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How to filter Core Data fetch requests using a predicate

Paul Hudson    @twostraws   

Updated for Xcode 12.0

Core Data fetch requests can use predicates in SwiftUI just like they can with UIKit, all by providing a predicate property to your @FetchRequest property wrapper.

If you followed my Core Data and SwiftUI set up instructions, you’ve already injected your managed object context into the SwiftUI environment.

Once that’s done, you can create a fetch request for one of your entities, passing in one or more sort descriptors and a predicate. These predicates are the same instances of NSPredicate that you would use without SwiftUI, which means you can use the same variety of string operations you would normally use.

For example, using the example data from my setup instructions we could create a predicate like this:

NSPredicate(format: "name == %@", "Python")

That will show details about Python, while ignoring other data.

We could use that inside a fetch request like this:

@FetchRequest(
    entity: ProgrammingLanguage.entity(),
    sortDescriptors: [
        NSSortDescriptor(keyPath: \ProgrammingLanguage.name, ascending: true),
    ],
    predicate: NSPredicate(format: "name == %@", "Python")
) var languages: FetchedResults<ProgrammingLanguage>

Because @FetchRequest uses standard Core Data predicates, you can create compound predicates too.

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