` 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 `
` tag.
figure.image-display
img(src="/resources/images/cookbooks/dependency-injection/highlight.png" alt="Highlighted bios")
:marked
.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.
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*.
.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.
### 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.
### 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.
### 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.*
### 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) — a `provide` function 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`:
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