Include SwiftUI views in existing UIKit application

Is is possible to build views with SwiftUI side by side with an existing UIKit application?

I have an existing application written in Objective-C. I've begun migrating to Swift 5. I'm wondering if I can use SwiftUI alongside my existing UIKit .xib views.

That is to say I want some views built with SwiftUI and some other views built with UIKit in the same app. Not mixing the two of course.

SomeObjCSwiftProject/
    SwiftUIViewController.swift
    SwiftUIView.xib
    UIKitViewController.swift
    UIKitView.xib

Working alongside each other

edit 05/06/19: Added information about UIHostingController as suggested by @Departamento B in his answer. Credits go to him!

Using SwiftUI within UIKit

One can use SwiftUI components in existing UIKit environments by wrapping a SwiftUI View into a UIHostingController like this:

let swiftUIView = SomeSwiftUIView() // swiftUIView is View
let viewCtrl = UIHostingController(rootView: swiftUIView)

It's also possible to override UIHostingController and customize it to one's needs, e. g. by setting the preferredStatusBarStyle manually if it doesn't work via SwiftUI as expected.

UIHostingController is documented here.

Using UIKit within SwiftUI

If an existing UIKit view should be used in a SwiftUI environment, the UIViewRepresentable protocol is there to help! It is documented here and can be seen in action in this official Apple tutorial.

Compatibility

Please note that you cannot use SwiftUI on iOS versions < iOS 13, as SwiftUI is only available on iOS 13 and above. See this post for more information.

If you want to use SwiftUI in a project with a target below iOS 13, you need to tag your SwiftUI structs with @available(iOS 13.0.0, *) attribute.

If you want to embed SwiftUI into a UIKit view controller, use a Container View.

class ViewController: UIViewController {
    @IBOutlet weak var theContainer: UIView!

    override func viewDidLoad() {
        super.viewDidLoad()

        let childView = UIHostingController(rootView: SwiftUIView())
        addChild(childView)
        childView.view.frame = theContainer.bounds
        theContainer.addSubview(childView.view)
        childView.didMove(toParent: self)
    }
}

Reference

UIHostingController

Although at the moment the documentation for the class has not been written, UIHostingController<Content> seems to be what you're looking for: https://developer.apple.com/documentation/swiftui/uihostingcontroller

I've just tried it in my app with the following line of code:

let vc = UIHostingController(rootView: BenefitsSwiftUIView())

Where BenefitsSwiftUIView is just the default "Hello World" View from SwiftUI. This works exactly as you expect it. It also works if you subclass UIHostingController.

One item I have not seen mentioned yet, and involves Xcode 11 beta 5 (11M382q) involves updating your app's info.plist file.

For my scenario, I am taking an existing Swift & UIKit based application and fully migrating it to be an iOS 13 & pure SwiftUI app, so backwards compatibility is not a concern for me.

After making the necessary changes to AppDelegate:

// MARK: UISceneSession Lifecycle
func application(_ application: UIApplication,
                 configurationForConnecting connectingSceneSession: UISceneSession,
                 options: UIScene.ConnectionOptions) -> UISceneConfiguration {
    return UISceneConfiguration(name: "Default Configuration",
                                sessionRole: connectingSceneSession.role)
}

And adding in a SceneDelegate class:

import UIKit
import SwiftUI

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        if let windowScene = scene as? UIWindowScene {
            let window = UIWindow(windowScene: windowScene)
            window.rootViewController = UIHostingController(rootView: HomeList())
            self.window = window
            window.makeKeyAndVisible()
        }
    }
}

I was encountering a problem where my SceneDelegate was not being called. This was fixed by adding the following into my info.plist file:

<key>UIApplicationSceneManifest</key>
<dict>
    <key>UIApplicationSupportsMultipleScenes</key>
    <false/>
    <key>UISceneConfigurations</key>
    <dict>
        <key>UIWindowSceneSessionRoleApplication</key>
        <array>
            <dict>
                <key>UISceneClassName</key>
                <string></string>
                <key>UISceneDelegateClassName</key>
                <string>$(PRODUCT_MODULE_NAME).SceneDelegate</string>
                <key>UISceneConfigurationName</key>
                <string>Default Configuration</string>
                <key>UISceneStoryboardFile</key>
                <string>LaunchScreen</string>
            </dict>
        </array>
    </dict>
</dict>

And a screenshot to see:

The main items to keep in sync are:

• Delegate Class Name so that Xcode knows where to find your SceneDelegate file
• Configuration Name so that the call in AppDelegate can load the correct UISceneConfiguration

After doing this, I was then able to load my newly created HomeList view (A SwiftUI object)