SwiftUI 为什么不能在视图之间传递发布者？

Question

我只是想做一些这样的测试↓

1. 从第一个视图创建一个发布者
2. 将其传递给第二个视图
3. 在第二个视图中将发布者与某些属性绑定并尝试在屏幕上显示它

代码是↓（第一个视图）

struct ContentView: View {
    
    let publisher = URLSession(configuration: URLSessionConfiguration.default)
        .dataTaskPublisher(for: URLRequest(url: URL(string: "https://v.juhe.cn/joke/content/list.php?sort=asc&page=&pagesize=&time=1418816972&key=aa73ebdd8672a2b9adc9dbb2923184c8")!))
        .map(\.data.description)
        .replaceError(with: "Error!")
        .receive(on: DispatchQueue.main)
        .eraseToAnyPublisher()
        
    
    var body: some View {
        NavigationView {
            List {
                NavigationLink(destination: ResponseView(publisher: publisher)) {
                    Text("Hello, World!")
                }
            }.navigationBarTitle("Title", displayMode: .inline)
        }
    }
}

（第二视图）

struct ResponseView: View {
    
    let publisher: AnyPublisher<String, Never>
    @State var content: String = ""
    
    var body: some View {
        HStack {
            VStack {
                Text(content)
                    .font(.system(size: 12))
                    .onAppear { _ = self.publisher.assign(to: \.content, on: self) }
                Spacer()
            }
            Spacer()
        }
        
    }
}

但代码不起作用。请求失败并显示消息 ↓

2020-11-11 11:08:04.657375+0800 PandaServiceDemo[83721:1275181] Task <6B53516E-5127-4C5E-AD5F-893F1AEE77E8>.<1> finished with error [-999] Error Domain=NSURLErrorDomain Code=-999 "cancelled" UserInfo={NSErrorFailingURLStringKey=https://v.juhe.cn/joke/content/list.php?sort=asc&page=&pagesize=&time=1418816972&key=aa73ebdd8672a2b9adc9dbb2923184c8, NSLocalizedDescription=cancelled, NSErrorFailingURLKey=https://v.juhe.cn/joke/content/list.php?sort=asc&page=&pagesize=&time=1418816972&key=aa73ebdd8672a2b9adc9dbb2923184c8}

有人能告诉我发生了什么吗？正确的方法是什么？

Answer 1

这里的问题是，订阅没有存储在任何地方。您必须将其存储在 AnyCancellable 变量中并保留订阅。

在调试组合相关问题时使用.print() 运算符。我觉得它真的很有用。

---

正确的做法是将发布者和订阅提取到ObservableObject 并注入到View 或使用@StateObject

class DataProvider: ObservableObject {
    @Published var content: String = ""
    private var bag = Set<AnyCancellable>()
    private let publisher: AnyPublisher<String, Never>
    init() {
        publisher = URLSession(configuration: URLSessionConfiguration.default)
            .dataTaskPublisher(for: URLRequest(url: URL(string: "https://v.juhe.cn/joke/content/list.php?sort=asc&page=&pagesize=&time=1418816972&key=aa73ebdd8672a2b9adc9dbb2923184c8")!))
            .map(\.data.description)
            .print()
            .replaceError(with: "Error!")
            .receive(on: DispatchQueue.main)
            .eraseToAnyPublisher()
    }
    
    func loadData() {
        publisher.assign(to: \.content, on: self).store(in: &bag)
    }
}

struct ContentView: View {
    @StateObject var dataProvider = DataProvider()
    
    var body: some View {
        NavigationView {
            List {
                NavigationLink(destination: ResponseView(dataProvider: dataProvider)) {
                    Text("Hello, World!")
                }
            }.navigationBarTitle("Title", displayMode: .inline)
        }
    }
}

struct ResponseView: View {
    let dataProvider: DataProvider
    
    var body: some View {
        HStack {
            VStack {
                Text(dataProvider.content)
                    .font(.system(size: 12))
                    .onAppear {
                        self.dataProvider.loadData()
                    }
                Spacer()
            }
            Spacer()
        }
        
    }
}

请注意，我们使用@StateObject 来确保DataProvider 实例在视图更新时不会被破坏。

Answer 2

订阅需要保存，否则会被反初始化并自动取消。

通常是这样完成的：

var cancellables: Set<AnyCancellable> = []
// ...
publisher
  .sink {...}
  .store(in: &cancellables)

所以，你可以像上面一样创建@State属性，也可以使用.onReceive：

let publisher: AnyPublisher<String, Never>

var body: some View {
   HStack {
     // ...
   }
   .onReceive(publisher) {
      content = $0
   }
}

---

您应该小心使用上述方法，因为如果 ResponseView 被重新初始化，它将获得发布者的副本（大多数发布者都是值类型），所以它会开始一个新的请求。

为避免这种情况，请将.share() 添加到发布者：

let publisher = URLSession(configuration: URLSessionConfiguration.default)
        .dataTaskPublisher(...)
        //...
        .share()
        .eraseToAnyPublisher()

Answer 3

就 SwiftUI 而言，您在做一些根本错误的事情：从视图创建发布者。这意味着 每次 实例化 ContentView 时都会创建一个新的发布者，并且出于各种方式和目的，这可能会发生很多次，SwiftUI 不保证 View 只会实例化一次。

您需要做的是将发布的内容提取到某个对象中，该对象要么从上游注入，要么由 SwiftUI 通过@StateObject 管理。

Answer 4

这个工作有两种方法：一种更好

方式一：

    import SwiftUI




struct ContentView: View {
    
    @State var urlForPublicsh: URL?
    
    
    var body: some View {
        
        VStack
        {
            Text(urlForPublicsh?.absoluteString ?? "nil")
                .padding()
            
            Button("Change the Publisher") {urlForPublicsh = URL(string: "https://stackoverflow.com")}
                .padding()
            
            
            SecondView(urlForPublicsh: $urlForPublicsh)
            
        }
        .onAppear()
        {
            urlForPublicsh = URL(string: "https://www.apple.com")
        }
  
    }
}
struct SecondView: View {
    @Binding var urlForPublicsh: URL?
    
    var body: some View {
        
        Text(urlForPublicsh?.absoluteString ?? "nil")
            .padding()
    }
}

方式 2：

    import SwiftUI

class UrlForPublicshModel: ObservableObject
{
    static let shared = UrlForPublicshModel()
    @Published var url: URL?
}



struct ContentView: View {
    

    @StateObject var urlForPublicshModel = UrlForPublicshModel.shared
    
    var body: some View {
        
        VStack
        {
            Text(urlForPublicshModel.url?.absoluteString ?? "nil")
                .padding()
            
            Button("Change the Publisher") {urlForPublicshModel.url = URL(string: "https://stackoverflow.com")}
                .padding()
            
            
            SecondView()
            
        }
        .onAppear()
        {
            urlForPublicshModel.url = URL(string: "https://www.apple.com")
        }
  
    }
}


struct SecondView: View {

    @ObservedObject var urlForPublicshModel = UrlForPublicshModel.shared
    
    var body: some View {
        
        Text(urlForPublicshModel.url?.absoluteString ?? "nil")
            .padding()

    }
}