Angular's DI

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Angular's DI
The last example introduced a hypothetical Injector object. Angular simplifies DI even further. With Angular, programmers almost never have to get bogged down with injection details.
Angular's DI system is (mostly) controlled through @NgModule. Specifically the providers and declarations array. (declarations is where we put components, pipes and directives; providers is where we put services)
For example:
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import { Injectable, NgModule } from '@angular/core';
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​
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@Component({
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  // ...
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})
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class ChatWidget {
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  constructor(private authService: AuthService, private authWidget: AuthWidget,
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    private chatSocket: ChatSocket) {}
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}
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​
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@NgModule({
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  declarations: [ ChatWidget ]
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})
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export class AppModule {
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};
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In the above example the AppModule is told about the ChatWidget class. Another way of saying this is that Angular has been provided a ChatWidget.
That seems pretty straightforward, but astute readers will be wondering how Angular knows how to build ChatWidget. What if ChatWidget was a string, or a plain function?
Angular assumes that it's being given a class.
What about AuthService, AuthWidget and ChatSocket? How is ChatWidget getting those?
It's not, at least not yet. Angular does not know about them yet. That can be changed easily enough:
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import { Injectable, NgModule } from '@angular/core';
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​
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@Component({
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  // ...
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})
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class ChatWidget {
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  constructor(private authService: AuthService, private authWidget: AuthWidget,
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    private chatSocket: ChatSocket) {}
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}
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​
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@Component({
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  // ...
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})
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class AuthWidget {}
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​
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@Injectable()
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class AuthService {}
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​
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@Injectable()
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class ChatSocket {}
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​
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@NgModule({
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  declarations[ ChatWidget, AuthWidget ]
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  providers: [ AuthService, ChatSocket ],
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})
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Okay, this is starting to look a little bit more complete. Although it's still unclear how ChatWidget is being told about its dependencies. Perhaps that is related to those odd @Injectable statements.
DI Framework
@Inject() and @Injectable
Last modified 3yr ago
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