angular-docs-cn/aio/content/guide/hierarchical-dependency-injection.md

Hierarchical injectors

Injectors in Angular have rules that you can leverage to achieve the desired visibility of injectables in your apps. By understanding these rules, you can determine in which NgModule, Component or Directive you should declare a provider.

Two injector hierarchies

There are two injector hierarchies in Angular:

1. ModuleInjector hierarchy—configure a ModuleInjector in this hierarchy using an @NgModule() or @Injectable() annotation.
2. ElementInjector hierarchy—created implicitly at each DOM element. An ElementInjector is empty by default unless you configure it in the providers property on @Directive() or @Component().

{@a register-providers-injectable}

ModuleInjector

The ModuleInjector can be configured in one of two ways:

• Using the @Injectable() providedIn property to refer to @NgModule(), or root.
• Using the @NgModule() providers array.

Tree-shaking and @Injectable()

Using the @Injectable() providedIn property is preferable to the @NgModule() providers array because with @Injectable() providedIn, optimization tools can perform tree-shaking, which removes services that your app isn't using and results in smaller bundle sizes.

Tree-shaking is especially useful for a library because the application which uses the library may not have a need to inject it. Read more about tree-shakable providers in DI Providers.

ModuleInjector is configured by the @NgModule.providers and NgModule.imports property. ModuleInjector is a flattening of all of the providers arrays which can be reached by following the NgModule.imports recursively.

Child ModuleInjectors are created when lazy loading other @NgModules.

Provide services with the providedIn property of @Injectable() as follows:

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

@Injectable({
  providedIn: 'root'  // <--provides this service in the root ModuleInjector
})
export class ItemService {
  name = 'telephone';
}

The @Injectable() decorator identifies a service class. The providedIn property configures a specific ModuleInjector, here root, which makes the service available in the root ModuleInjector.

Platform injector

There are two more injectors above root, an additional ModuleInjector and NullInjector().

Consider how Angular bootstraps the app with the following in main.ts:

platformBrowserDynamic().bootstrapModule(AppModule).then(ref => {...})

The bootstrapModule() method creates a child injector of the platform injector which is configured by the AppModule. This is the root ModuleInjector.

The platformBrowserDynamic() method creates an injector configured by a PlatformModule, which contains platform-specific dependencies. This allows multiple apps to share a platform configuration. For example, a browser has only one URL bar, no matter how many apps you have running. You can configure additional platform-specific providers at the platform level by supplying extraProviders using the platformBrowser() function.

The next parent injector in the hierarchy is the NullInjector(), which is the top of the tree. If you've gone so far up the tree that you are looking for a service in the NullInjector(), you'll get an error unless you've used @Optional() because ultimately, everything ends at the NullInjector() and it returns an error or, in the case of @Optional(), null. For more information on @Optional(), see the @Optional() section of this guide.

The following diagram represents the relationship between the root ModuleInjector and its parent injectors as the previous paragraphs describe.

While the name root is a special alias, other ModuleInjectors don't have aliases. You have the option to create ModuleInjectors whenever a dynamically loaded component is created, such as with the Router, which will create child ModuleInjectors.

All requests forward up to the root injector, whether you configured it with the bootstrapModule() method, or registered all providers with root in their own services.

@Injectable() vs. @NgModule()

If you configure an app-wide provider in the @NgModule() of AppModule, it overrides one configured for root in the @Injectable() metadata. You can do this to configure a non-default provider of a service that is shared with multiple apps.

Here is an example of the case where the component router configuration includes a non-default location strategy by listing its provider in the providers list of the AppModule.

ElementInjector

Angular creates ElementInjectors implicitly for each DOM element.

Providing a service in the @Component() decorator using its providers or viewProviders property configures an ElementInjector. For example, the following TestComponent configures the ElementInjector by providing the service as follows:

@Component({
  ...
  providers: [{ provide: ItemService, useValue: { name: 'lamp' } }]
})
export class TestComponent

Note: Please see the resolution rules section to understand the relationship between the ModuleInjector tree and the ElementInjector tree.

When you provide services in a component, that service is available via the ElementInjector at that component instance. It may also be visible at child component/directives based on visibility rules described in the resolution rules section.

When the component instance is destroyed, so is that service instance.

