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include ../_util-fns
:marked
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.
依赖注入是一个强大的管理代码依赖链的架构模式。在这个“食谱”中，我们会讨论Angular依赖注入的许多特性。
<a id="toc"></a>
:marked
## Table of contents
## 目录
[Application-wide dependencies](#app-wide-dependencies)
[应用程序全局依赖](#app-wide-dependencies)
[External module configuration](#external-module-configuration)
[外部模块设置](#external-module-configuration)
[*@Injectable* and nested service dependencies](#nested-dependencies)
[*@Injectable*和嵌套服务的依赖](#nested-dependencies)
[Limit service scope to a component subtree](#service-scope)
[限制服务作用范围到一个组件支树](#service-scope)
[Multiple service instances (sandboxing)](#multiple-service-instances)
[多个服务实例(sandboxing)](#multiple-service-instances)
[Qualify dependency lookup with *@Optional* and *@Host*](#qualify-dependency-lookup)
[使用*@Optional*和*@Host*装饰来认证依赖调用过程](#qualify-dependency-lookup)
[Inject the component's DOM element](#component-element)
[注入组件的DOM元素](#component-element)
[Define dependencies with providers](#providers)
[使用Provider定义依赖](#providers)
* [The *provide* function](#provide)
* [*provide*功能](#provide)
* [useValue - the *value provider*](#usevalue)
* [useClass - the *class provider*](#useclass)
* [useExisting - the *alias provider*](#useexisting)
* [useFactory - the *factory provider*](#usefactory)
[Provider token alternatives](#tokens)
* [class-interface](#class-interface)
* [OpaqueToken](#opaque-token)
[Inject into a derived class](#di-inheritance)
[Find a parent component by injection](#find-parent)
* [Find parent with a known component type](#known-parent)
* [Cannot find a parent by its base class](#base-parent)
* [Find a parent by its class-interface](#class-interface-parent)
* [Find a parent in a tree of parents (*@SkipSelf*)](#parent-tree)
* [A *provideParent* helper function](#provideparent)
[Break circularities with a forward class reference (*forwardRef*)](#forwardref)
[使用一个forward类引用(*forwardRef*)打破环状依赖](#forwardref)
:marked
**See the [live example](/resources/live-examples/cb-dependency-injection/ts/plnkr.html)**
of the code supporting this cookbook.
获取本“食谱”的代码支持，**请看[在线例子](/resources/live-examples/cb-dependency-injection/ts/plnkr.html)**。
.l-main-section
<a id="app-wide-dependencies"></a>
:marked
## Application-wide dependencies
## 应用程序全局依赖
Register providers for dependencies used throughout the application in the root application component, `AppComponent`.
在应用程序根组件`AppComponent`注册那些被应用程序全局使用的依赖providers。
In the following example, we import and register several services
(the `LoggerService`, `UserContext`, and the `UserService`)
in the `@Component` metadata `providers` array.
在下面的例子中，我们在`@Component`元数据的`providers`数组中，导入和注册了几个服务（`LoggerService`, `UserContext`和`UserService`)。
+makeExample('cb-dependency-injection/ts/app/app.component.ts','import-services','app/app.component.ts (excerpt)')(format='.')
:marked
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.
所有上面这些服务都是用类来实现的。服务类能充当它们自己的Providers，这就是为什么把它们列到一个`providers`数组里是唯一的注册要求。
.l-sub-section
:marked
A *provider* is something that can create or deliver a service.
Angular creates a service instance from a class provider by "new-ing" it.
Learn more about providers [below](#providers).
一个*provider*是用来新建或者送交服务的。Angular从一个类provider里面，通过“new-ing”来新建服务实例的。从[下面](#providers)学习更多关于provders的知识。
:marked
Now that we've registered these services,
Angular can inject them into the constructor of *any* component or service, *anywhere* in the application.
现在我们已经注册了这些服务，Angular能在应用程序的*任何地方*，将它们注入到*任何*组件和服务的构造函数里面。
+makeExample('cb-dependency-injection/ts/app/hero-bios.component.ts','ctor','app/hero-bios.component.ts (component constructor injection)')(format='.')
