这是一个很好的例子 guard
@IBAction func LoginBtn(sender: AnyObject) {
guard let userName = userNameTF.text where !userName.isEmpty else {
displayMyAlertMessage ("Forget to fill your user name")
return
}
guard let password = passwordTF.text where !password.isEmpty else {
displayMyAlertMessage ("Forget to fill your password")
return
}
// do something with userName and password
}
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