angular/aio/content/tutorial/first-app/first-app-lesson-09.md

First Angular app lesson 09 - Angular services

This tutorial lesson demonstrates how to create an Angular service and use dependency injection to include it in your app.

Time required: expect to spend about 15 minutes to complete this lesson.

Before you start

This lesson starts with the code from the previous lesson, so you can:

• Use the code that you created in Lesson 8 in your integrated development environment (IDE).
• Start with the code example from the previous lesson. Choose the from Lesson 8 where you can:
  • Use the live example in StackBlitz, where the StackBlitz interface is your IDE.
  • Use the download example and open it in your IDE.

If you haven't reviewed the introduction, visit the Introduction to Angular tutorial to make sure you have everything you need to complete this lesson.

If you have any trouble during this lesson, you can review the completed code for this lesson, in the for this lesson.

After you finish

• Your app has a service to serve the data to your app. At the end of this lesson, the service reads data from local, static data. In a later lesson, you update the service to get data from a web service.

Conceptual preview of interfaces

This tutorial introduces Angular services and dependency injection.

Angular services

Angular services provide a way for you to separate Angular app data and functions that can be used by multiple components in your app. To be used by multiple components, a service must be made injectable. Services that are injectable and used by a component become dependencies of that component. The component depends on those services and can't function without them.

Dependency injection

Dependency injection is the mechanism that manages the dependencies of an app's components and the services that other components can use.

Lesson steps

Perform these steps on the app code in your IDE.

Step 1 - Create a new service for your app

This step creates an injectable service for your app.

In the Terminal pane of your IDE:

1. In your project directory, navigate to the first-app directory.

2. In the first-app directory, run this command to create the new service.

   ng generate service housing --skip-tests

3. Run ng serve to build the app and serve it to http://localhost:4200.

4. Confirm that the app builds without error. Correct any errors before you continue to the next step.

Step 2 - Add static data to the new service

This step adds some sample data to your new service. In a later lesson, you replace the static data with a web interface to get data as you might in a real app. For now, your app's new service uses the data that has, so far, been created locally in HomeComponent.

In the Edit pane of your IDE:

1. In src/app/home/home.component.ts, from HomeComponent, copy the housingLocationList variable and its array value.

2. In src/app/housing.servivce.ts:

   1. Inside the HousingService class, paste the variable that you copied from HomeComponent in the previous step.

   2. Inside the HousingService class, paste these functions after the data you just copied. These functions allow dependencies to access the service's data.

      You will need these functions in a future lesson. For now, it is enough to understand that these functions return either a specific HousingLocation by id or the entire list.

   3. Add a file level import for the HousingLocation.

3. Confirm that the app builds without error. Correct any errors before you continue to the next step.

Step 3 - Inject the new service into HomeComponent

This step injects the new service into your app's HomeComponent so that it can read the app's data from a service. In a later lesson, you replace the static data with a live data source to get data as you might in a real app.

In the Edit pane of your IDE, in src/app/home/home.component.ts:

1. At the top of src/app/home/home.component.ts, add the inject to the items imported from @angular/common. This will import the inject function into the HomeComponent class.

2. Add a new file level import for the HousingService:

3. From HomeComponent, delete the housingLocationList delete the array entries and assign housingLocationList the value of empty array ([]). In a few steps you will update the code to pull the data from the HousingService.

4. In HomeComponent, add this code to inject the new service and initialize the data for the app. The constructor is the first function that runs when this component is created. The code in the constructor will assign the housingLocationList the value returned from the call to getAllHousingLocations.

5. Save the changes to src/app/home/home.component.ts and confirm your app builds without error. Correct any errors before you continue to the next step.

Lesson review

In this lesson, you added an Angular service to your app and injected it into the HomeComponent class. This compartmentalizes how your app gets its data. For now, the new service gets its data from a static array of data. In a later lesson, you refactor the service to get its data from a from an API endpoint.

If you are having any trouble with this lesson, you can review the completed code for it in the .

Next steps

• Lesson 10 - Add routes to the application

More information

For more information about the topics covered in this lesson, visit:

• Creating an injectable service
• Dependency injection in Angular
• ng generate service
• ng generate