+makeExample('cb-dependency-injection/ts/app/user-context.service.ts','ctor','app/user-context.service.ts (service constructor injection)')(format='.')
<a id="external-module-configuration"></a>
.l-main-section
:marked
## External module configuration
## 外部模块设置
We can register _certain_ module providers when bootstrapping rather than in the root application component.
与其在应用程序根组件里面，我们可以在引导过程中注册_某些_模块providers。
We'd do this when we expect to select or configure external modules that support our application
but (a) aren't conceptually part of the application and (b) that we could change later without
altering the essential logic of the application.
当使用支持我们的应用程序，并满足下面两个条件外来组件时，我们应该这样做（在引导过程中注册）：
a）在概念上不是我们程序的一部分
b）在未来，我们可能需要在不变化主要应用程序逻辑的情况下，更改或更换它。
For example, we might configure the Component Router with different
[location strategies](../guide/router.html#location-strategy) based on environmental factors.
The choice of location strategy doesn't matter to the application itself.
比如，我们可能用不同的[location strategies](../guide/router.html#location-strategy)，根据不同的环境因数设置不同的组件路由。这个location strategy不直接影响应用程序本身。
We could sneak in a fake HTTP backend with sample data during development rather than
allow http calls to a remote server (that might not yet exist).
We'll switch to the real backend in production.
The application shouldn't know or care one way or the other.
在开发过程中，我们可以偷偷把一个假的带有例子数据的HTTP后端嵌入进来，来取代对一个远程服务器（可能还不存在）进行http查询。我们在产品发布时再切换到真正的后端。应用程序不需要知道，也不在乎哪个后端。
See both examples in the following `main.ts`
where we list their service providers in an array in the second parameter of the `bootstrap` method.
在下面`main.ts`的两个例子中，我们在`bootstrap`类方法的第二个参数的数组中，我们列出了它们的service providers（服务提供者）。
+makeExample('cb-dependency-injection/ts/app/main.ts','bootstrap','app/main.ts')(format='.')
a(id="injectable")
a(id="nested-dependencies")
.l-main-section
:marked
## *@Injectable* and nested service dependencies
## *@Injectable*和嵌套服务依赖
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.
被注入服务的使用者不应该知道怎么创建这个服务。它不应该在在乎。新建和缓存这个服务是依赖注入的工作。
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.
有时候一个服务依赖其他服务...（其他服务）可能依赖另外的服务。按正确的顺序来解析这些嵌套依赖也是框架工具（Angualar 2依赖注入）的工作。
在每一步，依赖的使用者只是在它的构造函数里简单地声明它需要什么，框架工具会做剩余的事情。
For example, we inject both the `LoggerService` and the `UserContext` in the `AppComponent`.
比如，我们在`AppComponent`里注入`LoggerService`和`UserContext`。
+makeExample('cb-dependency-injection/ts/app/app.component.ts','ctor','app/app.component.ts')(format='.')
:marked
The `UserContext` in turn has dependencies on both the `LoggerService` (again) and
a `UserService` that gathers information about a particular user.
`UserContext`有两个依赖`LoggerService`（再次）和负责获取特定用户信息的`UserService`。
+makeExample('cb-dependency-injection/ts/app/user-context.service.ts','injectables','user-context.service.ts (injection)')(format='.')
:marked
When Angular creates an`AppComponent`, 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.
当Angular新建一个`AppComponent`，依赖注入框架工具创建一个`LoggerService`的实例，和开始创建`UserContextService`。`UserContextService`需要框架工具已经有了的`LoggerService`和还没创建的`UserService`。
`UserService`没有其他依赖，所以依赖注入框架工具可以直接`new`一个实例。
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.
依赖注入美丽的地方在于，`AppComponent`的作者不需要在乎这一切。作者只是在构造函数(`LoggerService`和`UserContextService`)里面简单地声明一下，框架工具来做剩下的工作。
Once all the dependencies are in place, the `AppComponent` displays the user information:
一旦所有依赖都准备好了，`AppComponent`显示用户信息：
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/logged-in-user.png" alt="Logged In User")
:marked
### *@Injectable()*
### *@Injectable()*
Notice the `@Injectable()`decorator on the `UserContextService` class.
