ios - onDismiss() 在绑定(bind)对象更新之前被调用

Question

我不太确定我在标题中使用了正确的短语，但这就是我正在努力实现的目标。我有一个带有按钮的页面，该按钮显示 DocumentPicker 以允许用户从他们的文档目录中选择项目。我已经像这样包装了 UIDocumentPickerViewController:

struct DocumentPicker: UIViewControllerRepresentable {
        
    class Coordinator: NSObject, UIDocumentPickerDelegate {
        var parent: DocumentPicker
        
        init(_ parent: DocumentPicker) {
            self.parent = parent
        }
        
        func documentPicker(_ controller: UIDocumentPickerViewController, didPickDocumentsAt urls: [URL]) {
            parent.urls = urls
            parent.presentationMode.wrappedValue.dismiss()
        }
    }
    
    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }
        
    @Environment(\.presentationMode) var presentationMode
    @Binding var urls: [URL]?
            
    func makeUIViewController(context: Context) -> UIDocumentPickerViewController {
        let pickerViewController = UIDocumentPickerViewController(forOpeningContentTypes: [UTType.movie])
        pickerViewController.allowsMultipleSelection = true
        pickerViewController.delegate = context.coordinator
        return pickerViewController
    }
    
    func updateUIViewController(_ uiViewController: UIDocumentPickerViewController, context: Context) { }
}

我的观点是这样的:

struct VideoList: View {

    @ObservedObject var playlistStore = PlaylistStore()
    
    @State private var showingDocumentPicker = false
    @State private var urls: [URL]?

    // MARK: - View

    var body: some View {

        NavigationView {
            List(playlistStore.items) { item in
                Text(item.title)
            }
            .navigationBarItems(trailing: Button("Add Items") { self.showingDocumentPicker = true })
        }
        .sheet(isPresented: $showingDocumentPicker, onDismiss: updatePlaylistStore) {
            DocumentPicker(urls: self.$urls)
        }
    }

    private func updatePlaylistStore() {
        guard let urls = urls else { return }
        playlistStore.addItems(items: urls)
        self.urls = nil
    }
}

当我选择项目并且工作表关闭时出现问题。 onDismiss 被调用，然后调用 updatePlaylistStore。但是，此时我刚刚选择的 URL 尚不可用。如果我再次添加更多项目，当第二次调用 onDismiss 时，我第一次选择的项目可用。由于我使用的是 DocumentPicker(urls: self.$urls) 我希望 URL 在被选择时立即更新。我在上下文中留下了一个名为 PlaylistStore() 的对象，但我一直在尝试打印 urls 的值，然后再添加到商店以检查我的问题与 PlaylistStore() 无关。谁能发现我做错了什么？任何指导表示赞赏!

Answer 1

有问题的代码基于对某些顺序或调用的假设，但没有关于该顺序的 promise 。

相反，我们应该只对发生的变化使用react。

这里是固定变体。使用 Xcode 12.1/iOS 14.1 测试

struct VideoList: View {
    
    @ObservedObject var playlistStore = PlaylistStore()
    
    @State private var showingDocumentPicker = false
    @State private var urls: [URL]?
    
    // MARK: - View
    
    var body: some View {
        
        NavigationView {
            List(playlistStore.items) { item in
                Text(item.title)
            }
            .navigationBarItems(trailing: Button("Add Items") { self.showingDocumentPicker = true })
        }
        .sheet(isPresented: $showingDocumentPicker) { // no needs in onDismiss
            DocumentPicker(urls: self.$urls)
        }
        .onChange(of: urls) { _ in       // update when urls changed !!
            updatePlaylistStore()
        }
    }
    
    private func updatePlaylistStore() {
        guard let urls = urls else { return }
        playlistStore.addItems(items: urls)
        self.urls = nil
    }
}