becomes the host. We look at this second, more interesting case in our next example.

Demonstration

The HeroBiosAndContactsComponent is a revision of the HeroBiosComponent that we looked at above.

@Component({
  selector: 'hero-bios-and-contacts',
  template: `
    <hero-bio [heroId]="1"> <hero-contact></hero-contact> </hero-bio>
    <hero-bio [heroId]="2"> <hero-contact></hero-contact> </hero-bio>
    <hero-bio [heroId]="3"> <hero-contact></hero-contact> </hero-bio>`,
  providers: [HeroService]
})
export class HeroBiosAndContactsComponent {
  constructor(logger: LoggerService) {
    logger.logInfo('Creating HeroBiosAndContactsComponent');
  }
}

Focus on the template:

template: `
  <hero-bio [heroId]="1"> <hero-contact></hero-contact> </hero-bio>
  <hero-bio [heroId]="2"> <hero-contact></hero-contact> </hero-bio>
  <hero-bio [heroId]="3"> <hero-contact></hero-contact> </hero-bio>`,

We've inserted a <hero-contact> element between the <hero-bio> tags. Angular projects (transcludes) the corresponding HeroContactComponent into the HeroBioComponent view, placing it in the <ng-content> slot of the HeroBioComponent template:

template: `
  <h4>{{hero.name}}</h4>
  <ng-content></ng-content>
  <textarea cols="25" [(ngModel)]="hero.description"></textarea>`,

It looks like this, with the hero's telephone number from HeroContactComponent projected above the hero description:

Here's the HeroContactComponent which demonstrates the qualifying decorators that we're talking about in this section:

@Component({
  selector: 'hero-contact',
  template: `
  <div>Phone #: {{phoneNumber}}
  <span *ngIf="hasLogger">!!!</span></div>`
})
export class HeroContactComponent {

  hasLogger = false;

  constructor(
      @Host() // limit to the host component's instance of the HeroCacheService
      private heroCache: HeroCacheService,

      @Host()     // limit search for logger; hides the application-wide logger
      @Optional() // ok if the logger doesn't exist
      private loggerService: LoggerService
  ) {
    if (loggerService) {
      this.hasLogger = true;
      loggerService.logInfo('HeroContactComponent can log!');
    }
  }

  get phoneNumber() { return this.heroCache.hero.phone; }

}

Focus on the constructor parameters

@Host() // limit to the host component's instance of the HeroCacheService
private heroCache: HeroCacheService,

@Host()     // limit search for logger; hides the application-wide logger
@Optional() // ok if the logger doesn't exist
private loggerService: LoggerService

The @Host() function decorating the heroCache property ensures that we get a reference to the cache service from the parent HeroBioComponent. Angular throws if the parent lacks that service, even if a component higher in the component tree happens to have that service.

A second @Host() function decorates the loggerService property. We know the only LoggerService instance in the app is provided at the AppComponent level. The host HeroBioComponent doesn't have its own LoggerService provider.

Angular would throw an error if we hadn't also decorated the property with the @Optional() function. Thanks to @Optional(), Angular sets the loggerService to null and the rest of the component adapts.

Here's the HeroBiosAndContactsComponent in action.

If we comment out the @Host() decorator, Angular now walks up the injector ancestor tree until it finds the logger at the AppComponent level. The logger logic kicks in and the hero display updates with the gratuitous "!!!", indicating that the logger was found.

On the other hand, if we restore the @Host() decorator and comment out @Optional, the application fails for lack of the required logger at the host component level.
EXCEPTION: No provider for LoggerService! (HeroContactComponent -> LoggerService)

Inject the component's element

On occasion we might need to access a component's corresponding DOM element. Although we strive to avoid it, many visual effects and 3rd party tools (such as jQuery) require DOM access.

