Any code that touches outside of the boundaries of a component should exist in a service; this includes inter-component communication, unless there's a parent-child relationship, and API calls of any kind and any code that cache or retrieve data from a cookie or the browser's localStorage. This is a critical architectural pattern that keeps your application maintainable in the long term. I expand upon this idea in my DevPro MVVM article at http://bit.ly/MVVMvsMVC.

To create an Angular service, do this:

1. In the terminal, execute npx ng g s weather --flat false
2. Observe the new weather folder created:

src/app
...
└── weather
   ├── weather.service.spec.ts
   └── weather.service.ts

A generated service has two parts:

• weather.service.spec.ts contains Jasmine-based unit tests that you can extend to test your service functionality.
• weather.service.ts contains the @Injectable decorator above the class definition, which makes it possible to inject this service into other components, leveraging Angular's provider system. This will ensure that our service will be a singleton, meaning only instantiated once, no matter how many times it is injected elsewhere.

The service is generated, but it's not automatically provided. To do this, follow these steps:

1. Open app.module.ts
2. Type in WeatherService inside the providers array
3. Use the auto-fixer to import the class for you:

src/app/app.module.ts
...
import { WeatherService } from './weather/weather.service'
...
@NgModule({
  ...
  providers: [WeatherService],
  ...