@Directive() and @Component()

A component is a special type of directive, which means that just as @Directive() has a providers property, @Component() does too. This means that directives as well as components can configure providers, using the providers property. When you configure a provider for a component or directive using the providers property, that provider belongs to the ElementInjector of that component or directive. Components and directives on the same element share an injector.

{@a resolution-rules}

Resolution rules

When resolving a token for a component/directive, Angular resolves it in two phases:

1. Against the ElementInjector hierarchy (its parents)
2. Against the ModuleInjector hierarchy (its parents)

When a component declares a dependency, Angular tries to satisfy that dependency with its own ElementInjector. If the component's injector lacks the provider, it passes the request up to its parent component's ElementInjector.

The requests keep forwarding up until Angular finds an injector that can handle the request or runs out of ancestor ElementInjectors.

If Angular doesn't find the provider in any ElementInjectors, it goes back to the element where the request originated and looks in the ModuleInjector hierarchy. If Angular still doesn't find the provider, it throws an error.

If you have registered a provider for the same DI token at different levels, the first one Angular encounters is the one it uses to resolve the dependency. If, for example, a provider is registered locally in the component that needs a service, Angular doesn't look for another provider of the same service.

Resolution modifiers

Angular's resolution behavior can be modified with @Optional(), @Self(), @SkipSelf() and @Host(). Import each of them from @angular/core and use each in the component class constructor when you inject your service.

For a working app showcasing the resolution modifiers that this section covers, see the resolution modifiers example.

Types of modifiers

Resolution modifiers fall into three categories:

1. What to do if Angular doesn't find what you're looking for, that is @Optional()
2. Where to start looking, that is @SkipSelf()
3. Where to stop looking, @Host() and @Self()

By default, Angular always starts at the current Injector and keeps searching all the way up. Modifiers allow you to change the starting (self) or ending location.

Additionally, you can combine all of the modifiers except @Host() and @Self() and of course @SkipSelf() and @Self().

{@a optional}

@Optional()

@Optional() allows Angular to consider a service you inject to be optional. This way, if it can't be resolved at runtime, Angular simply resolves the service as null, rather than throwing an error. In the following example, the service, OptionalService, isn't provided in the service, @NgModule(), or component class, so it isn't available anywhere in the app.

@Self()

Use @Self() so that Angular will only look at the ElementInjector for the current component or directive.

A good use case for @Self() is to inject a service but only if it is available on the current host element. To avoid errors in this situation, combine @Self() with @Optional().

For example, in the following SelfComponent, notice the injected LeafService in the constructor.

In this example, there is a parent provider and injecting the service will return the value, however, injecting the service with @Self() and @Optional() will return null because @Self() tells the injector to stop searching in the current host element.

Another example shows the component class with a provider for FlowerService. In this case, the injector looks no further than the current ElementInjector because it finds the FlowerService and returns the yellow flower 🌼.

@SkipSelf()

@SkipSelf() is the opposite of @Self(). With @SkipSelf(), Angular starts its search for a service in the parent ElementInjector, rather than in the current one. So if the parent ElementInjector were using the value 🌿 (fern) for emoji , but you had 🍁 (maple leaf) in the component's providers array, Angular would ignore 🍁 (maple leaf) and use 🌿 (fern).

To see this in code, assume that the following value for emoji is what the parent component were using, as in this service:

Imagine that in the child component, you had a different value, 🍁 (maple leaf) but you wanted to use the parent's value instead. This is when you'd use @SkipSelf():

In this case, the value you'd get for emoji would be 🌿 (fern), not 🍁 (maple leaf).

@SkipSelf() with @Optional()

Use @SkipSelf() with @Optional() to prevent an error if the value is null. In the following example, the Person service is injected in the constructor. @SkipSelf() tells Angular to skip the current injector and @Optional() will prevent an error should the Person service be null.

class Person {
  constructor(@Optional() @SkipSelf() parent?: Person) {}
}

@Host()

@Host() lets you designate a component as the last stop in the injector tree when searching for providers. Even if there is a service instance further up the tree, Angular won't continue looking. Use @Host() as follows:

Since HostComponent has @Host() in its constructor, no matter what the parent of HostComponent might have as a flower.emoji value, the HostComponent will use 🌼 (yellow flower).