请注意在`UserContextService`类里面的`@Injectable()`装饰器。
+makeExample('cb-dependency-injection/ts/app/user-context.service.ts','injectable','user-context.service.ts (@Injectable)')(format='.')
:marked
That decorator makes it possible for Angular to identify the types of its two dependencies, `LoggerService` and `UserService`.
该装饰器让Angular有能力辨认它的两个依赖 `LoggerService` 和 `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.
技术上讲，这个`@Injectable()`装饰器只在一个服务类有_自己的依赖_的时候，才是_不可缺少_的。`LoggerService`不依赖任何东西。该日志在没有`@Injectable()`的时候应该也工作，生成的代码也小一些。
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.
但是该服务在我们添加依赖给它的那一刻就会停止工作，要修复它，我们就必须要添加`@Injectable()`。为了保持一致性和防止将来的麻烦，我们从一开始就添加`@Injectable()`。
.alert.is-helpful
:marked
Although we recommend applying `@Injectable` to all service classes, do not feel bound by it.
Some developers prefer to add it only where needed and that's a reasonable policy too.
虽然我们推荐在所有服务中使用`@Injectable`，你不需要一定要这么做。一些开发者宁可在需要添加的地方才添加，这也是一个合理的策略。
.l-sub-section
:marked
The `AppComponent` class had two dependencies as well but no `@Injectable()`.
It didn't need `@Injectable()` because that component class has the `@Component` decorator.
In Angular with TypeScript, a *single* decorator &mdash; *any* decorator &mdash; is sufficient to identify dependency types.
`AppComponent`类有两个依赖，但是没有`@Injectable()`。它不需要`@Injectable()`是因为组件类有`@Component`装饰器。
在用TypeScript的Angular（应用程序）里，一个 *单独的* 装饰器 &mdash; *任何* 装饰器 &mdash; 来辨识依赖类别就足够了。
<a id="service-scope"></a>
.l-main-section
:marked
## Limit service scope to a component subtree
## 限制服务作用范围到一个组件支树
All injected service dependencies are singletons meaning that,
for a given dependency injector ("injector"), there is only one instance of service.
所有注入的服务依赖都是单例（singletons)，意思是，在任意一个依赖注入器("injector")中，每个服务只有一个唯一的实例。
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.
但是一个Angular应用程序有多个依赖注入器，组织在一个与组件树平行的树形结构中。所以，在任何组件级别，一个特定的服务能被*提供*（和被建立）。如果在多个组件中注入，可以被新建或提供多次。
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.
默认情况下，在一个组件中注入的服务依赖，在所有该组件的子组件中都可见，而且Angular会注入同样的服务实例，到所有要求该服务的子组件中。
Accordingly, dependencies provided in the root `AppComponent` can be injected into *any* component *anywhere* in the application.
同样，在根部`AppComponent`提供的依赖能被注入到*任何*组件，到应用程序的*任何地方*。
That isn't always desireable.
Sometimes we want to restrict service availability to a particular region of the application.
这不一定总是想要的。有时候我们想要把服务的有效性限制到应用程序的一个特定区域。
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:
通过*在该分支的子级根部组件*中提供该服务， 我们能把一个注入服务的作用范围局限到一个应用程序结构的该*分支*中。
这里我们通过列入`providers`数列，在`HeroesBaseComponent`中提供了`HeroService`：
+makeExample('cb-dependency-injection/ts/app/sorted-heroes.component.ts','injection','app/sorted-heroes.component.ts (HeroesBaseComponent excerpt)')
:marked
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).
当Angular新建`HeroBaseComponent`的时候，它会同时建立一个`HeroService`实例，该实例只在该组件和其子组件（如果有）中可见。
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.
我们也可以在应用程序的另外的地方的*不同的*组件里提供`HeroService`。这样的结果是一个*不同的*该服务的实例，在一个*不同的*注入器内存在。
.l-sub-section
:marked
We examples of such scoped `HeroService` singletons appear throughout the accompanying sample code,
including the `HeroBiosComponent`, `HeroOfTheMonthComponent`, and `HeroesBaseComponent`.
