我有以下代码.提示是一个UIAlertController.
self.presentViewController(prompt,animated: true,completion: {
prompt.textFields[0].becomeFirstResponder()
})
@H_301_4@但它给了我这个错误：无法找到成员’becomeFirstResponder’.
@H_301_4@然而,如果我把它放在它的工作正常：
self.presentViewController(prompt,completion: {
let foo = 0
prompt.textFields[0].becomeFirstResponder()
})
@H_301_4@当我添加如上所述的无用代码行时,为什么错误会消失？
解决方法
根据关于If语句和强制解包的Swift编程语言一书的部分,
@H_301_4@“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.@H_301_4@UIAlertController不必具有textField,因为textFields数组是可选的,你必须先打开它才能调用数组内部对象的函数,所以看起来应该是这样的：
Once you’re sure that the optional does contain a value,you can access its underlying value”
self.presentViewController(prompt,completion: {
if let textFields = prompt.textFields {
textFields[0].becomeFirstResponder()
}
})
