swift - void 函数中意外的非 void 返回值 - Swift

Question

不让我返回用户名值，因为它在闭包中。有人可以向我解释如何使用@escaping 和 void 闭包

错误:void 函数中出现意外的非 void 返回值

func grabUsername () -> String {
    let uid = Auth.auth().currentUser?.uid
    let database = Firestore.firestore().collection("Users").document(uid!)
    database.getDocument { (docSnapshot, error) in
           guard let docSnapshot = docSnapshot, docSnapshot.exists else {return}
           let mydata = docSnapshot.data()
           let username = mydata!["Username"] as? String ?? ""
           return username
    }
}

Answer 1

getDocument 是异步方法，回调闭包没有返回值，所以你不能从它返回，相反你必须在某处应用结果，例如。在属性(property)中

所以解决方案可以如下

var username: String = ""

// ... other your code

func grabUsername () { // << async, no immediate return
    let uid = Auth.auth().currentUser?.uid
    let database = Firestore.firestore().collection("Users").document(uid!)
    database.getDocument { (docSnapshot, error) in
           guard let docSnapshot = docSnapshot, docSnapshot.exists else {return}
           let mydata = docSnapshot.data()
           let username = mydata!["Username"] as? String ?? ""

           DispatchQueue.main.async {
              self.username = username // << store result
           }
    }
}