Each of these components has its own `HeroService` instance managing its own independent collection of heroes.
我们这个局限范围的`HeroService`单例的例子，贯穿伴随例子代码，包括`HeroBiosComponent`, `HeroOfTheMonthComponent`和`HeroBaseComponent`。
这些组件中每个都有自己的`HeroService`实例，管理自己独立的英雄库。
.l-main-section
.alert.is-helpful
:marked
### Take a break!
### 休息一下！
This much Dependency Injection knowledge may be all that many Angular developers
ever need to build their applications. It doesn't always have to be more complicated.
对一些Angular开发者来说，这么多依赖注入知识可能是所有需要的全部知识。不一定都要更加复杂。
<a id="multiple-service-instances"></a>
.l-main-section
:marked
## Multiple service instances (sandboxing)
## 多个服务实例（sandboxing)
Sometimes we want multiple instances of a service at *the same level of the component hierarchy*.
在*同一个级别的组件树*里，我们有时需要一个服务的多个实例。
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.
一个管理自己伴随组件实例状态的服务是个好例子。
对每个组件，我们都需要该服务的单独实例。每个服务有自己的工作-状态，与其他组件的服务-和-状态隔离。我们叫这个*sandboxing*，因为每个服务和组件实例都在自己的沙盒里运行。
<a id="hero-bios-component"></a>
Imagine a `HeroBiosComponent` that presents three instances of the `HeroBioComponent`.
想象一下，一个`HeroBioComponent`，显示三个`HeroBioComponent`的实例。
+makeExample('cb-dependency-injection/ts/app/hero-bios.component.ts','simple','ap/hero-bios.component.ts')
:marked
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.
每个`HeroBioComponent`都能编辑一个英雄的生平。一个`HeroBioComponent`依赖一个`HeroCacheService`来对该英雄进行读取、缓存和执行其他持久性质的操作。
+makeExample('cb-dependency-injection/ts/app/hero-cache.service.ts','service','app/hero-cache.service.ts')
:marked
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.
很明显，这三个`HeroBioComponent`实例不能共享一样的`HeroCacheService`。要不然它们会相互竞争冲突，取决于哪个英雄在缓存里面。
Each `HeroBioComponent` gets its *own* `HeroCacheService` instance
by listing the `HeroCacheService` in its metadata `providers` array.
通过在自己的元数据（metadata)提供者(providers)数组里面列出`HeroCacheService`, 每个`HeroBioComponent`有自己*拥有*的`HeroCacheService`实例。
+makeExample('cb-dependency-injection/ts/app/hero-bio.component.ts','component','app/hero-bio.component.ts')
:marked
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.
此父级`HeroBioComponent`绑定一个变量到`HeroId`。`ngOnInit`传递该`id`到服务，服务再获取和缓存英雄。`hero`属性的getter从服务里面获取缓冲的英雄，并在模板里面显示该数据绑定的属性。
Find this example in [live code](/resources/live-examples/cb-dependency-injection/ts/plnkr.html)
and confirm that the three `HeroBioComponent` instances have their own cached hero data.
在[在线代码](/resources/live-examples/cb-dependency-injection/ts/plnkr.html)找到这个例子，确认三个`HeroBioComponent`实例有自己拥有的缓冲的英雄数据。
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/hero-bios.png" alt="Bios")
a(id="optional")
a(id="qualify-dependency-lookup")
.l-main-section
:marked
## Qualify dependency lookup with *@Optional* and *@Host*
## 使用*@Optional*和*@Host*装饰来认证依赖调用过程
We learned that dependencies can be registered at any level in the component hierarchy.
我们学习了依赖可以在任何组件级别注册依赖。
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.
当一个组件请求一个依赖，Angular以该组件注入器开始，然后往上面以及的注入器树走，知道它找到第一个合适的provider。如果Angular不能再这个过程中找不到合适的依赖，它就抛出一个错误。
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.
