Swift 3：在if/else语句中设置变量

Question

我想为我的警告框定义一个动态变量。我从我的服务器得到一个 json，比如 ok，error，username_in_use，等等

let response = parseJSON["message"] as? String

if response == "username_in_use" {
    let error_msg = "Username in already use!"
} else if response == "email_in_use" {
    let error_msg = "Email address in already use!"
} else {
    let error_msg = "Unknown Error!"
}

alertView.showTitle(
    alertTitle: error_msg
)

但我收到了这条消息：

Use of unresolved identifier 'error_msg'

如何为提醒标题设置动态值？

感谢您的帮助，抱歉我的英语不好。

Answer 1

您应该在 if 语句之外声明 let error_msg，以便在将其传递给警报时可见。

   let response = parseJSON["message"] as? String

   var error_msg = ""

   if response == "username_in_use" {
       error_msg = "Username in already use!"
   } else if response == "email_in_use" {
       error_msg = "Email address in already use!"
   } else {
       error_msg = "Unknown Error!"
   }

   alertView.showTitle(alertTitle: error_msg)

Answer 2

在您的代码中，error_msg 的范围仅限于 if 语句中的块。您可以在 if 块范围之外声明 error_msg，例如

let response = parseJSON["message"] as? String
var error_msg:String

if response == "username_in_use" {
    error_msg = "Username in already use!"
} else if response == "email_in_use" {
    error_msg = "Email address in already use!"
} else {
    error_msg = "Unknown Error!"
}

alertView.showTitle(
    alertTitle: error_msg
)

Answer 3

这是 switch 语句的完美用例：

let errorMsg: String

switch response {
case "username_in_use": errorMsg = "Username in already use!"
case "email_in_use": errorMsg = "Email address in already use!"
default: errorMsg = "Unknown Error!"
}

Answer 4

如果只想使用parseJSON字典中键"message"对应的可能存在的（Any?）值直接进一步processing/translation，则需要' 必须存储此值，但可以内联使用它以继续将其处理为有效的错误消息。例如，使用 switch 语句：

var error_msg: String = "Unknown error"
switch parseJSON["message"] {
    case let val as String where val == "username_in_use": error_msg = "Username in already use!"
    case let val as String where val == "email_in_use": error_msg = "Email address in already use!"
    case _: ()
}

// ...
alertView.showTitle(
    alertTitle: error_msg
)

或者，使用辅助字典将 JSON "message" 代码（值）映射到错误消息，轻松允许使用 nil 合并运算符来提供默认（未知错误）消息：

let error_msgs = [
    "username_in_use" : "Username in already use!",
    "email_in_use"    : "Email address in already use!"]

let error_msg = error_msgs[parseJSON["message"] as? String ?? ""] ?? "Unknown Error!"

// ...
alertView.showTitle(
    alertTitle: error_msg
)

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