Code displays how to switch between Landscape and Portrait views on SwiftUI depending on the current device orientation. This is achieved by attaching "UIDevice.current.orientation" to a State variable and using a .onReceive modifier to detect when the current orientation is changed through the Notification Center Publisher.

Orientation.swift

//
//  Orientation.swift
//  Code Template
//
//  Created by Pao Yu on 2020-04-18.
//  Copyright © 2020 Yu Pao. All rights reserved.
//

import SwiftUI

struct MainView: View {
    
    @State var orientation: UIDeviceOrientation = UIDevice.current.orientation
    
    var body: some View {
        
        GeometryReader { device in
                        
            ZStack {
                
                if self.mvm.orientation.isLandscape {
                    LandscapeView()
                } else if self.mvm.orientation.isPortrait {
                    PortraitView()
                } else if device.size.width > device.size.height {
                    LandscapeView()
                } else if device.size.width < device.size.height {
                    PortraitView()
                } else {
                    LandscapeView()
                }
                
            }   .edgesIgnoringSafeArea(.all)
                .onReceive(NotificationCenter.Publisher(center: .default, name: UIDevice.orientationDidChangeNotification)) { _ in self.mvm.detectOrientation() }
                .onAppear() { self.viewDidLoad() }
        }
        
    }
    
    func viewDidLoad() {
        // Load code goes here.
    }
    
    func detectOrientation() {
        orientation = UIDevice.current.orientation
    }
    
}

struct LandscapeView: View {
    var body: some View {
        HStack{
            Text("Landscape")
        }
    }
}

struct PortraitView: View {
    var body: some View {
        HStack {
            Text("Portrait")
        }
    }
}