在大部分时候，我们*想要*这个行为。
但是有时候，我们需要限制这个（依赖）搜索，并且/或者提供一个缺失的依赖。
单独使用或者一起使用`@Host`和`@Optional`认证装饰器，我们能修改Angular的搜索行为。
The `@Optional` decorator tells Angular to continue when it can't find the dependency.
Angular sets the injection parameter to `null` instead.
当Angular找不到依赖时，`@Optional`装饰器告诉Angular继续执行。Angular设置该注入参数为`null`（取代抛出错误得行为）。
The `@Host` decorator stops the upward search at the *host component*.
`@Host`装饰器将往上搜索的行为停止到*主持组件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.
该主持组件一般为请求依赖的组件。但是当这个组件被投放到一个*父级*组件后，该父级组件变成了主持。我们考虑一会，更多有趣的案例还在后面的例子里。
### Demonstration
### 示范
The `HeroBiosAndContactsComponent` is a revision of the `HeroBiosComponent` that we looked at [above](#hero-bios-component).
该`HeroBiosAndContactsComponent`是[上面](#hero-bios-component)我们看过的`HeroBiosComponent`的修改版。
+makeExample('cb-dependency-injection/ts/app/hero-bios.component.ts','hero-bios-and-contacts','app/hero-bios.component.ts (HeroBiosAndContactsComponent)')
:marked
Focus on the template:
注意它的模板：
+makeExample('cb-dependency-injection/ts/app/hero-bios.component.ts','template')(format='.')
:marked
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:
我们在`<hero-bio>`标签里插入了`<hero-contact>`元素。Angular*投放*（*transcludes*)对应的`HeroContactComponent`到`HeroBioComponent`视图里，将它放到`HeroBioComponent`模板的`<ng-content>`槽里面。
+makeExample('cb-dependency-injection/ts/app/hero-bio.component.ts','template','app/hero-bio.component.ts (template)')(format='.')
:marked
It looks like this, with the heroe's telephone number from `HeroContactComponent` projected above the hero description:
从`HeroContactComponent`来的英雄的电话号码被投放到上面的英雄说明里面，看起来像这样：
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/hero-bio-and-content.png" alt="bio and contact")
:marked
Here's the `HeroContactComponent` which demonstrates the qualifying decorators that we're talking about in this section:
下面是`HeroContactComponent`，示范了我们在本节一直在讨论的认证装饰器(Qualifying decorators)：
+makeExample('cb-dependency-injection/ts/app/hero-contact.component.ts','component','app/hero-contact.component.ts')
:marked
Focus on the constructor parameters
请注意看构造函数的参数。
+makeExample('cb-dependency-injection/ts/app/hero-contact.component.ts','ctor-params','app/hero-contact.component.ts')(format='.')
:marked
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.
`@Host()`函数装饰`_heroCache`属性，确保我们从其父级`HeroBioComponent`得到一个缓冲服务的引用。如果该父级不存在这个服务，Angular则抛错，就算组件树里再上级有一个组件拥有这个服务（Angular也抛错）。
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.
另外一个`@Host()`函数装饰`_loggerService`属性，我们知道在应用程序中，只有一个`LoggerService`实例，在`AppComponent`级别提供的。该主持`HeroBioComponent`没有自己的`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.
如果我们没有同时使用`@Optional()`来装饰该属性的话，Angular会抛错。感谢`@Optional()`，Angular将`loggerService`设置为null，并继续改组件的执行。
.l-sub-section
:marked
We'll come back to the `elementRef` property shortly.
我们将很快回到`elementRef`属性。
:marked
Here's the `HeroBiosAndContactsComponent` in action.
下面是`HeroBiosAndContactsComponent`的执行结果：
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/hero-bios-and-contacts.png" alt="Bios with contact into")
:marked
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 gratuituous "!!!", indicating that the logger was found.
如果我们
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/hero-bio-contact-no-host.png" alt="Without @Host")
:marked
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.
<br>
`EXCEPTION: No provider for LoggerService! (HeroContactComponent -> LoggerService)`
<a id="component-element"></a>
:marked
## 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](../guide/attribute-directives.html) chapter.
+makeExample('cb-dependency-injection/ts/app/highlight.directive.ts','','app/highlight.directive.ts')
:marked
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.