Logical structure of the template

When you provide services in the component class, services are visible within the ElementInjector tree relative to where and how you provide those services.

Understanding the underlying logical structure of the Angular template will give you a foundation for configuring services and in turn control their visibility.

Components are used in your templates, as in the following example:

<app-root>
    <app-child></app-child>
</app-root>

Note: Usually, you declare the components and their templates in separate files. For the purposes of understanding how the injection system works, it is useful to look at them from the point of view of a combined logical tree. The term logical distinguishes it from the render tree (your application DOM tree). To mark the locations of where the component templates are located, this guide uses the <#VIEW> pseudo element, which doesn't actually exist in the render tree and is present for mental model purposes only.

The following is an example of how the <app-root> and <app-child> view trees are combined into a single logical tree:

<app-root>
  <#VIEW>
    <app-child>
     <#VIEW>
       ...content goes here...
     </#VIEW>
    </app-child>
  <#VIEW>
</app-root>

Understanding the idea of the <#VIEW> demarcation is especially significant when you configure services in the component class.

Providing services in @Component()

How you provide services via an @Component() (or @Directive()) decorator determines their visibility. The following sections demonstrate providers and viewProviders along with ways to modify service visibility with @SkipSelf() and @Host().

A component class can provide services in two ways:

1. with a providers array

@Component({
  ...
  providers: [
    {provide: FlowerService, useValue: {emoji: '🌺'}}
  ]
})

2. with a viewProviders array

@Component({
  ...
  viewProviders: [
    {provide: AnimalService, useValue: {emoji: '🐶'}}
  ]
})

To understand how the providers and viewProviders influence service visibility differently, the following sections build a step-by-step and compare the use of providers and viewProviders in code and a logical tree.

NOTE: In the logical tree, you'll see @Provide, @Inject, and @NgModule, which are not real HTML attributes but are here to demonstrate what is going on under the hood.

• @Inject(Token)=>Value demonstrates that if Token is injected at this location in the logical tree its value would be Value.
• @Provide(Token=Value) demonstrates that there is a declaration of Token provider with value Value at this location in the logical tree.
• @NgModule(Token) demonstrates that a fallback NgModule injector should be used at this location.

Example app structure

The example app has a FlowerService provided in root with an emoji value of 🌺 (red hibiscus).

Consider a simple app with only an AppComponent and a ChildComponent. The most basic rendered view would look like nested HTML elements such as the following:

<app-root> <!-- AppComponent selector -->
    <app-child> <!-- ChildComponent selector -->
    </app-child>
</app-root>

However, behind the scenes, Angular uses a logical view representation as follows when resolving injection requests:

<app-root> <!-- AppComponent selector -->
    <#VIEW>
        <app-child> <!-- ChildComponent selector -->
            <#VIEW>
            </#VIEW>
        </app-child>
    </#VIEW>
</app-root>

The <#VIEW> here represents an instance of a template. Notice that each component has its own <#VIEW>.

Knowledge of this structure can inform how you provide and inject your services, and give you complete control of service visibility.

Now, consider that <app-root> simply injects the FlowerService:

Add a binding to the <app-root> template to visualize the result:

The output in the view would be:

Emoji from FlowerService: 🌺

In the logical tree, this would be represented as follows:

<app-root @NgModule(AppModule)
        @Inject(FlowerService) flower=>"🌺">
  <#VIEW>
    <p>Emoji from FlowerService: {{flower.emoji}} (🌺)</p>
    <app-child>
      <#VIEW>
      </#VIEW>
     </app-child>
  </#VIEW>
</app-root>

When <app-root> requests the FlowerService, it is the injector's job to resolve the FlowerService token. The resolution of the token happens in two phases:

1. The injector determines the starting location in the logical tree and an ending location of the search. The injector begins with the starting location and looks for the token at each level in the logical tree. If the token is found it is returned.
2. If the token is not found, the injector looks for the closest parent @NgModule() to delegate the request to.

In the example case, the constraints are:

1. Start with <#VIEW> belonging to <app-root> and end with <app-root>.

• Normally the starting point for search is at the point of injection. However, in this case <app-root> @Components are special in that they also include their own viewProviders, which is why the search starts at <#VIEW> belonging to <app-root>. (This would not be the case for a directive matched at the same location).
• The ending location just happens to be the same as the component itself, because it is the topmost component in this application.