To illustrate, we've written a simplified version of the HighlightDirective from the Attribute Directives chapter.

import { Directive, ElementRef, HostListener, Input } from '@angular/core';

@Directive({
  selector: '[myHighlight]'
})
export class HighlightDirective {

  @Input('myHighlight') highlightColor: string;

  private el: HTMLElement;

  constructor(el: ElementRef) {
    this.el = el.nativeElement;
  }

  @HostListener('mouseenter') onMouseEnter() {
    this.highlight(this.highlightColor || 'cyan');
  }

  @HostListener('mouseleave') onMouseLeave() {
    this.highlight(null);
  }

  private highlight(color: string) {
    this.el.style.backgroundColor = color;
  }
}

The directive sets the background to a highlight color when the user mouses over the DOM element to which it is applied.

Angular set the constructor's el parameter to the injected ElementRef which is a wrapper around that DOM element. Its nativeElement property exposes the DOM element for the directive to manipulate.

The sample code applies the directive's myHighlight attribute to two <div> tags, first without a value (yielding the default color) and then with an assigned color value.

<div id="highlight"  class="di-component"  myHighlight>
  <h3>Hero Bios and Contacts</h3>
  <div myHighlight="yellow">
    <hero-bios-and-contacts></hero-bios-and-contacts>
  </div>
</div>

The following image shows the effect of mousing over the <hero-bios-and-contacts> tag.

Define dependencies with providers

In this section we learn to write providers that deliver dependent services.

Background

We get a service from a dependency injector by giving it a token.

We usually let Angular handle this transaction for us by specifying a constructor parameter and its type. The parameter type serves as the injector lookup token. Angular passes this token to the injector and assigns the result to the parameter. Here's a typical example:

constructor(logger: LoggerService) {
  logger.logInfo('Creating HeroBiosComponent');
}

Angular asks the injector for the service associated with the LoggerService and and assigns the returned value to the logger parameter.

Where did the injector get that value? It may already have that value in its internal container. If it doesn't, it may be able to make one with the help of a provider. A provider is a recipe for delivering a service associated with a token.

A new injector has no providers. Angular initializes the injectors it creates with some providers it cares about. We have to register our own application providers manually, usually in the providers array of the Component or Directive metadata:

providers: [LoggerService, UserContextService, UserService]

Defining providers

The simple class provider is the most typical by far. We mention the class in the providers array and we're done.

providers: [HeroService]

It's that simple because the most common injected service is an instance of a class. But not every dependency can be satisfied by creating a new instance of a class. We need other ways to deliver dependency values and that means we need other ways to specify a provider.

The HeroOfTheMonthComponent example demonstrates many of the alternatives and why we need them.

It's visually simple: a few properties and the output of a logger. The code behind it gives us plenty to talk about.

import { Component, Inject } from '@angular/core';

import { DateLoggerService,
         MinimalLogger }     from './date-logger.service';
import { Hero }              from './hero';
import { HeroService }       from './hero.service';
import { LoggerService }     from './logger.service';
import { RUNNERS_UP,
         runnersUpFactory }  from './runners-up';

@Component({
  selector: 'hero-of-the-month',
  template: template,
  providers: [
    { provide: Hero,          useValue:    someHero },
    { provide: TITLE,         useValue:   'Hero of the Month' },
    { provide: HeroService,   useClass:    HeroService },
    { provide: LoggerService, useClass:    DateLoggerService },
    { provide: MinimalLogger, useExisting: LoggerService },
    { provide: RUNNERS_UP,    useFactory:  runnersUpFactory(2), deps: [Hero, HeroService] }
  ]
})
export class HeroOfTheMonthComponent {
  logs: string[] = [];

  constructor(
      logger: MinimalLogger,
      public heroOfTheMonth: Hero,
      @Inject(RUNNERS_UP) public runnersUp: string,
      @Inject(TITLE) public title: string)
  {
    this.logs = logger.logs;
    logger.logInfo('starting up');
  }
}

The provide object literal

The provide object literal takes a token and a definition object. The token is usually a class but it doesn't have to be.

The definition object has one main property, (e.g. useValue) that indicates how the provider should create or return the provided value.

useValue - the value provider

Set the useValue property to a fixed value that the provider can return as the dependency object.

Use this technique to provide runtime configuration constants such as web-site base addresses and feature flags. We often use a value provider in a unit test to replace a production service with a fake or mock.

