Скачать или смотреть Fixing NSFetchRequest to Get the Correct Count in SwiftUI

Learn how to resolve the issue of `NSFetchRequest` returning an incorrect count in your SwiftUI app while managing Core Data.
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This video is based on the question https://stackoverflow.com/q/63972191/ asked by the user 'Igor R.' ( https://stackoverflow.com/u/5248465/ ) and on the answer https://stackoverflow.com/a/63979642/ provided by the user 'andrewbuilder' ( https://stackoverflow.com/u/1883707/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Fixing NSFetchRequest to Get the Correct Count in SwiftUI

In iOS development using SwiftUI, one common issue developers face is when the NSFetchRequest returns an incorrect count of entities, even after performing delete operations. This problem can be especially tricky when you're managing data persistence with Core Data. If you've encountered a situation where, after deleting a record, your app continues to report the outdated count, you’ve come to the right place.

Understanding the Problem

Let's say you are using NSFetchRequest to count the number of books in your database and perform operations like deleting entries. You expect that after removing a book, the count should reflect the change. However, to your surprise, the count remains unchanged, leading to potential bugs and inconsistencies in your app. Here's a simplified structure of the code that demonstrates this issue:

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The function above seems straightforward: fetch all Book entities and return their count. However, when you delete a book and ask for the count again, you still see the old value. This inconsistency can confuse users and diminish the app's trustworthiness.

Why This Happens

The main issue arises from the way SwiftUI handles data flow. SwiftUI relies on data invalidation to refresh views. Therefore, if your user interface does not respond to these changes in data seamlessly, you will encounter a state where the countResult represents outdated information — in your case, the count of books.

Solution: Utilize @ FetchRequest

Instead of manually fetching and counting records every time you need the count, a better approach is to use the @ FetchRequest property wrapper. This wrapper provides a reactive way to manage your data, automatically updating views when the underlying data changes.

Step-by-Step Implementation

Set Up the @ FetchRequest: In your view, declare the @ FetchRequest for the Book entity. Here's how you do it:

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Note: The sortDescriptors parameter is required; if you don’t want to sort the fetched results, you can provide an empty array [].

Update Your View: Use the count directly in your SwiftUI view. This allows the user interface to automatically reflect the current state of your data.

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With this implementation, the Text view will update every time a book is added or deleted, indicating the current count of books seamlessly.

Optional: Keep the Button Action: If you wish to maintain a button in your view to perform other actions, you can embed the count within the button label itself:

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Conclusion

By leveraging SwiftUI's @ FetchRequest, not only do you resolve the issue of outdated counts, but you also make your code cleaner and more efficient. The user interface will robustly reflect real-time data changes, leading to a better user experience. SwiftUI elegantly manages visual data updates, allowing you to focus on building great features rather than troubleshooting outdated state issues.

In summary, if you’re facing problems with NSFetchRequest returning the wrong count, remember to utilize @ FetchRequest for a more dynamic approach in your SwiftUI applications.