如何从完成处理程序返回一个值 [英] how to return a value from completion handler

问题描述

我想将从完成处理程序获得的值作为字符串放入文本视图，但出现错误.这是我尝试做的:

I want to put the value I get from the completion handler into Text view as a string, but I get an error. Here is what I try to do:

    func final(name: String, completion: @escaping (_ message: String)-> Void){
        guard let uid = AuthViewModel.shared.userSession?.uid else {return}
        Firestore.firestore().collection("users").document(uid).collection("chats").document(name).collection("messages").whereField("read", isEqualTo: false).getDocuments { (snapshot, _) in
            guard let documents = snapshot?.documents.compactMap({ $0.documentID }) else {return}
            
            let unread = documents.count
            let unreadString = String(unread)
            completion(unreadString)
        }
    }

试着把它放在这样的文本中:

Try to put it in a text like so:

Text(model.final(name: name, completion: { (message) in
 String(message)
 }))

这是我得到的错误

Type '()' cannot conform to 'StringProtocol'; only struct/enum/class types can conform to protocols

推荐答案

对于简单的解决方案，使用一条消息 @State var.并更新您的视图.这是演示

For simple solution use one message @State var. and update your view. Here is demo

struct ContentView: View {
    
    @State private var message: String = ""
    
    var body: some View {
        Text(message)
            .onAppear {
                model.final(name: "Name") { message in
                    self.message = message
                }
            }
    }
}

这篇关于如何从完成处理程序返回一个值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！