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Learn how to implement a custom async validator in Angular, even when using template-driven forms. This guide provides clear steps and code examples for better validation of input fields.
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How to Create a Custom Async Validator in Angular

When working with Angular forms, validation plays a crucial role in ensuring that the data collected is valid and reliable. Developers often encounter scenarios where they need to validate inputs against backend services, which can be achieved through custom async validators. In this post, we will walk you through the process of creating a custom async validator in Angular, especially when using template-driven forms.

Understanding the Problem

Imagine you have a scenario where users enter details into multiple fields such as first name, last name, email, and telephone across several rows of input fields. You want to validate these inputs against a backend service to ensure their correctness. Here’s a simplified version of how you want to validate the user information on the frontend:

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

You might think of using Angular’s reactive forms, but what if you’re sticking with ngModel for your templates? Fear not! You can still create powerful validation using custom async validators.

Solution: Creating a Custom Async Validator

Step 1: Implementing the Async Validator Directive

To get started, you need to create a custom directive that implements the AsyncValidator. This involves defining our validation logic within the directive and registering it with Angular's dependency injection using NG_ASYNC_VALIDATORS.
Here's a sample code to guide you:

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

Step 2: Using the Validator in Your Template

Once you've defined your directive, it's time to apply it to your input fields. You simply need to add the directive as an attribute to the relevant input field. For instance:

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

Important Considerations

Control Extraction: The validate method only takes the control as an argument, so you’ll need to extract any necessary information from this control to perform your validation.

Asynchronous Operation: Your validation logic should return an Observable. Make sure that it communicates effectively with your backend service to retrieve validation results.

Conclusion

By creating a custom async validator, you can effectively validate user input fields directly against your backend service, even within template-driven forms using ngModel. This adds significant flexibility and maintains robust data integrity throughout your Angular application.

Implementing custom async validators can enhance your application’s user experience by providing real-time validation feedback, thus guiding users to correct their input before submission.

With this guide, you should now be well-equipped to create custom async validators tailored to your Angular project needs!