SwiftUI: Change “About View” in macOS app

You need to add a Credits.rtf file to your Bundle. This will be automatically detect and be inserted in the About Dialog.

You can find more here

You can do it but it does require creating an AppDelegate. Here's what your AppFile should look like:

struct MyApp: App {
    @NSApplicationDelegateAdaptor(AppDelegate.self) var appDelegate

    var body: some Scene {
        WindowGroup {
            MainView()
        }
        .commands {
            CommandGroup(replacing: CommandGroupPlacement.appInfo) {
                Button(action: {
                    appDelegate.showAboutPanel()
                }) {
                    Text("About My App")
                }
            }
        }
    }
}

and your AppDelegate Should Look like this:

class AppDelegate: NSObject, NSApplicationDelegate {
    private var aboutBoxWindowController: NSWindowController?

    func showAboutPanel() {
        if aboutBoxWindowController == nil {
            let styleMask: NSWindow.StyleMask = [.closable, .miniaturizable,/* .resizable,*/ .titled]
            let window = NSWindow()
            window.styleMask = styleMask
            window.title = "About My App"
            window.contentView = NSHostingView(rootView: AboutView())
            aboutBoxWindowController = NSWindowController(window: window)
        }

        aboutBoxWindowController?.showWindow(aboutBoxWindowController?.window)
    }
}

Then, just make a SwiftUI View named AboutView and it'll display that in your About Box. For Example:

struct AboutView: View {
    var body: some View {
        VStack {
            Spacer()
            HStack {
                Spacer()
                Text("Hello, World!")
                Spacer()
            }
            Spacer()
        }
        .frame(minWidth: 300, minHeight: 300)
    }
}

import Foundation
import SwiftUI

struct AboutView: View {
    var body: some View {
        VStack(spacing: 10) {
            Image(nsImage: NSImage(named: "AppIcon")!)
            
            Text("\(Bundle.main.appName)")
                .font(.system(size: 20, weight: .bold))
                // Xcode 13.0 beta 2
                //.textSelection(.enabled)
            
            Link("\(AboutView.offSiteAdr.replace(of: "http://", to: ""))", destination: AboutView.offCiteUrl )
            
            Text("Ver: \(Bundle.main.appVersionLong) (\(Bundle.main.appBuild)) ")
                // Xcode 13.0 beta 2
                //.textSelection(.enabled)
            
            Text(Bundle.main.copyright)
                .font(.system(size: 10, weight: .thin))
                .multilineTextAlignment(.center)
        }
        .padding(20)
        .frame(minWidth: 350, minHeight: 300)
    }
}

///////////////////////////////////
/// HELPERS
//////////////////////////////////
class AppDelegate: NSObject, NSApplicationDelegate {
    private var aboutBoxWindowController: NSWindowController?
    
    func showAboutWnd() {
        if aboutBoxWindowController == nil {
            let styleMask: NSWindow.StyleMask = [.closable, .miniaturizable,/* .resizable,*/ .titled]
            let window = NSWindow()
            window.styleMask = styleMask
            window.title = "About \(Bundle.main.appName)"
            window.contentView = NSHostingView(rootView: AboutView())
            window.center()
            aboutBoxWindowController = NSWindowController(window: window)
        }
        
        aboutBoxWindowController?.showWindow(aboutBoxWindowController?.window)
    }
}

extension AboutView {
    private static var offSiteAdr: String { "http://www.taogit.com" }
    private static var offEmail: String { "someUser@gmail.com" }
    
    public static var offCiteUrl: URL { URL(string: AboutView.offSiteAdr )! }
    public static var offEmailUrl: URL { URL(string: "mailto:\(AboutView.offEmail)")! }
}
extension Bundle {
    public var appName: String { getInfo("CFBundleName")  }
    //public var displayName: String {getInfo("CFBundleDisplayName")}
    //public var language: String {getInfo("CFBundleDevelopmentRegion")}
    //public var identifier: String {getInfo("CFBundleIdentifier")}
    public var copyright: String {getInfo("NSHumanReadableCopyright").replace(of: "\\\\n", to: "\n") }
    
    public var appBuild: String { getInfo("CFBundleVersion") }
    public var appVersionLong: String { getInfo("CFBundleShortVersionString") }
    //public var appVersionShort: String { getInfo("CFBundleShortVersion") }
    
    fileprivate func getInfo(_ str: String) -> String { infoDictionary?[str] as? String ?? "⚠️" }
}

And assign to menu line:

struct MyApp: App {
    @NSApplicationDelegateAdaptor(AppDelegate.self) var appDelegate

    var body: some Scene {
        WindowGroup {
            MainView()
        }
        // Replacement of standard About window
        .commands {
            CommandGroup(replacing: CommandGroupPlacement.appInfo) {
                Button("About \(Bundle.main.appName)") { appDelegate.showAboutWnd() }
            }
        }
    }
}

result:

Bonus: Support of "\n" in Copyright