The HeroOfTheMonthComponent example has two value providers. The first provides an instance of the Hero class; the second specifies a literal string resource:

    { provide: Hero,          useValue:    someHero },
    { provide: TITLE,         useValue:   'Hero of the Month' },

The Hero provider token is a class which makes sense because the value is a Hero and the consumer of the injected hero would want the type information.

The TITLE provider token is not a class. It's a special kind of provider lookup key called an OpaqueToken. We often use an OpaqueToken when the dependency is a simple value like a string, a number, or a function.

The value of a value provider must be defined now. We can't create the value later. Obviously the title string literal is immediately available. The someHero variable in this example was set earlier in the file:

const someHero = new Hero(42, 'Magma', 'Had a great month!', '555-555-5555');

The other providers create their values lazily when they're needed for injection.

useClass - the class provider

The useClass provider creates and returns new instance of the specified class.

Use this technique to substitute an alternative implementation for a common or default class. The alternative could implement a different strategy, extend the default class, or fake the behavior of the real class in a test case.

We see two examples in the HeroOfTheMonthComponent:

{ provide: HeroService,   useClass:    HeroService },
{ provide: LoggerService, useClass:    DateLoggerService },

The first provider is the de-sugared, expanded form of the most typical case in which the class to be created (HeroService) is also the provider's injection token. We wrote it in this long form to de-mystify the preferred short form.

The second provider substitutes the DateLoggerService for the LoggerService. The LoggerService is already registered at the AppComponent level. When this component requests the LoggerService, it receives the DateLoggerService instead.

The DateLoggerService inherits from LoggerService; it appends the current date/time to each message:

@Injectable()
export class DateLoggerService extends LoggerService implements MinimalLogger
{
  logInfo(msg: any)  { super.logInfo(stamp(msg)); }
  logDebug(msg: any) { super.logInfo(stamp(msg)); }
  logError(msg: any) { super.logError(stamp(msg)); }
}

function stamp(msg: any) { return msg + ' at ' + new Date(); }

useExisting - the alias provider

The useExisting provider maps one token to another. In effect, the first token is an alias for the service associated with second token, creating two ways to access the same service object.

{ provide: MinimalLogger, useExisting: LoggerService },

Narrowing an API through an aliasing interface is one important use case for this technique. We're aliasing for that very purpose here. Imagine that the LoggerService had a large API (it's actually only three methods and a property). We want to shrink that API surface to just the two members exposed by the MinimalLogger class-interface:

// class used as a restricting interface (hides other public members)
export abstract class MinimalLogger {
  logInfo: (msg: string) => void;
  logs: string[];
}

The constructor's logger parameter is typed as MinimalLogger so only its two members are visible in TypeScript:

Angular actually sets the logger parameter to the injector's full version of the LoggerService which happens to be the DateLoggerService thanks to the override provider registered previously via useClass. The following image, which displays the logging date, confirms the point:

useFactory - the factory provider

The useFactory provider creates a dependency object by calling a factory function as seen in this example.

{ provide: RUNNERS_UP,    useFactory:  runnersUpFactory(2), deps: [Hero, HeroService] }

Use this technique to create a dependency object with a factory function whose inputs are some combination of injected services and local state.

The dependency object doesn't have to be a class instance. It could be anything. In this example, the dependency object is a string of the names of the runners-up to the "Hero of the Month" contest.

The local state is the number 2, the number of runners-up this component should show. We execute runnersUpFactory immediately with 2.

The runnersUpFactory itself isn't the provider factory function. The true provider factory function is the function that runnersUpFactory returns.

export function runnersUpFactory(take: number) {
  return (winner: Hero, heroService: HeroService): string => {
    /* ... */
  };
};

That returned function takes a winning Hero and a HeroService as arguments.

Angular supplies these arguments from injected values identified by the two tokens in the deps array. The two deps values are tokens that the injector uses to provide these factory function dependencies.

After some undisclosed work, the function returns the string of names and Angular injects it into the runnersUp parameter of the HeroOfTheMonthComponent.