+makeExample('cb-dependency-injection/ts/app/app.component.html','highlight','app/app.component.html (highlight)')(format='.')
:marked
The following image shows the effect of mousing over the `<hero-bios-and-contacts>` tag.
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/highlight.png" alt="Highlighted bios")
:marked
<a id="providers"></a>
.l-main-section
:marked
## 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:
+makeExample('cb-dependency-injection/ts/app/hero-bios.component.ts','ctor','app/hero-bios.component.ts (component constructor injection)')(format='.')
:marked
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.
It 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*.
.l-sub-section
:marked
If the injector doesn't have a provider for the requested *token*, it delegates the request
to its parent injector, where the process repeats until there are no more injectors.
If the search is futile, the injector throws an error ... unless the request was [optional](#optional).
Let's return our attention to providers themselves.
:marked
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:
+makeExample('cb-dependency-injection/ts/app/app.component.ts','providers','app/app.component.ts (providers)')
:marked
### Defining providers
The simple class provider is the most typical by far.
We mention the class in the `providers` array and we're done.
+makeExample('cb-dependency-injection/ts/app/hero-bios.component.ts','class-provider','app/hero-bios.component.ts (class provider)')(format='.')
:marked
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.
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/hero-of-month.png" alt="Hero of the month" width="300px")
:marked
It's visually simple: a few properties and the output of a logger. The code behind it gives us plenty to talk about.
+makeExample('cb-dependency-injection/ts/app/hero-of-the-month.component.ts','hero-of-the-month','hero-of-the-month.component.ts')
.l-main-section
a(id='provide')
:marked
#### The *provide* function
The imported Angular `provide` function creates an instance of
the Angular [Provider](../api/core/Provider-class.html) class.
The `provide` function takes a *token* and a *definition object*.
The *token* is usually a class but [it doesn't have to be](#tokens).
The *definition* object has one main property, (e.g. `useValue`) that indicates how the provider
should create or return the provided value.
.l-main-section
a(id='usevalue')
:marked
#### 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:
+makeExample('cb-dependency-injection/ts/app/hero-of-the-month.component.ts','use-value')(format='.')
:marked
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](#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:
+makeExample('cb-dependency-injection/ts/app/hero-of-the-month.component.ts','some-hero')
:marked
The other providers create their values *lazily* when they're needed for injection.
.l-main-section
a(id='useclass')
:marked
#### 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`:
+makeExample('cb-dependency-injection/ts/app/hero-of-the-month.component.ts','use-class')(format='.')
:marked
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.
.l-sub-section
:marked
This component and its tree of child components receive the `DateLoggerService` instance.
Components outside the tree continue to receive the original `LoggerService` instance.
:marked
The `DateLoggerService` inherits from `LoggerService`; it appends the current date/time to each message:
+makeExample('cb-dependency-injection/ts/app/date-logger.service.ts','date-logger-service','app/date-logger.service.ts')(format='.')
.l-main-section
a(id='useexisting')
:marked
#### 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***.
+makeExample('cb-dependency-injection/ts/app/hero-of-the-month.component.ts','use-existing')
:marked
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-interface):
+makeExample('cb-dependency-injection/ts/app/date-logger.service.ts','minimal-logger','app/date-logger.service.ts (MinimalLogger)')(format='.')
:marked
The constructor's `logger` parameter is typed as `MinimalLogger` so only its two members are visible in TypeScript:
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/minimal-logger-intellisense.png" alt="MinimalLogger restricted API")
:marked
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:
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/date-logger-entry.png" alt="DateLoggerService entry" width="300px")
.l-main-section
a(id='usefactory')
:marked
#### useFactory - the *factory provider*
The `useFactory` provider creates a dependency object by calling a factory function
as seen in this example.
+makeExample('cb-dependency-injection/ts/app/hero-of-the-month.component.ts','use-factory')
:marked
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.
+makeExample('cb-dependency-injection/ts/app/runners-up.ts','factory-synopsis','runners-up.ts (excerpt)')(format='.')
