Core Data and SwiftUI polymorphism issues - swift

Question

Having troubles putting down together SwiftUI and generic types for handling Core Data.
Consider following example:
Parent is abstract. Foo and Bar are children of Parent and they have some custom attributes.
Now what I want to do, is roughly that:
protocol EntityWithView {
associatedtype T: View
func buildView() -> T
}
extension Parent: EntityWithView {
func buildView() -> some View {
fatalError("Re-implement in child")
}
}
extension Foo {
override func buildView() -> some View {
return Text(footribute)
}
}
extension Bar {
override func buildView() -> some View {
return Text(atrribar)
}
}
struct ViewThatUsesCoreDataAsModel: View {
let entities: [Parent]
var body: some View {
ForEach(entities) { entity in
entity.buildView()
}
}
}
I would want to add polymorphic builder to my core data entities that shape data or build views, that confirm to common interface so I can use them without casting/typing.
Problem that compiler throws errors if I try to modify generated Core data entity directly not through extension, and confirming to protocol though extension doesn't allow overriding.

Answer 1

Ok, this is head-breaking (at least for Preview, which gone crazy), but it works in run-time. Tested with Xcode 11.4 / iOS 13.4.
As we need to do all in extension the idea is to use dispatching via Obj-C messaging, the one actually available pass to override implementation under such requirements.
Note: use Simulator or Device
Complete test module
protocol EntityWithView {
associatedtype T: View
var buildView: T { get }
}
extension Parent {
// allows to use Objective-C run-time messaging by complete
// type erasing.
// By convention subclasses
#objc func generateView() -> Any {
AnyView(EmptyView()) // << safe
//fatalError("stub in base") // << alternate
}
}
extension Parent: EntityWithView {
var buildView: some View {
// restory SwiftUI view type from dispatched message
guard let view = self.generateView() as? AnyView else {
fatalError("Dev error - subview must generate AnyView")
}
return view
}
}
extension Foo {
#objc override func generateView() -> Any {
AnyView(Text(footribute ?? ""))
}
}
extension Bar {
#objc override func generateView() -> Any {
AnyView(Text(attribar ?? ""))
}
}
struct ViewThatUsesCoreDataAsModel: View {
let entities: [Parent]
var body: some View {
VStack {
ForEach(entities, id: \.self) { entity in
entity.buildView
}
}
}
}
struct DemoGeneratingViewInCoreDataExtension: View {
#Environment(\.managedObjectContext) var context
var body: some View {
ViewThatUsesCoreDataAsModel(entities: [
Foo(context: context),
Bar(context: context)
])
}
}