xcode 从“字符串”到“字符串”的条件向下转换总是成功 - Swift 错误
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Conditional downcast from 'String' to 'String' always succeeds - Swift Error
提问by Kyle Clegg
I'm trying to essentially do an valid check on a String in Swift, however I'm getting an error Conditional downcast from 'String' to 'String' always succeeds.
我试图对 Swift 中的 String 进行本质上的有效检查，但是我收到了一个错误Conditional downcast from 'String' to 'String' always succeeds。
zipCode is created:
创建邮政编码：
var zipCode = String()
Checking for a valid string at a later time:
稍后检查有效字符串：
if let code = zipCode as? String {
println("valid")
}
Can someone help me understand what I'm doing wrong?
有人可以帮助我了解我做错了什么吗？
回答by vacawama
If zipCodecan be "unset", then you need to declare it as an optional:
如果zipCode可以“未设置”，则需要将其声明为可选：
var zipCode: String?
This syntax (which is known as optional binding):
此语法（称为可选绑定）：
if let code = zipCode {
print("valid")
// use code here
}
is used for checking if an optional variable has a value, or if it is still unset (nil). If zipCodeis set, then codewill be a constant of type Stringthat you can use to safely access the contents of zipCodeinside the ifblock.
用于检查可选变量是否有值，或者它是否仍然未设置 ( nil)。如果zipCode设置，code则将是一个类型的常量String，您可以使用它来安全地访问块zipCode内部的内容if。