:marked
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`.
.l-sub-section
:marked
The function retrieves candidate heroes from the `HeroService`,
takes `2` of them to be the runners-up, and returns their concatenated names.
Look at the [live example](/resources/live-examples/cb-dependency-injection/ts/plnkr.html)
for the full source code.
a(id="tokens")
.l-main-section
:marked
## 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.
<a id="class-interface"></a>
### class-interface
In the previous *Hero of the Month* example, we used the `MinimalLogger` class
as the token for a provider of a `LoggerService`.
+makeExample('cb-dependency-injection/ts/app/hero-of-the-month.component.ts','use-existing')
:marked
The `MinimalLogger` is an abstract class.
+makeExample('cb-dependency-injection/ts/app/date-logger.service.ts','minimal-logger')(format='.')
:marked
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`
+makeExample('cb-dependency-injection/ts/app/date-logger.service.ts','date-logger-service-signature')(format='.')
:marked
`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.
.l-sub-section
:marked
#### Why *MinimalLogger* is a class and not an interface
We can't use an interface as a provider token because
interfaces are not JavaScript objects.
They exist only in the TypeScript design space.
They disappear after the code is transpiled to JavaScript.
A provider token must be a real JavaScript object of some kind:
a function, an object, a string ... a class.
Using a class as an interface gives us the characteristics of an interface in a JavaScript object.
The minimize memory cost, the class should have *no implementation*.
The `MinimalLogger` transpiles to this unoptimized, pre-minified JavaScript:
+makeExample('cb-dependency-injection/ts/app/date-logger.service.ts','minimal-logger-transpiled')(format='.')
:marked
It never grows larger no matter how many members we add *as long as they are typed but not implemented*.
a(id='opaque-token')
:marked
### 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.
+makeExample('cb-dependency-injection/ts/app/hero-of-the-month.component.ts','provide-opaque-token')(format='.')
:marked
We created the `TITLE` token like this:
+makeExample('cb-dependency-injection/ts/app/hero-of-the-month.component.ts','opaque-token')(format='.')
a(id="di-inheritance")
.l-main-section
:marked
## 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.
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/sorted-heroes.png" alt="Sorted Heroes")
:marked
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.
+makeExample('cb-dependency-injection/ts/app/sorted-heroes.component.ts','heroes-base','app/sorted-heroes.component.ts (HeroesBaseComponent)')
.l-sub-section
:marked
We strongly prefer simple constructors. They should do little more than initialize variables.
This rule makes the component safe to construct under test without fear that it will do something dramatic like talk to the server.
That's why we call the `HeroService` from within the `ngOnInit` rather than the constructor.
We explain the mysterious `_afterGetHeroes` below.
:marked
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.
+makeExample('cb-dependency-injection/ts/app/sorted-heroes.component.ts','sorted-heroes','app/sorted-heroes.component.ts (SortedHeroesComponent)')
:marked
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*.
a(id="find-parent")
.l-main-section
:marked
## 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.
<a id="known-parent"></a>
### 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`:
a(id='alex')
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','alex-1','parent-finder.component.ts (AlexComponent v.1)')(format='.')
:marked
*Cathy* reports whether or not she has access to *Alex*
after injecting an `AlexComponent` into her constructor:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','cathy','parent-finder.component.ts (CathyComponent)')(format='.')
:marked
We added the [@Optional](#optional) qualifier for safety but
the [live example](/resources/live-examples/cb-dependency-injection/ts/plnkr.html)
confirms that the `alex` parameter is set.
<a id="base-parent"></a>
### 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.
.l-sub-section
:marked
Looking for components that implement an interface would be better.
That's not possible because TypeScript interfaces disappear from the transpiled JavaScript
which doesn't support interfaces. There's no artifact we could look for.
:marked
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](#alex)
we see that the `Alex` component *extends* (*inherits*) from a class named `Base`.
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','alex-class-signature','parent-finder.component.ts (Alex class signature)')(format='.')
:marked
The `CraigComponent` tries to inject `Base` into its `alex` constructor parameter and reports if it succeeded.
