How to Instantiate UIHostingController with CoreData??

Question

As a long-time Obj-C dev (since iPhone OS 2) I decided that SwiftUI was the nudge to use Swift! ¯\_(ツ)_/¯
Thus, I am still trying to understand how a language can be so type-vague in code, and so type-pedantic in the compiler!!!

Pulling my hair out trying to get Swift/SwiftUI to instantiate a UIHostingController<>, for use with CoreData

Code Block class MyCoreDataHostingController : UIHostingController<MyCoreDataView> {
	required init?(coder: NSCoder) {//Instantiate From Storyboard
		let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
		let contentView = MyCoreDataView().environment(\.managedObjectContext, context)
		super.init(coder: coder, rootView: contentView)
		//Cannot convert value of type 'some View' to expected argument type 'MyCoreDataView'
	}
}
struct MyCoreDataView: View {
	var body: some View {
		Text("Core Data View")
	}
}

How can I FORCE Swift to INFER the correct/appropriate type?

What I have tried so far.

Code Block 5 let contentView = MyCoreDataView()

Compiles/runs, but does not include CoreData.

Code Block 6 super.init(coder: coder, rootView: contentView as! MyCoreDataView)

Compiles, but crashes at instantiation.

Could not cast value of type 'SwiftUI.ModifiedContent<MyHostingController.MyCoreDataView, SwiftUI.EnvironmentKeyWritingModifier<_C.NSManagedObjectContext>>' (...)
to 'MyHostingController.MyCoreDataView' (...).

... or is this simply not possible? (yet)

1.3k

Posted by

Linasses

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Answer 1

As you see, modifiers for SwiftUI View returns some opaque type some View which is not revealed to programmers but Swift compiler claims matching the type every here and there...

One possible solution is creating a wrapper View:

Code Block class MyCoreDataHostingController : UIHostingController<MyCoreDataViewEnvironmentWrapper> {
    required init?(coder: NSCoder) {//Instantiate From Storyboard
        let contentView = MyCoreDataViewEnvironmentWrapper()
        super.init(coder: coder, rootView: contentView)
    }
}
struct MyCoreDataViewEnvironmentWrapper: View {
    private let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    
    var body: some View {
        MyCoreDataView().environment(\.managedObjectContext, context)
    }
}

Or you can choose AnyView as the wrapper:

Code Block class MyCoreDataHostingController : UIHostingController<AnyView> {
    required init?(coder: NSCoder) {//Instantiate From Storyboard
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        let contentView = AnyView(MyCoreDataView().environment(\.managedObjectContext, context))
        super.init(coder: coder, rootView: contentView)
    }
}

I'm not sure if this works for your case, but please try.

Posted by

OOPer

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Answer 2

As you see, modifiers for SwiftUI View returns some opaque type some View which is not revealed to programmers but Swift compiler claims matching the type every here and there...

One possible solution is creating a wrapper View:

Code Block class MyCoreDataHostingController : UIHostingController<MyCoreDataViewEnvironmentWrapper> {
    required init?(coder: NSCoder) {//Instantiate From Storyboard
        let contentView = MyCoreDataViewEnvironmentWrapper()
        super.init(coder: coder, rootView: contentView)
    }
}
struct MyCoreDataViewEnvironmentWrapper: View {
    private let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    
    var body: some View {
        MyCoreDataView().environment(\.managedObjectContext, context)
    }
}

Or you can choose AnyView as the wrapper:

Code Block class MyCoreDataHostingController : UIHostingController<AnyView> {
    required init?(coder: NSCoder) {//Instantiate From Storyboard
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        let contentView = AnyView(MyCoreDataView().environment(\.managedObjectContext, context))
        super.init(coder: coder, rootView: contentView)
    }
}

I'm not sure if this works for your case, but please try.

Posted by

OOPer

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Answer 3

Thanks OOPer,
BOTH methods worked: I still have to check if the CoreData environment setting works with either/both, but very promising, and then decide the best/cleanest approach.

The <AnyView> (https://developer.apple.com/documentation/swiftui/anyview) solution was particularly inspired. And works with the ManagedObjectContext passing, so using that. 👍

Posted by

Linasses

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How to Instantiate UIHostingController with CoreData??

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