Solving the Mystifying Case of Mat-List-Option Deselecting Itself: A Step-by-Step Guide

Are you tired of dealing with the frustrating issue of mat-list-options deselecting themselves when recreated in a virtual-scroll-viewport? You’re not alone! This pesky problem has been plaguing Angular developers for far too long. But fear not, dear reader, for we’re about to embark on a thrilling adventure to vanquish this nuisance once and for all.

The Culprit: ngFor TrackBy

The root of this issue lies in the way Angular uses the ngFor directive to render the list options. Specifically, the trackBy function is the primary suspect behind this confounding behavior. By default, ngFor uses the object’s identity to track changes, which can lead to unintended consequences when dealing with dynamic data.

A Glimpse into the Problem

Imagine you have a list of items, and each item has a unique identifier. You’re using ngFor to render these items as mat-list-options inside a virtual-scroll-viewport. Everything seems fine at first, but when you recreate the list (e.g., after a data refresh), the selected option mysteriously deselects itself.

<mat-list-option *ngFor="let item of items; trackBy: trackById" [value]="item">{{ item.name }}</mat-list-option>

In this example, the trackBy function is used to track the items by their unique identifier. However, when the list is recreated, Angular will recreate the entire list, causing the selected option to lose its selection state.

Finding the Solution

To overcome this hurdle, we’ll employ a clever trick: utilize a custom trackBy function that incorporates the selected option’s value. This will ensure that Angular correctly tracks the selected item even when the list is recreated.

Step 1: Create a Custom TrackBy Function

In your component, define a custom trackBy function that takes into account the selected option’s value:

trackBySelectedItem(index: number, item: any): string {
  return `${item.id}_${this.selectedItemId}`;
}

In this example, we’re concatenating the item’s ID with the selected item’s ID (which we’ll store in a component property, `selectedItemId`). This unique string will serve as the tracking identifier for each item.

Step 2: Update the ngFor Directive

Modify the ngFor directive to use the custom trackBy function:

<mat-list-option *ngFor="let item of items; trackBy: trackBySelectedItem">{{ item.name }}</mat-list-option>

By using the custom trackBy function, we’re ensuring that Angular correctly tracks the selected item even when the list is recreated.

Step 3: Store the Selected Item’s ID

In your component, create a property to store the selected item’s ID:

selectedItemId: string;

Update this property whenever the selected item changes:

(selectionChange)="selectedItemId = $event.option.value.id">

In this example, we’re updating the `selectedItemId` property whenever the selection changes.

Additional Tips and Tricks

To further solidify our solution, let’s cover some additional best practices:

Using a Unique Identifier

Make sure each item in your list has a unique identifier. This identifier should be used in the custom trackBy function to ensure accurate tracking.

<mat-list-option *ngFor="let item of items; trackBy: trackBySelectedItem">
  {{ item.name }} (ID: {{ item.id }})
</mat-list-option>

Preserving the Selected State

When recreating the list, preserve the selected state by storing the selected item’s ID before recreating the list:

const selectedItemId = this.selectedItemId;
this.items = [...this.items]; // recreate the list
this.selectedItemId = selectedItemId;

By doing so, you ensure that the selected item remains selected even after the list is recreated.

Handling Dynamic Data

If your data is dynamic, consider using an observable to fetch the data. This will allow you to recreate the list without losing the selected state:

this.dataService.getItems().subscribe(items => {
  this.items = items;
});

By using an observable, you can confidently recreate the list without worrying about losing the selected item’s state.

Conclusion

With these steps and tips, you should now be equipped to tackle the pesky issue of mat-list-options deselecting themselves when recreated in a virtual-scroll-viewport. By using a custom trackBy function and preserving the selected state, you’ll ensure a seamless user experience for your users. Remember to stay vigilant and adapt these strategies to your specific use case.

Problem	Solution
Mat-list-options deselecting themselves when recreated in a virtual-scroll-viewport	Use a custom trackBy function that incorporates the selected option’s value
Loss of selected state when recreating the list	Store the selected item’s ID and preserve it when recreating the list
Dynamic data causing issues	Use an observable to fetch the data and recreate the list

With these strategies in place, you’ll be well on your way to creating a seamless and intuitive user experience. Happy coding!

• Remember to adapt these strategies to your specific use case
• Stay vigilant and test thoroughly to ensure the solution works as intended
• Don’t hesitate to reach out if you encounter any issues or have further questions

1. Review the code examples and adapt them to your project
2. Test the solution thoroughly to ensure it works as intended
3. Share your findings and experiences with the Angular community

By following these steps and tips, you’ll be well-equipped to tackle the issue of mat-list-options deselecting themselves when recreated in a virtual-scroll-viewport. Happy coding, and may your users rejoice in a seamless and intuitive experience!

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