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','craig','parent-finder.component.ts (CraigComponent)')(format='.')
:marked
Unfortunately, this does not work.
The [live example](/resources/live-examples/cb-dependency-injection/ts/plnkr.html)
confirms that the `alex` parameter is null.
*We cannot inject a parent by its base class.*
<a id="class-interface-parent"></a>
### Find a parent by its class-interface
We can find a parent component with a [class-interface](#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](#known-parent).
We write an [*alias provider*](#useexisting) &mdash; a `provide` function with a `useExisting` definition &mdash;
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`:
a(id="alex-providers")
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','alex-providers','parent-finder.component.ts (AlexComponent providers)')(format='.')
:marked
[Parent](#parent-token) is the provider's *class-interface* token.
The [*forwardRef*](#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:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','carol-class','parent-finder.component.ts (CarolComponent class)')(format='.')
:marked
Here's *Alex* and family in action:
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/alex.png" alt="Alex in action")
a(id="parent-tree")
:marked
### 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*:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','barry','parent-finder.component.ts (BarryComponent)')(format='.')
:marked
*Barry*'s `providers` array looks just like [*Alex*'s](#alex-providers).
If we're going to keep writing [*alias providers*](#useexisting) like this we should create a [helper function](#provideparent).
For now, focus on *Barry*'s constructor:
+makeTabs(
'cb-dependency-injection/ts/app/parent-finder.component.ts, cb-dependency-injection/ts/app/parent-finder.component.ts',
'barry-ctor, carol-ctor',
'Barry\'s constructor, Carol\'s constructor')(format='.')
:marked
:marked
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:
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/alice.png" alt="Alice in action")
a(id="parent-token")
:marked
### The *Parent* class-interface
We [learned earlier](#class-interface) 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* .
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','parent','parent-finder.component.ts (Parent class-interface)')(format='.')
:marked
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:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','alice-class-signature','parent-finder.component.ts (AliceComponent class signature)')(format='.')
:marked
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`:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','alex-class-signature','parent-finder.component.ts (AlexComponent class signature)')(format='.')
.l-sub-section
:marked
The `AlexComponent` *should* implement `Parent` as a matter of proper style.
It doesn't in this example *only* to demonstrate that the code will compile and run without the interface
a(id="provideparent")
:marked
### A *provideParent* helper function
Writing variations of the same parent *alias provider* gets old quickly,
especially this awful mouthful with a [*forwardRef*](#forwardref):
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','alex-providers')(format='.')
:marked
We can extract that logic into a helper function like this:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','provide-the-parent')(format='.')
:marked
Now we can add a simpler, more meaningful parent provider to our components:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','alice-providers')(format='.')
:marked
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*.
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','provide-parent')(format='.')
:marked
And here's how we could use it with a different parent type:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','beth-providers')(format='.')
:marked
a(id="forwardref")
.l-main-section
:marked
## 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.
In the [*Alex/Cathy* example](#known-parent) above:
* the `AlexComponent` lists the `CathyComponent` in its component metadata `directives` array
so it can display *Cathy* in its template.
* the `CathyComponent` constructor injects the parent `AlexComponent` which means that the `alex` parameter
of its constructor has the `AlexComponent` type.
*Alex* refers to *Cathy* and *Cathy* refers to *Alex*. We're stuck. We must define one of them first.
We defined *Alex* first and built its `C_DIRECTIVES` array with a forward reference to *Cathy*:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','C_DIRECTIVES','parent-finder.component.ts (C_DIRECTIVES)')(format='.')
:marked
.l-sub-section
:marked
Defining *Alex* and *Cathy* in separate files won't help.
*Alex* would have to import *Cathy* and *Cathy* would have to import *Alex*.
We *had* to define *Alex* first because,
while we can add `forwardRef(CathyComponent)` to *Alex*'s `directives` array,
we can't write `public alex: forwardRef(AlexComponent))` in *Cathy*'s constructor.
:marked
We face a similar 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.
Again we break the circularity with `forwardRef`:
+makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','alex-providers','parent-finder.component.ts (AlexComponent providers)')(format='.')
:marked
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