Update Docs - services and dipendency ijnection

README.md

@@ -5,8 +5,7 @@
 - [The Basic](docs/thebasic.md)
 - [Components & Databinding](docs/component_databinding.md)
 - [Directives](docs/directives.md)
-- [Services](docs/services.md)
-- [Dependency Injection](docs/dependency_injection.md)
+- [Services and Dependency Injection](docs/services_dipendencyinjection.md)
 - [Routing](docs/routing.md)
 - [Observables](docs/observables.md)
 - [Forms](docs/forms.md)

docs/services_dipendencyinjection.md

@@ -0,0 +1,57 @@
+# Services and dependency injection
+
+A service prevents code duplication (LogService) or handlich with data (UserService - Data Sotrage).
+
+## Create a logging service from
+
+Naming: `logging.service.ts`
+
+> Instanciate a service manually with method `new` is the wrong way! Angular offers a much better solution.
+
+## Dependency injection
+
+Angular is responsable to instanciate the service which is passed as argument to the constructor. Which tels Angular thet the LoggingService is required.
+
+```ts
+import { LoggingService } from '../logging.service.ts';
+
+@Component({
+    ...
+    providers: [LoggingService]
+})
+
+export class NewAccountComponent {
+
+    constructor(private loggingService: LoggingService) {}
+
+    function(){
+        // Calling the loggingService
+        // logStatusChange is a function in the loggingService which accepts a string as argument.
+        this.loggingService.logStatusChange(accountStatus);
+    }
+}
+```
+
+`private loggingService` makes directly a property of type `LoggingService` and with the `provider` in the `@Component`-decorator, angular instanciate the logging service.
+
+## Hierarchical Injector
+
+By instanciate a service, Angular provides the same Instance to the component and all its childs.
+
+| Level               | Description                                                                                   |
+| ------------------- | --------------------------------------------------------------------------------------------- |
+| AppModule           | Service is available **Application-wide**                                                     |
+| AppComponent        | Same instance of service is available for **all Components** (but **not for other services**) |
+| Any other Component | Same instance of Service is available for **the Component and all its  chilf component**      |
+
+By declare the service in a lower component, will create a new instance and override the inherited from the parents one.
+
+It is necessary to declare it in the `impot` and in the `constructor`.
+
+`app.module.ts` is the highest level to declare a service. This opens the possibility to inject services into an other service.
+
+To the service who will be injected a service, is necessaty to provide metadata. Its keyword is `@Injectable()`
+
+Services can also be used to avoiding data/event/listener chaining.
+
+By Implementing the event in the service, and the listener in the component, can mitigate the chaining.