Скачать или смотреть Resolving @ Input() Not Updating UI Issue in Angular Components

Learn how to effectively solve the `@ Input()` data binding problem in Angular by understanding change detection and exploring optimized solutions.
---
This video is based on the question https://stackoverflow.com/q/65204496/ asked by the user 'ossys' ( https://stackoverflow.com/u/382560/ ) and on the answer https://stackoverflow.com/a/65205320/ provided by the user 'Misha Borisov' ( https://stackoverflow.com/u/13532354/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Angular @ Input() Not Updating UI in Child

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding @ Input() Not Updating UI in Angular Components

When working with Angular components, developers often encounter issues with data not updating in child components when using the @ Input() decorator. A specific case involves a child component that fails to refresh its UI based on a list that's loaded via HTTP. In this post, we'll break down the problem and provide a well-structured solution to ensure your components update correctly when data changes.

The Problem

Consider the following situation where you have a child component designed to display a table based on data passed from its parent component. The crucial part is that this data is retrieved asynchronously, typically through an HTTP service. However, the UI of the child component does not seem to reflect the updated data until a mouseover event occurs, leading to questions about whether Angular's change detection mechanism is functioning as expected.

Code Example

Here’s a minimal representation of the child and parent components involved:
Child Component (child.component.ts):

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

Child Template (child.component.html):

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

Parent Component (parent.component.ts):

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

Parent Template (parent.component.html):

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

Why the UI Doesn't Update

In Angular, the @ Input() properties are bound to the parent component's data. However, if the change detection mechanism does not detect any changes, the child component won't update automatically. This issue can arise particularly in cases where the data is being modified in an asynchronous manner, such as inside an Observable subscription.

What You Observed

You noticed that when setting a static array, the child component updated correctly. The problem only occurred when you updated the data fetched from an API, leading to the discovery that invoking changeDetectorRef.detectChanges() allowed the UI to refresh. While this workaround works, it feels less than ideal and raises the question of whether there’s a better, more Angular-like way to handle this situation.

Solutions to Consider

1. Using the Spread Operator

One effective solution is to force Angular's change detection mechanism to recognize that the value has changed by updating the reference of the data value. You can achieve this by using the spread operator when assigning the new data:

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

This approach signals to Angular that there’s a new array, prompting the change detection process to trigger the UI update in the child component.

2. Change Detection Strategy

Another approach involves modifying the change detection strategy of your child component. If performance is a concern and you want to reduce the number of checks Angular performs, you might consider setting the change detection strategy to OnPush for the child component. However, ensure that you handle updates correctly in this case as it can change the way your component becomes reactive to input changes:

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

Bear in mind, with OnPush, Angular only checks the component when its inputs have changed based on reference, or when an event occurs within the component.

3. Manual Change Detection as a Last Resort

While invoking changeDetectorRef.detectChanges() can resolve the issue, it should be a last resort. If you're frequently needing this workaround, it's better to review your component's change detection strategy and data flow.

Conclusion

Resolving issues with @ Input() data not updating UI in Angular components boils down to understanding Angular’s change dete