2. The AppModule acts as the fallback injector when the injection token can't be found in the ElementInjectors.

Using the providers array

Now, in the ChildComponent class, add a provider for FlowerService to demonstrate more complex resolution rules in the upcoming sections:

Now that the FlowerService is provided in the @Component() decorator, when the <app-child> requests the service, the injector has only to look as far as the <app-child>'s own ElementInjector. It won't have to continue the search any further through the injector tree.

The next step is to add a binding to the ChildComponent template.

To render the new values, add <app-child> to the bottom of the AppComponent template so the view also displays the sunflower:

Child Component
Emoji from FlowerService: 🌻

In the logical tree, this would be represented as follows:

<app-root @NgModule(AppModule)
        @Inject(FlowerService) flower=>"🌺">
  <#VIEW>
    <p>Emoji from FlowerService: {{flower.emoji}} (🌺)</p>
    <app-child @Provide(FlowerService="🌻")
               @Inject(FlowerService)=>"🌻"> <!-- search ends here -->
      <#VIEW> <!-- search starts here -->
        <h2>Parent Component</h2>
        <p>Emoji from FlowerService: {{flower.emoji}} (🌻)</p>
      </#VIEW>
     </app-child>
  </#VIEW>
</app-root>

When <app-child> requests the FlowerService, the injector begins its search at the <#VIEW> belonging to <app-child> (<#VIEW> is included because it is injected from @Component()) and ends with <app-child>. In this case, the FlowerService is resolved in the <app-child>'s providers array with sunflower 🌻. The injector doesn't have to look any further in the injector tree. It stops as soon as it finds the FlowerService and never sees the 🌺 (red hibiscus).

{@a use-view-providers}

Using the viewProviders array

Use the viewProviders array as another way to provide services in the @Component() decorator. Using viewProviders makes services visible in the <#VIEW>.

The steps are the same as using the providers array, with the exception of using the viewProviders array instead.

For step-by-step instructions, continue with this section. If you can set it up on your own, skip ahead to Modifying service availability.

The example app features a second service, the AnimalService to demonstrate viewProviders.

First, create an AnimalService with an emoji property of 🐳 (whale):

Following the same pattern as with the FlowerService, inject the AnimalService in the AppComponent class:

Note: You can leave all the FlowerService related code in place as it will allow a comparison with the AnimalService.

Add a viewProviders array and inject the AnimalService in the <app-child> class, too, but give emoji a different value. Here, it has a value of 🐶 (puppy).

Add bindings to the ChildComponent and the AppComponent templates. In the ChildComponent template, add the following binding:

Additionally, add the same to the AppComponent template:

Now you should see both values in the browser:

AppComponent
Emoji from AnimalService: 🐳

Child Component
Emoji from AnimalService: 🐶

The logic tree for this example of viewProviders is as follows:

<app-root @NgModule(AppModule)
        @Inject(AnimalService) animal=>"🐳">
  <#VIEW>
    <app-child>
      <#VIEW
       @Provide(AnimalService="🐶")
       @Inject(AnimalService=>"🐶")>
       <!-- ^^using viewProviders means AnimalService is available in <#VIEW>-->
       <p>Emoji from AnimalService: {{animal.emoji}} (🐶)</p>
      </#VIEW>
     </app-child>
  </#VIEW>
</app-root>

Just as with the FlowerService example, the AnimalService is provided in the <app-child> @Component() decorator. This means that since the injector first looks in the ElementInjector of the component, it finds the AnimalService value of 🐶 (puppy). It doesn't need to continue searching the ElementInjector tree, nor does it need to search the ModuleInjector.

providers vs. viewProviders

To see the difference between using providers and viewProviders, add another component to the example and call it InspectorComponent. InspectorComponent will be a child of the ChildComponent. In inspector.component.ts, inject the FlowerService and AnimalService in the constructor:

You do not need a providers or viewProviders array. Next, in inspector.component.html, add the same markup from previous components:

Remember to add the InspectorComponent to the AppModule declarations array.

Next, make sure your child.component.html contains the following:

The first two lines, with the bindings, are there from previous steps. The new parts are <ng-content> and <app-inspector>. <ng-content> allows you to project content, and <app-inspector> inside the ChildComponent template makes the InspectorComponent a child component of ChildComponent.