Provider token alternatives: the class-interface and OpaqueToken

Angular dependency injection is easiest when the provider token is a class that is also the type of the returned dependency object (what we usually call the service).

But the token doesn't have to be a class and even when it is a class, it doesn't have to be the same type as the returned object. That's the subject of our next section.

class-interface

In the previous Hero of the Month example, we used the MinimalLogger class as the token for a provider of a LoggerService.

{ provide: MinimalLogger, useExisting: LoggerService },

The MinimalLogger is an abstract class.

// class used as a restricting interface (hides other public members)
export abstract class MinimalLogger {
  logInfo: (msg: string) => void;
  logs: string[];
}

We usually inherit from an abstract class. But LoggerService doesn't inherit from MinimalLogger. No class inherits from it. Instead, we use it like an interface.

Look again at the declaration for DateLoggerService

export class DateLoggerService extends LoggerService implements MinimalLogger

DateLoggerService inherits (extends) from LoggerService, not MinimalLogger. The DateLoggerService implements MinimalLogger as if MinimalLogger were an interface.

We call a class used in this way a class-interface. The key benefit of a class-interface is that we can get the strong-typing of an interface and we can use it as a provider token in the same manner as a normal class.

A class-interface should define only the members that its consumers are allowed to call. Such a narrowing interface helps decouple the concrete class from its consumers. The MinimalLogger defines just two of the LoggerClass members.

var MinimalLogger = (function () {
  function MinimalLogger() {}
  return MinimalLogger;
}());
exports("MinimalLogger", MinimalLogger);

OpaqueToken

Dependency objects can be simple values like dates, numbers and strings or shapeless objects like arrays and functions.

Such objects don't have application interfaces and therefore aren't well represented by a class. They're better represented by a token that is both unique and symbolic, a JavaScript object that has a friendly name but won't conflict with another token that happens to have the same name.

The OpaqueToken has these characteristics. We encountered them twice in the Hero of the Month example, in the title value provider and in the runnersUp factory provider.

{ provide: TITLE,         useValue:   'Hero of the Month' },
{ provide: RUNNERS_UP,    useFactory:  runnersUpFactory(2), deps: [Hero, HeroService] }

We created the TITLE token like this:

import { OpaqueToken } from '@angular/core';

export const TITLE = new OpaqueToken('title');

Inject into a derived class

We must take care when writing a component that inherits from another component. If the base component has injected dependencies, we must re-provide and re-inject them in the derived class and then pass them down to the base class through the constructor.

In this contrived example, SortedHeroesComponent inherits from HeroesBaseComponent to display a sorted list of heroes.

The HeroesBaseComponent could stand on its own. It demands its own instance of the HeroService to get heroes and displays them in the order they arrive from the database.

@Component({
  selector: 'unsorted-heroes',
  template: `<div *ngFor="let hero of heroes">{{hero.name}}</div>`,
  providers: [HeroService]
})
export class HeroesBaseComponent implements OnInit {
  constructor(private heroService: HeroService) { }

  heroes: Array<Hero>;

  ngOnInit() {
    this.heroes = this.heroService.getAllHeroes();
    this.afterGetHeroes();
  }

  // Post-process heroes in derived class override.
  protected afterGetHeroes() {}

}

Users want to see the heroes in alphabetical order. Rather than modify the original component, we sub-class it and create a SortedHeroesComponent that sorts the heroes before presenting them. The SortedHeroesComponent lets the base class fetch the heroes. (we said it was contrived).

Unfortunately, Angular cannot inject the HeroService directly into the base class. We must provide the HeroService again for this component, then pass it down to the base class inside the constructor.

