ios - 在关闭中找不到成员

Question

我有以下代码。 prompt 是一个 UIAlertController。

self.presentViewController(prompt, animated: true, completion: {
     prompt.textFields[0].becomeFirstResponder()
})

但它给了我这个错误:Could not find member 'becomeFirstResponder'。

然而，如果我把它放进去，它就可以正常工作:

self.presentViewController(prompt, animated: true, completion: {
     let foo = 0
     prompt.textFields[0].becomeFirstResponder()
})

为什么当我添加一行无用的代码(如上面)时错误消失了？

Answer 1

根据 Swift 编程语言书籍关于 If 语句和强制展开的部分，

“You can use an if statement to find out whether an optional contains a value. If an optional does have a value, it evaluates to true; if it has no value at all, it evaluates to false. Once you’re sure that the optional does contain a value, you can access its underlying value”

UIAlertController 不必有 textFields，所以因为 textFields 数组是可选的，你必须先打开它才能调用数组内对象的函数，所以它应该看起来像像这样:

self.presentViewController(prompt, animated: true, completion: {
    if let textFields = prompt.textFields {
       textFields[0].becomeFirstResponder()
    }
})