Next, add the following to app.component.html to take advantage of content projection.

The browser now renders the following, omitting the previous examples for brevity:

//...Omitting previous examples. The following applies to this section.

Content projection: This is coming from content. Doesn't get to see
puppy because the puppy is declared inside the view only.

Emoji from FlowerService: 🌻
Emoji from AnimalService: 🐳

Emoji from FlowerService: 🌻
Emoji from AnimalService: 🐶

These four bindings demonstrate the difference between providers and viewProviders. Since the 🐶 (puppy) is declared inside the <#VIEW>, it isn't visible to the projected content. Instead, the projected content sees the 🐳 (whale).

The next section though, where InspectorComponent is a child component of ChildComponent, InspectorComponent is inside the <#VIEW>, so when it asks for the AnimalService, it sees the 🐶 (puppy).

The AnimalService in the logical tree would look like this:

<app-root @NgModule(AppModule)
        @Inject(AnimalService) animal=>"🐳">
  <#VIEW>
    <app-child>
      <#VIEW
       @Provide(AnimalService="🐶")
       @Inject(AnimalService=>"🐶")>
       <!-- ^^using viewProviders means AnimalService is available in <#VIEW>-->
       <p>Emoji from AnimalService: {{animal.emoji}} (🐶)</p>
       <app-inspector>
        <p>Emoji from AnimalService: {{animal.emoji}} (🐶)</p>
       </app-inspector>
      </#VIEW>
      <app-inspector>
        <#VIEW>
          <p>Emoji from AnimalService: {{animal.emoji}} (🐳)</p>
        </#VIEW>
      </app-inspector>
     </app-child>
  </#VIEW>
</app-root>

The projected content of <app-inspector> sees the 🐳 (whale), not the 🐶 (puppy), because the 🐶 (puppy) is inside the <app-child> <#VIEW>. The <app-inspector> can only see the 🐶 (puppy) if it is also within the <#VIEW>.

{@a modify-visibility}

Modifying service visibility

This section describes how to limit the scope of the beginning and ending ElementInjector using the visibility decorators @Host(), @Self(), and @SkipSelf().

Visibility of provided tokens

Visibility decorators influence where the search for the injection token begins and ends in the logic tree. To do this, place visibility decorators at the point of injection, that is, the constructor(), rather than at a point of declaration.

To alter where the injector starts looking for FlowerService, add @SkipSelf() to the <app-child> @Inject declaration for the FlowerService. This declaration is in the <app-child> constructor as shown in child.component.ts:

  constructor(@SkipSelf() public flower : FlowerService) { }

With @SkipSelf(), the <app-child> injector doesn't look to itself for the FlowerService. Instead, the injector starts looking for the FlowerService at the <app-root>'s ElementInjector, where it finds nothing. Then, it goes back to the <app-child> ModuleInjector and finds the 🌺 (red hibiscus) value, which is available because the <app-child> ModuleInjector and the <app-root> ModuleInjector are flattened into one ModuleInjector. Thus, the UI renders the following:

Emoji from FlowerService: 🌺

In a logical tree, this same idea might look like this:

<app-root @NgModule(AppModule)
        @Inject(FlowerService) flower=>"🌺">
  <#VIEW>
    <app-child @Provide(FlowerService="🌻")>
      <#VIEW @Inject(FlowerService, SkipSelf)=>"🌺">
      <!-- With SkipSelf, the injector looks to the next injector up the tree -->
      </#VIEW>
      </app-child>
  </#VIEW>
</app-root>

Though <app-child> provides the 🌻 (sunflower), the app renders the 🌺 (red hibiscus) because @SkipSelf() causes the current injector to skip itself and look to its parent.

If you now add @Host() (in addition to the @SkipSelf()) to the @Inject of the FlowerService, the result will be null. This is because @Host() limits the upper bound of the search to the <#VIEW>. Here's the idea in the logical tree:

<app-root @NgModule(AppModule)
        @Inject(FlowerService) flower=>"🌺">
  <#VIEW> <!-- end search here with null-->
    <app-child @Provide(FlowerService="🌻")> <!-- start search here -->
      <#VIEW @Inject(FlowerService, @SkipSelf, @Host, @Optional)=>null>
      </#VIEW>
      </app-parent>
  </#VIEW>
</app-root>

Here, the services and their values are the same, but @Host() stops the injector from looking any further than the <#VIEW> for FlowerService, so it doesn't find it and returns null.