@Component({
  selector: 'sorted-heroes',
  template: `<div *ngFor="let hero of heroes">{{hero.name}}</div>`,
  providers: [HeroService]
})
export class SortedHeroesComponent extends HeroesBaseComponent {
  constructor(heroService: HeroService) {
    super(heroService);
  }

  protected afterGetHeroes() {
    this.heroes = this.heroes.sort((h1, h2) => {
      return h1.name < h2.name ? -1 :
            (h1.name > h2.name ? 1 : 0);
    });
  }
}

Now take note of the afterGetHeroes method. Our first instinct was to create an ngOnInit method in SortedHeroesComponent and do the sorting there. But Angular calls the derived class's ngOnInit before calling the base class's ngOnInit so we'd be sorting the heroes array before they arrived. That produces a nasty error.

Overriding the base class's afterGetHeroes method solves the problem

These complications argue for avoiding component inheritance.

Find a parent component by injection

Application components often need to share information. We prefer the more loosely coupled techniques such as data binding and service sharing. But sometimes it makes sense for one component to have a direct reference to another component perhaps to access values or call methods on that component.

Obtaining a component reference is a bit tricky in Angular. Although an Angular application is a tree of components, there is no public API for inspecting and traversing that tree.

There is an API for acquiring a child reference (checkout Query, QueryList, ViewChildren, and ContentChildren).

There is no public API for acquiring a parent reference. But because every component instance is added to an injector's container, we can use Angular dependency injection to reach a parent component.

This section describes some techniques for doing that.

Find a parent component of known type

We use standard class injection to acquire a parent component whose type we know.

In the following example, the parent AlexComponent has several children including a CathyComponent:

@Component({
  selector: 'alex',
  template: `
    <div class="a">
      <h3>{{name}}</h3>
      <cathy></cathy>
      <craig></craig>
      <carol></carol>
    </div>`,
})
export class AlexComponent extends Base
{
  name= 'Alex';
}

Cathy reports whether or not she has access to Alex after injecting an AlexComponent into her constructor:

@Component({
  selector: 'cathy',
  template: `
  <div class="c">
    <h3>Cathy</h3>
    {{alex ? 'Found' : 'Did not find'}} Alex via the component class.<br>
  </div>`
})
export class CathyComponent {
  constructor( @Optional() public alex: AlexComponent ) { }
}

We added the @Optional qualifier for safety but the confirms that the alex parameter is set.

Cannot find a parent by its base class

What if we do not know the concrete parent component class?

A re-usable component might be a child of multiple components. Imagine a component for rendering breaking news about a financial instrument. For sound (cough) business reasons, this news component makes frequent calls directly into its parent instrument as changing market data stream by.

The app probably defines more than a dozen financial instrument components. If we're lucky, they all implement the same base class whose API our NewsComponent understands.

We're not claiming this is good design. We are asking can a component inject its parent via the parent's base class?

The sample's CraigComponent explores this question. Looking back we see that the Alex component extends (inherits) from a class named Base.

export class AlexComponent extends Base

The CraigComponent tries to inject Base into its alex constructor parameter and reports if it succeeded.

@Component({
  selector: 'craig',
  template: `
  <div class="c">
    <h3>Craig</h3>
    {{alex ? 'Found' : 'Did not find'}} Alex via the base class.
  </div>`
})
export class CraigComponent {
  constructor( @Optional() public alex: Base ) { }
}

Unfortunately, this does not work. The confirms that the alex parameter is null. We cannot inject a parent by its base class.

Find a parent by its class-interface

We can find a parent component with a class-interface.

The parent must cooperate by providing an alias to itself in the name of a class-interface token.

Recall that Angular always adds a component instance to its own injector; that's why we could inject Alex into Carol earlier.

We write an alias provider — a provide object literal with a useExisting definition — that creates an alternative way to inject the same component instance and add that provider to the providers array of the @Component metadata for the AlexComponent:

providers: [{ provide: Parent, useExisting: forwardRef(() => AlexComponent) }],

Parent is the provider's class-interface token. The forwardRef breaks the circular reference we just created by having the AlexComponent refer to itself.

Carol, the third of Alex's child components, injects the parent into its parent parameter, the same way we've done it before:

export class CarolComponent {
  name= 'Carol';
  constructor( @Optional() public parent: Parent ) { }
}

Here's Alex and family in action:

Find the parent in a tree of parents

Imagine one branch of a component hierarchy: Alice -> Barry -> Carol. Both Alice and Barry implement the Parent class-interface.

