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Learn how to use Angular Observables to continuously check the printer status with effective polling techniques. This guide explains polling, RxJS operators, and practical examples for your Angular application.
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Keep Checking: Angular Observable from Promise

In the world of modern web applications, real-time data updates are essential, especially when dealing with devices like printers. An Angular app needing to monitor printer status can present some unique challenges, particularly when interfacing with services that return data via promises. If you’re developing an Angular app that needs to keep checking a printer’s status, you might find yourself in a situation where you need to turn a promise into a continuously polling observable. In this post, we will explore how to achieve this.

The Problem

Many devices and services encapsulate their status checks as promises. In this specific case, the printer status is retrieved via a promise, but in order to keep your Angular application updated with the current printer state (like “on” or “off”), you need a mechanism for continuous polling. Simply calling the promise once won’t suffice, as you'll only receive the status response one time.

Example Scenario

If your printer initially sends back a status "on", it will only be logged once when you perform that initial check. It won’t inform you if you later turn the printer "off" unless you re-poll the status.

To illustrate the issue, you might have a setup in your PrinterService.ts similar to this:

[[See Video to Reveal this Text or Code Snippet]]

And in your HomePage.ts, you are subscribing to this function:

[[See Video to Reveal this Text or Code Snippet]]

While this works, it only retrieves the status once. It doesn’t inherently keep track of any changes over time.

The Solution

To keep checking the status of your printer, you will need to implement a polling mechanism. This is where RxJS comes in handy. Below, we break down the solution step-by-step.

1. Understanding Polling with Timer

Polling involves repeatedly checking for data at regular intervals. We can achieve this by using RxJS operators such as timer.

2. Implementing the Polling Functionality

We will set up a timer that emits an event every specified interval, allowing us to call the getStatus method consistently.

[[See Video to Reveal this Text or Code Snippet]]

3. Explanation of Operators

timer(0, 500): This tells RxJS to start emitting events immediately (0 milliseconds) and then every 500 milliseconds thereafter.

switchMap(_ = printerService.getStatus()): This operator subscribes to the latest status returned by getStatus every time the timer emits.

distinctUntilChanged(): This is crucial! It ensures that the observable only emits values when the printer status actually changes, preventing unnecessary updates and clutter in your console output.

4. Advanced Customization

For more structured output, you might want to return an object that contains the status so you can manage state clearly:

[[See Video to Reveal this Text or Code Snippet]]

Here, the scan operator is used to carry the previous status along with the current one, ensuring that the distinct check works correctly.

Conclusion

By employing observables with RxJS in your Angular application, you can efficiently monitor the status of a printer or any similar device that relies on promises for status reporting. With the combination of timer, switchMap, and distinctUntilChanged, you can set up a real-time monitoring system that only updates your application when there are actual changes, keeping everything efficient and responsive.

Implement these strategies in your app, and you’ll effortlessly keep track of the printer's status in the background without any manual refresh.