Detecting Form Changes in Angular with updateOn: 'submit'

Описание: Learn how to effectively detect user changes in Angular forms even with `updateOn: 'submit'` setting. Discover a practical solution using custom directives and RxJS.
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This video is based on the question https://stackoverflow.com/q/66972979/ asked by the user 'Dan King' ( https://stackoverflow.com/u/1490986/ ) and on the answer https://stackoverflow.com/a/66980077/ provided by the user 'Mehdi Shakeri' ( https://stackoverflow.com/u/11604867/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Detecting Form Changes in Angular with updateOn: 'submit'

When building web applications with Angular, managing and monitoring form state is crucial for creating a seamless user experience. A common scenario arises when a user has made changes to a form, but those changes aren't saved immediately due to the specific updateOn: 'submit' configuration. If the user attempts to leave the page, it's essential to alert them about unsaved changes. In this guide, we will explore how to effectively detect changes in such forms and ensure that users can be warned if they accidentally navigate away with unsaved changes.

The Problem

The core issue at hand is that when you set your Angular form to updateOn: 'submit', Angular will only register changes to form fields when the form is submitted. This means that any changes made by users are not detected in real-time, preventing you from notifying them of unsaved changes until after submission. The initial implementation using the valueChanges observable works fine when tracking changes, but fails in this configuration.

User Experience Impact

Users might accidentally navigate away from the form.

Unsaved data can be lost if proper measures aren't implemented.

Providing feedback or warnings can enhance user confidence in your application.

A Solution

To overcome this issue, we can create a custom directive that listens for changes outside of Angular's built-in form change detection. Below are the steps to implement an effective solution.

Step 1: Create a Custom Directive

First, we will create a directive that listens for input events on the form fields. This directive will enable us to capture changes made by the user in real-time regardless of the updateOn setting.

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

In the code above:

We use the HostListener decorator to listen for input events on the associated form element.

We can then implement logic that runs whenever the input changes, such as marking the form as dirty.

Step 2: Debouncing Input Changes

For better performance and to prevent overly frequent updates, we can use the fromEvent method from RxJS along with a debounce strategy. This allows us to ensure that our logic only runs after the user has stopped typing for a specified duration, improving performance and user experience.

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

Key Considerations

Unsubscribe Caution: Ensure not to unsubscribe from the observable in the ngOnDestroy method unless you are using takeUntil. This is needed to avoid potential memory leaks.

Compliance with Angular's Forms API: Maintain compatibility with Angular’s forms functionality by utilizing control status methods like markAsDirty or any other relevant methods as necessary.

Conclusion

By using a custom directive to handle input events, you can successfully detect unsaved changes in a form that uses updateOn: 'submit'. This prevents the loss of data and improves user experience by properly alerting users about unsaved changes. With these insights, your Angular applications can offer a more robust and user-friendly experience, ensuring that users are always aware of their input status. Happy coding!