Creating an Impure Pipe in Angular

Angular pipes are typically pure, meaning they only run when their input values change. However, sometimes you need a pipe that re-evaluates on every change detection cycle, even if the input hasn't strictly changed (e.g., when relying on a global service or external data). This challenge asks you to create an impure pipe that fetches data from a service and displays it, demonstrating how to build and use impure pipes in Angular.

Problem Description

You need to create an Angular pipe named DataDisplayPipe that fetches data from a provided service and formats it for display. The service, DataService, simulates fetching data (e.g., current time) and updates it periodically. The pipe should display the data provided by the service. Because the data changes frequently, the pipe must be impure to reflect these updates in the view.

Key Requirements:

• The pipe should accept a DataService as input.
• The pipe should subscribe to an observable provided by the DataService that emits the data to be displayed.
• The pipe should format the data (in this case, simply displaying it as a string).
• The pipe must be marked as impure.
• The pipe should handle the case where the DataService is null or undefined gracefully (e.g., display a default message).
• The pipe should unsubscribe from the observable in the ngOnDestroy lifecycle hook to prevent memory leaks.

Expected Behavior:

The view using the pipe should display the data fetched from the DataService. The data should update automatically whenever the DataService emits a new value. The pipe should not throw errors if the service is not provided.

Edge Cases to Consider:

• DataService being null or undefined.
• The observable from DataService emitting errors.
• Memory leaks if the subscription is not properly managed.

Examples

Example 1:

Input: DataService providing a stream of timestamps (e.g., every second).
Output: The view displays the latest timestamp from the DataService.
Explanation: The DataDisplayPipe subscribes to the DataService's observable and updates the view with the latest timestamp.

Example 2:

Input: DataService is null.
Output: The view displays a message like "Data unavailable."
Explanation: The DataDisplayPipe handles the null DataService case and displays a default message.

Constraints

• The DataService is assumed to provide an observable that emits strings.
• The pipe should be implemented using Angular's Pipe decorator.
• The pipe should not perform any complex data transformations beyond simple string conversion.
• The pipe should be performant enough to handle frequent updates without causing noticeable lag in the view. Avoid unnecessary computations within the pipe.

Notes

• Remember to use the PurePipe decorator with pure: false to create an impure pipe.
• Properly managing subscriptions is crucial to prevent memory leaks. Use the ngOnDestroy lifecycle hook to unsubscribe.
• Consider using RxJS operators like catchError to handle potential errors from the DataService.
• The DataService is provided as a dependency injection. You don't need to create it; assume it's available.