连接 <number>：无法在没有连接的情况下确定回退状态

Question

使用结合而不是闭包的 API。使用 URLSession 任务和 .resume 获取带有闭包的资源不会出错，而使用 Combine 获取会给出：

Connection 57: unable to determine fallback status without a connection

struct Item: View {
    @ObservedObject var item: Item
    @State var thumbnail: UIImage?
    
    var body: some View {
        NavigationLink(destination: ItemView(item: item, thumbnail: thumbnail)) {
            ItemRowView(item: item, thumbnail: thumbnail)
        }
        .onAppear {
             let url = URL(string: "valid address")!
             let cancellable = URLSession.shared.dataTaskPublisher(for: url)
                 .receive(on: DispatchQueue.main)
                 .sink(receiveCompletion: { print ("Received completion: \($0).") },
                       receiveValue: { print ("Received data: \($0.data).")})
        }
    }
}

Unable to determine interface type without an established connection Xcode 也有类似的问题。但是，在我的情况下不应该有任何 TLS 错误，因为端点有一个有效的证书并且在没有组合的情况下可以正常工作。

由于不清楚缺少什么回退状态以及为什么没有连接，您能否解释一下错误本身？

Answer 1

似乎cancellable 必须分配给视图的状态变量，如下所示：

struct Item: View {
    @ObservedObject var item: Item
    @State var thumbnail: UIImage?
    @State var cancellable: AnyCancellable?
    
    var body: some View {
        NavigationLink(destination: ItemView(item: item, thumbnail: thumbnail)) {
            ItemRowView(item: item, thumbnail: thumbnail)
        }
        .onAppear {
             let url = URL(string: "valid address")!
             cancellable = URLSession.shared.dataTaskPublisher(for: url)
                 .receive(on: DispatchQueue.main)
                 .sink(receiveCompletion: { print ("Received completion: \($0).") },
                       receiveValue: { print ("Received data: \($0.data).")})
        }
    }
}

否则请求不会被执行。