Understanding the functionality of Async Pipe and Observable in Angular

The Async pipe in Angular subscribes to an Observable or Promise and returns the latest value it has emitted. If a new value is emitted, the Async Pipe marks the component that uses it for checking, and this results in the component's template getting updated with the new value. This means that the statement "Async Pipe subscribes to an observer and updates expression whenever there is data sent from the observer" is True.

An Observable is a function that produces a stream of values to an observer over time. When you subscribe to an Observable, you are essentially listening for these values and can operate on them. Using Async Pipe with an Observable allows components in Angular to reactively update when the values that the Observable streams change.

Let's take a quick example. Consider the case where you are creating a real-time data dashboard with Angular. You would create an Observable that streams the data to your component via a service. You can now use the Async Pipe in your component's template to automatically update the UI every time a new data set is streamed.

<!-- dashboard.component.html -->
<h1>Latest Data</h1>
<p>{{ dataObservable | async }}</p>

In this case, dataObservable is an Observable that streams the latest data. Thanks to the Async Pipe (| async), the paragraph tag will automatically update itself whenever new data is sent from the Observable.

In terms of best practices, one of the main benefits of using Async Pipe is that it automatically takes care of subscribing and unsubscribing from Observables reducing the chances of memory leaks. It's recommended to leverage Async Pipe when working with Observables for this reason.

However, keep in mind that any error that occurs in an Observable will be swallowed by the async pipe if not handled. Therefore, error handling should be implemented at the observable level where errors are thrown, ensuring the application can respond appropriately.

In conclusion, the Async pipe is a powerful tool for handling reactive programming in Angular, assisting in automatically subscribing, updating, and unsubscribing from Observables.