import UIKit
import SwiftUI

class KeyboardViewController: UIInputViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        struct ContentView: View {
            
            @State var enlarge = false
            
            var body: some View {
                Color.red
                    .frame(height: enlarge ? 400 : 200)
                    .onTapGesture {
                        enlarge.toggle()
                    }
            }
            
        }
        
        //  Key fix #1:
        //  Override UIHostingController.viewWillLayoutSubviews's method
        
        class HostingController: UIHostingController<ContentView> {
            
            override func viewWillLayoutSubviews() {
                super.viewWillLayoutSubviews()
                
                if #available(iOS 15.0, *) {
                    //  Workaround for an iOS 15 SwiftUI bug(?):
                    //  The intrinsicContentSize of UIView is not updated
                    //  when the internal SwiftUI view changes size.
                    
                    view.invalidateIntrinsicContentSize()
                }
            }
            
        }
        
        let hostingController = HostingController(rootView: ContentView())
        
        let hostingView = hostingController.view!
        
        //  Key fix #2:
        //  Add the hosting controller as a child view controller
        
        addChild(hostingController)
        
        view.addSubview(hostingView)
        
        hostingController.view.translatesAutoresizingMaskIntoConstraints = false
        
        //  Key fix #3:
        //  Constraining the top & bottom of the hosting view to its parent
        
        NSLayoutConstraint.activate([
            hostingView.topAnchor.constraint(equalTo: view.topAnchor),
            hostingView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
            hostingView.leftAnchor.constraint(equalTo: view.leftAnchor),
            hostingView.rightAnchor.constraint(equalTo: view.rightAnchor)
        ])
        
        //  Key fix #4:
        //  Let the hosting controller know it has finished
        //  moving to the parent view controller.
        
        hostingController.didMove(toParent: self)
    }

}