Backport Foundation `Predicate`s

[helje5]

The Foundation Predicate is available in the open source Foundation. I think it requires iOS 17+ because it uses variadic generics. But that's not actually necessary for the SwiftData APIs which only pass in a single model type.

This should probably go into an own package.

[HealsCodes]

Has there ever been any progress towards this?

I've been playing around with Predicates + ManagedModels but always run into a dead-end seemingly because Model properties aren't exposed to the underlying Objective-C runtime so code like this doesn't work:

@Model
final class MyModel: NSManagedObject {
  var enabled: Bool

  convenience init() {
    self.enabled = false
  }
}

// the following works if MyModel is created using the traditional NSManagedObject / @NSManaged means

let p = #Predicate<MyModel> { $0.enabled == true } 
let predicate = NSPredicate(p) // returns nil, because apparently MyModel.enabled isn't bridged

Likewise a simple NSExpression(forKeyPath: \MyModel.enabled) fails with "Foundation/NSObject.swift:132: Fatal error: Could not extract a String from KeyPath \MyModel.enabled"

The same issue breaks interoperability with packages like PredicateKit which relies on being able to convert AnyKeyPath to String using ObjC bridging internals.

[helje5]

I haven't seen a Predicate back port yet, but that seems unrelated to your issue (and this becomes less necessary as time goes by). I've created #36 for this.