Note: The example app uses @Optional() so the app does not throw an error, but the principles are the same.

@SkipSelf() and viewProviders

The <app-child> currently provides the AnimalService in the viewProviders array with the value of 🐶 (puppy). Because the injector has only to look at the <app-child>'s ElementInjector for the AnimalService, it never sees the 🐳 (whale).

Just as in the FlowerService example, if you add @SkipSelf() to the constructor for the AnimalService, the injector won't look in the current <app-child>'s ElementInjector for the AnimalService.

export class ChildComponent {

// add @SkipSelf()
  constructor(@SkipSelf() public animal : AnimalService) { }

}

Instead, the injector will begin at the <app-root> ElementInjector. Remember that the <app-child> class provides the AnimalService in the viewProviders array with a value of 🐶 (puppy):

@Component({
  selector: 'app-child',
  ...
  viewProviders:
  [{ provide: AnimalService, useValue: { emoji: '🐶' } }]
})

The logical tree looks like this with @SkipSelf() in <app-child>:

  <app-root @NgModule(AppModule)
          @Inject(AnimalService=>"🐳")>
    <#VIEW><!-- search begins here -->
      <app-child>
        <#VIEW
         @Provide(AnimalService="🐶")
         @Inject(AnimalService, SkipSelf=>"🐳")>
         <!--Add @SkipSelf -->
        </#VIEW>
        </app-child>
    </#VIEW>
  </app-root>

With @SkipSelf() in the <app-child>, the injector begins its search for the AnimalService in the <app-root> ElementInjector and finds 🐳 (whale).

@Host() and viewProviders

If you add @Host() to the constructor for AnimalService, the result is 🐶 (puppy) because the injector finds the AnimalService in the <app-child> <#VIEW>. Here is the viewProviders array in the <app-child> class and @Host() in the constructor:

@Component({
  selector: 'app-child',
  ...
  viewProviders:
  [{ provide: AnimalService, useValue: { emoji: '🐶' } }]

})
export class ChildComponent {
  constructor(@Host() public animal : AnimalService) { }
}

@Host() causes the injector to look until it encounters the edge of the <#VIEW>.

  <app-root @NgModule(AppModule)
          @Inject(AnimalService=>"🐳")>
    <#VIEW>
      <app-child>
        <#VIEW
         @Provide(AnimalService="🐶")
         @Inject(AnimalService, @Host=>"🐶")> <!-- @Host stops search here -->
        </#VIEW>
        </app-child>
    </#VIEW>
  </app-root>

Add a viewProviders array with a third animal, 🦔 (hedgehog), to the app.component.ts @Component() metadata:

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ],
  viewProviders: [{ provide: AnimalService, useValue: { emoji: '🦔' } }]
})

Next, add @SkipSelf() along with @Host() to the constructor for the Animal Service in child.component.ts. Here are @Host() and @SkipSelf() in the <app-child> constructor :

export class ChildComponent {

  constructor(
  @Host() @SkipSelf() public animal : AnimalService) { }

}

When @Host() and SkipSelf() were applied to the FlowerService, which is in the providers array, the result was null because @SkipSelf() starts its search in the <app-child> injector, but @Host() stops searching at <#VIEW>—where there is no FlowerService. In the logical tree, you can see that the FlowerService is visible in <app-child>, not its <#VIEW>.

However, the AnimalService, which is provided in the AppComponent viewProviders array, is visible.

The logical tree representation shows why this is:

<app-root @NgModule(AppModule)
        @Inject(AnimalService=>"🐳")>
  <#VIEW @Provide(AnimalService="🦔")
         @Inject(AnimalService, @SkipSelf, @Host, @Optional)=>"🦔">
    <!-- ^^@SkipSelf() starts here,  @Host() stops here^^ -->
    <app-child>
      <#VIEW @Provide(AnimalService="🐶")
             @Inject(AnimalService, @SkipSelf, @Host, @Optional)=>"🐶">
               <!-- Add @SkipSelf ^^-->
      </#VIEW>
      </app-child>
  </#VIEW>
</app-root>

@SkipSelf(), causes the injector to start its search for the AnimalService at the <app-root>, not the <app-child>, where the request originates, and @Host() stops the search at the <app-root> <#VIEW>. Since AnimalService is provided via the viewProviders array, the injector finds 🦔 (hedgehog) in the <#VIEW>.

