我用UIActivityViewController在我的手机上保存了一个文件。它很简单，也很好用。

import SwiftUI

struct ContentView: View {

    var body: some View {
        Button(action: wirtieFile) {
            Image(systemName: "square.and.arrow.up")
        }
    }

    func wirtieFile() -> Void{
        let file = "test.txt"
        let dir = NSURL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(file)
        let contents = "test..."
        do {
           try contents.write(to: dir!, atomically: true, encoding: .utf8)
        } catch {
           print(error.localizedDescription)
        }
        var filesToShare = [Any]()
        filesToShare.append(dir!)
        let av = UIActivityViewController(activityItems: filesToShare, applicationActivities: nil)
        UIApplication.shared.windows.first?.rootViewController?.present(av, animated: true, completion: nil)
    }
}

正如我已经在评论中发布的那样，你不能以编程方式将文件保存到你的应用程序包之外。你可以使用一个UIDocumentInteractionController，并要求用户选择文件应该写入的位置。
因此，如果您使用SwiftUI，这会比常规的故事板方法稍微复杂一些，正如您在post中所看到的，因为您需要为UIDocumentInteractionController实现UIViewControllerRepresentable：

struct DocumentInteractionController: UIViewControllerRepresentable {

    fileprivate var isExportingDocument: Binding<Bool>
    fileprivate let viewController = UIViewController()
    fileprivate let documentInteractionController: UIDocumentInteractionController

    init(_ isExportingDocument: Binding<Bool>, url: URL) {
        self.isExportingDocument = isExportingDocument
        documentInteractionController = .init(url: url)
    }

    func makeUIViewController(context: UIViewControllerRepresentableContext<DocumentInteractionController>) -> UIViewController { viewController }

    func updateUIViewController(_ controller: UIViewController, context: UIViewControllerRepresentableContext<DocumentInteractionController>) {
        if isExportingDocument.wrappedValue && documentInteractionController.delegate == nil {
            documentInteractionController.uti = documentInteractionController.url?.typeIdentifier ?? "public.data, public.content"
            documentInteractionController.name = documentInteractionController.url?.localizedName
            documentInteractionController.presentOptionsMenu(from: controller.view.frame, in: controller.view, animated: true)
            documentInteractionController.delegate = context.coordinator
            documentInteractionController.presentPreview(animated: true)
        }
    }

    func makeCoordinator() -> Coordintor { .init(self) }
}

及其协调员：

class Coordintor: NSObject, UIDocumentInteractionControllerDelegate {
    let documentInteractionController: DocumentInteractionController
    init(_ controller: DocumentInteractionController) {
        documentInteractionController = controller
    }
    func documentInteractionControllerViewControllerForPreview(_ controller: UIDocumentInteractionController) -> UIViewController { documentInteractionController.viewController }

    func documentInteractionControllerDidDismissOptionsMenu(_ controller: UIDocumentInteractionController) {
        controller.delegate = nil
        documentInteractionController.isExportingDocument.wrappedValue = false
    }
}

现在您可以创建DocumentInteraction视图及其预览：

struct DocumentInteraction: View {
    @State private var isExportingDocument = false
    var body: some View {
        VStack {
            Button("Export Document") { self.isExportingDocument = true }
            .background(DocumentInteractionController($isExportingDocument,
                url: Bundle.main.url(forResource: "sample", withExtension: "pdf")!))
        }
    }
}

struct DocumentInteraction_Previews: PreviewProvider {
    static var previews: some View { DocumentInteraction() }
}

您还需要这些助手：

extension URL {
    var typeIdentifier: String? { (try? resourceValues(forKeys: [.typeIdentifierKey]))?.typeIdentifier }
    var localizedName: String? { (try? resourceValues(forKeys: [.localizedNameKey]))?.localizedName }
}

Sample project

尝试添加“应用程序支持iTunes文件共享”和“支持就地打开文档”到“info.plist”。并将它们的值更改为true。