Скачать или смотреть Resolving the Value of type [ViewModel] has no member nsPredicate Error in SwiftUI

Learn how to effectively handle `nsPredicate` issues when using Core Data with SwiftUI through NSFetchedResultsController in your macOS app.
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This video is based on the question https://stackoverflow.com/q/71748707/ asked by the user 'swiftnoob' ( https://stackoverflow.com/u/16297405/ ) and on the answer https://stackoverflow.com/a/71748797/ provided by the user 'vadian' ( https://stackoverflow.com/u/5044042/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the Value of type [ViewModel] has no member nsPredicate Error in SwiftUI

If you’re developing a macOS application using Swift and Core Data, encountering the error message "Value of type '[MyBookmarkViewModel]' has no member 'nsPredicate'" can be frustrating. This error commonly arises when you're trying to filter a list of bookmarks but mistakenly reference the wrong property in your ViewModel. Let’s go through the issue at hand, detail the cause of the error, and present a clear solution to help you effectively manage your Core Data queries.

Understanding the Problem

This error generally occurs when you attempt to assign a predicate to a collection of ViewModels (vm.myBookmarks) instead of the Core Data fetch request directly. In your code, this line:

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

is the culprit. It leads to the compiler confusion because the array of MyBookmarkViewModel does not have a member named nsPredicate. Instead, predicates are typically associated with fetch requests.

Solution Overview

To resolve this issue, follow these steps:

Make the NSFetchedResultsController Public: Ensure your NSFetchedResultsController is accessible.

Assign the Predicate to the Fetch Request: Rather than modifying the array of ViewModels, directly modify the predicate of the fetch request associated with your fetched results controller.

Step 1: Make NSFetchedResultsController Public

In your MyBookmarksViewModel, change the declaration of fetchedResultsController to be public:

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

By doing this, you allow access to the fetch request outside of the class, enabling you to modify its properties.

Step 2: Assign the Predicate to the Controller's Fetch Request

Modify the query logic where you're handling the search functionality. Instead of trying to assign a predicate directly to vm.myBookmarks, you will now work with vm.fetchedResultsController.fetchRequest:

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

You can even simplify this further using the conditional operator:

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

This snippet succinctly assigns a valid predicate based on whether the searchText is empty or not.

Alternative: Using a Helper Function

If you prefer not to change the visibility of the NSFetchedResultsController, you can create a helper function within your ViewModel for managing the predicate:

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

You would then call this function from your query binding, resulting in a cleaner architecture that encapsulates predicate management within your ViewModel.

Conclusion

By properly managing how predicates are assigned to your Core Data fetch requests instead of trying to assign them to a collection of ViewModels, you can effectively avoid the error "Value of type '[MyBookmarkViewModel]' has no member 'nsPredicate'". This approach not only resolves the issue but also sets a foundation for cleaner, more maintainable code in your SwiftUI applications. Happy coding!