How to Fix ngFor Updates in Angular When Using Observables

Discover how to resolve the issue of constant `ngFor` updates in Angular when working with Observables, and ensure efficient memory usage in your applications.
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Understanding the Problem with ngFor in Angular

If you've ever faced performance issues in your Angular application, particularly with ngFor, you're not alone. A common issue developers encounter is when their ngFor does not seem to function correctly, leading to unnecessary re-renders and increased RAM usage. Specifically, you may find that your ngFor keeps updating even though the underlying array hasn't changed after its initial load. Let's explore this issue and how to resolve it seamlessly.

The Scenario

Imagine you have a component displaying a list of announcements using ngFor. You also want to show related comments for each announcement. Here’s a brief overview of how you might structure your code:

Example Structure

In your HTML template, you might have two main ngFor divs, something like:

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

When you load this component, the ngFor appears to be continuously updating, likely for memory-intensive reasons because the inner ngFor is not being managed correctly.

The Root Cause

The problem stems from how you are handling the loading of comments. In your TypeScript code, you have a method called loadComments that retrieves comments from a service. However, this method is designed poorly, leading to old data being returned. The crucial line of code in your method is:

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

The recommendation here is to modify your loadComments method to properly return an Observable instead of relying on previously loaded comments. This change ensures that each call to loadComments fetches the latest data.

Proposed Solution

Step 1: Update the loadComments Method

Change the loadComments method to return an Observable of comments like this:

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

This ensures that each time loadComments is called, it makes a fresh API call to fetch the current comments instead of returning stale data.

Step 2: Modify the Template to Use async Pipe

Next, update your HTML to utilize Angular's async pipe, which will automatically handle the subscription to your Observable and keep track of its latest value. Here’s how you can do this:

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

By making these changes, your template will only re-render when the comments are actually updated, reducing unnecessary rendering and helping with performance issues you were encountering.

Conclusion

By refactoring how comments are loaded in your Angular component, you not only solve the problem of unnecessary updates but also enhance the overall efficiency of your application. Using Observables correctly, along with Angular’s async pipe, helps ensure that your application behaves as expected without putting undue strain on memory or processing power. Keep this approach in mind as you work with lists and dynamic data in your Angular projects, and you'll find your applications running smoother and faster.