{@a component-injectors}

ElementInjector use case examples

The ability to configure one or more providers at different levels opens up useful possibilities. For a look at the following scenarios in a working app, see the heroes use case examples.

Scenario: service isolation

Architectural reasons may lead you to restrict access to a service to the application domain where it belongs. For example, the guide sample includes a VillainsListComponent that displays a list of villains. It gets those villains from a VillainsService.

If you provided VillainsService in the root AppModule (where you registered the HeroesService), that would make the VillainsService visible everywhere in the application, including the Hero workflows. If you later modified the VillainsService, you could break something in a hero component somewhere.

Instead, you can provide the VillainsService in the providers metadata of the VillainsListComponent like this:

By providing VillainsService in the VillainsListComponent metadata and nowhere else, the service becomes available only in the VillainsListComponent and its sub-component tree.

VillainService is a singleton with respect to VillainsListComponent because that is where it is declared. As long as VillainsListComponent does not get destroyed it will be the same instance of VillainService but if there are multilple instances of VillainsListComponent, then each instance of VillainsListComponent will have its own instance of VillainService.

Scenario: multiple edit sessions

Many applications allow users to work on several open tasks at the same time. For example, in a tax preparation application, the preparer could be working on several tax returns, switching from one to the other throughout the day.

This guide demonstrates that scenario with an example in the Tour of Heroes theme. Imagine an outer HeroListComponent that displays a list of super heroes.

To open a hero's tax return, the preparer clicks on a hero name, which opens a component for editing that return. Each selected hero tax return opens in its own component and multiple returns can be open at the same time.

Each tax return component has the following characteristics:

• Is its own tax return editing session.
• Can change a tax return without affecting a return in another component.
• Has the ability to save the changes to its tax return or cancel them.

Suppose that the HeroTaxReturnComponent had logic to manage and restore changes. That would be a pretty easy task for a simple hero tax return. In the real world, with a rich tax return data model, the change management would be tricky. You could delegate that management to a helper service, as this example does.

The HeroTaxReturnService caches a single HeroTaxReturn, tracks changes to that return, and can save or restore it. It also delegates to the application-wide singleton HeroService, which it gets by injection.

Here is the HeroTaxReturnComponent that makes use of HeroTaxReturnService.

The tax-return-to-edit arrives via the @Input() property, which is implemented with getters and setters. The setter initializes the component's own instance of the HeroTaxReturnService with the incoming return. The getter always returns what that service says is the current state of the hero. The component also asks the service to save and restore this tax return.

This won't work if the service is an application-wide singleton. Every component would share the same service instance, and each component would overwrite the tax return that belonged to another hero.

To prevent this, configure the component-level injector of HeroTaxReturnComponent to provide the service, using the providers property in the component metadata.

The HeroTaxReturnComponent has its own provider of the HeroTaxReturnService. Recall that every component instance has its own injector. Providing the service at the component level ensures that every instance of the component gets its own, private instance of the service, and no tax return gets overwritten.

The rest of the scenario code relies on other Angular features and techniques that you can learn about elsewhere in the documentation. You can review it and download it from the .

Scenario: specialized providers

Another reason to re-provide a service at another level is to substitute a more specialized implementation of that service, deeper in the component tree.

Consider a Car component that depends on several services. Suppose you configured the root injector (marked as A) with generic providers for CarService, EngineService and TiresService.

You create a car component (A) that displays a car constructed from these three generic services.

Then you create a child component (B) that defines its own, specialized providers for CarService and EngineService that have special capabilities suitable for whatever is going on in component (B).

Component (B) is the parent of another component (C) that defines its own, even more specialized provider for CarService.

Behind the scenes, each component sets up its own injector with zero, one, or more providers defined for that component itself.

When you resolve an instance of Car at the deepest component (C), its injector produces an instance of Car resolved by injector (C) with an Engine resolved by injector (B) and Tires resolved by the root injector (A).

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More on dependency injection

For more information on Angular dependency injection, see the DI Providers and DI in Action guides.