概要

AppKitではファイルを開く/保存する際にNSOpenPanel/NSSavePanelを使用していましたが、SwiftUIではfileImporter/fileExporterが利用できます。

参考

SwiftUI macOSアプリでファイルを開く
How to export files using fileExporter()
fileExporter(isPresented:document:contentType:defaultFilename:onCompletion:)
fileImporter(isPresented:allowedContentTypes:allowsMultipleSelection:onCompletion:)
fileImporterでデフォルトのディレクトリ指定ができないなど、NSOpenPanelと比べて色々とパラメータが少ないように思います。細かい設定をしたい場合はそちらを使うのがいいかもしれません。

実装

まずファイルのアクセス権が必要となるので、SandboxのUser Selected FileをRead/Writeに設定します。

また保存するファイルの形式に合わせて、FileDocumentのサブクラスを作る必要があります。

import SwiftUI
import UniformTypeIdentifiers

/// refs: https://www.hackingwithswift.com/quick-start/swiftui/how-to-export-files-using-fileexporter
struct TextFile: FileDocument {
    // tell the system we support only plain text
    static var readableContentTypes = [UTType.plainText]

    // by default our document is empty
    var text = ""

    // a simple initializer that creates new, empty documents
    init(initialText: String = "") {
        text = initialText
    }

    // this initializer loads data that has been saved previously
    init(configuration: ReadConfiguration) throws {
        if let data = configuration.file.regularFileContents {
            text = String(decoding: data, as: UTF8.self)
        }
    }

    // this will be called when the system wants to write our data to disk
    func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
        let data = Data(text.utf8)
        return FileWrapper(regularFileWithContents: data)
    }
}

呼び出し例は以下の通りとなります。

import SwiftUI

struct ContentView: View {
    
    @State private var text = ""
    @State private var isSaveButtonDisable = true
    @State private var exporterPresented = false
    @State private var importerPresented = false
    
    var body: some View {
        
        VStack {
            textField()
            HStack {
                Spacer()
                saveButton()
                openButton()
            }
        }
        .padding()
        .fileExporter(isPresented: $exporterPresented,
                      document: TextFile(initialText: text),
                      contentType: .plainText,
                      defaultFilename: "Untitled.txt",
                      onCompletion: { result in
            switch result {
            case .success(let url):
                print("\(url)に保存が完了しました！")
            case .failure(let error):
                print(error.localizedDescription)
            }
        })
        .fileImporter(isPresented: $importerPresented,
                      allowedContentTypes: [.plainText],
                      allowsMultipleSelection: false) { result in
            switch result {
            case .success(let urls):
                guard let url = urls.first else { return }
                do {
                    let contents = try String(contentsOf: url)
                    text = contents
                } catch {
                    print(error.localizedDescription)
                }
            case .failure(let error):
                print(error.localizedDescription)
            }
        }
    }
}

// MARK: - ViewBuilder

extension ContentView {
    
    @ViewBuilder
    private func textField() -> some View {
        TextField("Enter here!", text: $text, axis: .vertical)
            .lineLimit(3, reservesSpace: true)
            .textFieldStyle(.roundedBorder)
            .onChange(of: text) { newValue in
                isSaveButtonDisable = newValue.isEmpty
            }
    }
    
    @ViewBuilder
    private func saveButton() -> some View {
        Button {
            exporterPresented.toggle()
        } label: {
            Text("Save...")
        }
        .disabled(isSaveButtonDisable)
    }
    
    @ViewBuilder
    private func openButton() -> some View {
        Button {
            importerPresented.toggle()
        } label: {
            Text("Open...")
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}