Barry is the problem. He needs to reach his parent, Alice, and also be a parent to Carol. That means he must both inject the Parent class-interface to get Alice and provide a Parent to satisfy Carol.

Here's Barry:

const templateB = `
  <div class="b">
    <div>
      <h3>{{name}}</h3>
      <p>My parent is {{parent?.name}}</p>
    </div>
    <carol></carol>
    <chris></chris>
  </div>`;

@Component({
  selector:   'barry',
  template:   templateB,
  providers:  [{ provide: Parent, useExisting: forwardRef(() => BarryComponent) }]
})
export class BarryComponent implements Parent {
  name = 'Barry';
  constructor( @SkipSelf() @Optional() public parent: Parent ) { }
}

Barry's providers array looks just like Alex's. If we're going to keep writing alias providers like this we should create a helper function.

For now, focus on Barry's constructor:

constructor( @SkipSelf() @Optional() public parent: Parent ) { }

constructor( @Optional() public parent: Parent ) { }

It's identical to Carol's constructor except for the additional @SkipSelf decorator.

@SkipSelf is essential for two reasons:

1. It tell the injector to start its search for a Parent dependency in a component above itself, which is what parent means.

2. Angular throws a cyclic dependency error if we omit the @SkipSelf decorator.

   Cannot instantiate cyclic dependency! (BethComponent -> Parent -> BethComponent)

Here's Alice, Barry and family in action:

The Parent class-interface

We learned earlier that a class-interface is an abstract class used as an interface rather than as a base class.

Our example defines a Parent class-interface .

export abstract class Parent { name: string; }

The Parent class-interface defines a name property with a type declaration but no implementation., The name property is the only member of a parent component that a child component can call. Such a narrowing interface helps decouple the child component class from its parent components.

A component that could serve as a parent should implement the class-interface as the AliceComponent does:

export class AliceComponent implements Parent

Doing so adds clarity to the code. But it's not technically necessary. Although the AlexComponent has a name property (as required by its Base class) its class signature doesn't mention Parent:

export class AlexComponent extends Base

A provideParent helper function

Writing variations of the same parent alias provider gets old quickly, especially this awful mouthful with a forwardRef:

providers: [{ provide: Parent, useExisting: forwardRef(() => AlexComponent) }],

We can extract that logic into a helper function like this:

// Helper method to provide the current component instance in the name of a `parentType`.
const provideParent =
  (component: any) => {
    return { provide: Parent, useExisting: forwardRef(() => component) };
  };

Now we can add a simpler, more meaningful parent provider to our components:

providers:  [ provideParent(AliceComponent) ]

We can do better. The current version of the helper function can only alias the Parent class-interface. Our application might have a variety of parent types, each with its own class-interface token.

Here's a revised version that defaults to parent but also accepts an optional second parameter for a different parent class-interface.

// Helper method to provide the current component instance in the name of a `parentType`.
// The `parentType` defaults to `Parent` when omitting the second parameter.
const provideParent =
  (component: any, parentType?: any) => {
    return { provide: parentType || Parent, useExisting: forwardRef(() => component) };
  };

And here's how we could use it with a different parent type:

providers:  [ provideParent(BethComponent, DifferentParent) ]

Break circularities with a forward class reference (forwardRef)

The order of class declaration matters in TypeScript. We can't refer directly to a class until it's been defined.

This isn't usually a problem, especially if we adhere to the recommended one class per file rule. But sometimes circular references are unavoidable. We're in a bind when class 'A refers to class 'B' and 'B' refers to 'A'. One of them has to be defined first.

The Angular forwardRef function creates an indirect reference that Angular can resolve later.

The Parent Finder sample is full of circular class references that are impossible to break.

We face this dilemma when a class makes a reference to itself as does the AlexComponent in its providers array. The providers array is a property of the @Component decorator function which must appear above the class definition.

We break the circularity with forwardRef:

providers: [{ provide: Parent, useExisting: forwardRef(() => AlexComponent) }],