Solving the Angular Multi-Select Component Issue: valueChanges Fired but Nothing Selected

Описание: Encountering issues with Angular Material's multi-select component? Discover the solution that solves the `valueChanges` firing without any selections!
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This video is based on the question https://stackoverflow.com/q/64339709/ asked by the user 'Ida Lin' ( https://stackoverflow.com/u/13693520/ ) and on the answer https://stackoverflow.com/a/64371824/ provided by the user 'Ida Lin' ( https://stackoverflow.com/u/13693520/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving the Angular Multi-Select Component Issue

If you've recently dived into Angular (particularly Angular Material), you might have stumbled upon a perplexing issue related to the multi-select component. Imagine setting up your form, only to find that despite having valid values in your formControl, nothing appears selected in the drop-down. This might leave you scratching your head. In this guide, we'll dissect this common problem and reveal the solution that restored functionality to the multi-select component.

The Problem

You may have implemented a custom material multi-select component with an additional "All" option that allows users to select or deselect all choices at once. Upon initializing the form, this "All" option selected correctly. However, after submitting the form and retrieving data from a store like NgRx, you discover that even with the patched values present, nothing displays as selected in the multi-select.

Key Observations

The valueChanges observable fires appropriately.

The correct values are being retrieved and patched into the form.

The system correctly responds when resubmitting the form after making selections.

Understanding the Cause

The crux of the issue lies in how Angular compares values within form controls. When using custom objects as values (like instances of a class), Angular considers the reference of these objects, not their actual value. This means that even if the contents of the objects are identical, they may not register as equal unless explicitly told to do so.

The Solution

After considerable troubleshooting, the solution became apparent: Angular's compareWith directive! Adding this directive allows you to define a custom comparison function to determine equality between the form control's value and the options available in the multi-select. Here’s a step-by-step approach to implementing the fix.

Step 1: Update the Template

Modify your multi-select template to include the compareWith directive:

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

Step 2: Define the Comparison Function

In your component class, create a custom comparison function:

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

Why This Works

By implementing the compareWith directive and specifying the compareFn, Angular can now accurately assess whether the selected options exist in the options list based on the defined keys of each SelectOption object. This ensures that the form control correctly registers selected values even when stored as custom objects.

Conclusion

Navigating through Angular's intricacies can be daunting, particularly when working with forms and custom components. The issue of valueChanges firing without displaying selected options can be resolved by leveraging Angular's capabilities, like the compareWith directive.

With this approach, you can now keep your multi-select component fully functional and up to expectation. If you find yourself stuck in a similar situation, remember to check for object references and consider implementing a custom comparison strategy.

Thank you for exploring this common Angular issue with me! If you have further questions, don’t hesitate to drop them in the comments below.