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In this video we will discuss
1. How to call ASP.NET Web API service using Angular 2 http service. Though this example demonstrates calling ASP.NET Web API service, we can use this same approach to call any web service built using any server side technology.
2. We will also briefly discuss Observable pattern

In our previous video we have created ASP.NET EmployeeWebAPIService. Here are the steps to call the Web API service using the Angular builtin http service.

Step 1 - Import the angular HTTP module : The first step is to import HttpModule which is present in a separate javascript file - @angular/http. After the HttpModule is imported, include it in the imports array of the NgModule() decorator of our root module "AppModule" which is in "app.module.ts" file. With this change we can now start using the angular built-in http service throught our application to access web services over HTTP.

import { HttpModule } from '@angular/http';

@NgModule({
imports: [
HttpModule],
declarations: [
],
bootstrap: [AppComponent]
})
export class AppModule { }

Step 2 - Modify angular EmployeeService to issue a GET request using the builtin http service :
1. The angular EmployeeService is in employee.service.ts file.

2. Use the EmployeeService class constructor to inject Angular Http service. The injected http service can then be used anywhere in this class to call a web service over http.

3. Since this Angular EmployeeService class has an injected dependency, @Injectable() decorator is required on this class. If there are no injectable dependencies then we may omit the @Injectable() decorator, but angular strongly recomends to use the @Injectable() decorator irrespective of there are injectible dependencies or not for consistency and future proof.

4. Notice in the getEmployees() method, we are using the get() method of the angular http service to issue a get request over http.

5. If you right click on get() method and go to it's definition you will notice that this method return Observable[Response].

6. Observable[Response] is not that useful to us, so we have set the return type of getEmployees() method to Observable[IEmployee[]]

7. To convert Observable[Response] to Observable[IEmployee[]] we are using the map operator provided by rxjs.

At the moment, we are not handling exceptions. We will discuss how to handle exceptions in our upcoming videos.

What is an Observable
1. Observable is an asynchronous pattern. In the Observable pattern we have an Observable and an Observer. Observer observes the Observable. In many implementations an Observer is also called as a Subscriber.

2. An Observable can have many Observers (also called Subscribers).

3. Observable emits items or notifications over time to which an Observer (also called Subscriber) can subscribe.

4. When a subscriber subscribes to an Observable, the subscriber also specifies a callback function.

5. This subscriber callback function is notified as and when the Observable emits items or notifications.

6. Within this callback function we write code to handle data itmes or notifications received from the Observable.

Step 3 - Subscribe to the Observable returned by angular EmployeeService : EmployeeListComponent needs the employees data returned by the service. So in the ngOnInit() method of "employeeList.component.ts" use the subscribe method as shown below.

ngOnInit() {
this._employeeService.getEmployees()
.subscribe(employeesData =] this.employees = employeesData);
}

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