Module 13 - Advanced topics in Angulars Dependency Injection


fun-with-more-di	Advanced Topics in Angulars Dependency Injection

Injection Token

We understood that the type of the requested injection does not always accurately describe the intent of the required object. Sometime the type can be something like "string" or "number" and you to further describe the requested item
Injection Tokens Allow us to define something that needs to be injected, regardless of its type
- We create an injection token by exporting a constant of type InjectionToken<T> where T is the data type.
- We provide it by using the injection token in the provide property of the provider object.
- We consume it using the @Inject(token) decorator
- We can also mark it as optional using the @Optional() decorator

We have already seen the useClass and useExisting providers in Module 05
We saw how to privde an exact value using the useValue provider.
We saw how to use the useValue provider to provide a token of type function, so that the function can later be called lazily.
We saw how to use the useFactory provider, to calculate the value using a method that is called when the value is first needed to be provided.
We saw how to use the useFactory provider, to provide a value of type function (a function that returns a function).
We saw that the useFactory provider may also receive injectable prameters
- These are set using the deps property of the provider.
- In the deps property, we list the tokens that can be used to populate each parameter
- We saw that we can use closure to create a function value, that uses on of these parameters.

We saw that we can define a token of type array, and then provide each item seperately.
We saw that when several modules provide the same token, only one provider takes effect.
But, when we use the multi:true settings, the various providers are accumulated, and all the values are injected.

We saw the APP_INITIALIZER token, which is of type function, that either returns void, Promise<void or Observable<void>.
We saw that the actualy type that angular expects is an array of initialization methods.
We saw that each module may supply several initializers using the multi settings.
We saw that in order to initalize a service, we need to:
- use the useFactory provider
- supply the service as parameter using the deps property
- return a function object that calls the init method
- notice that we take advantage of the closure feature.
We saw that angular will then await the returned promises and will start rendering the application only once the initializers are all complete.