ios - 在 Obj-C 中使用 Switch 语句

Question

下面是一个 Switch/Case 语句，当无法发送电子邮件时显示错误消息。在大多数情况下，一切似乎都是正确的，但是当我将 UIAlertView 放入 Switch 语句时，我在 Xcode 中收到错误消息:

switch (result) {
    case MFMailComposeResultCancelled:
        NSLog(@"Result: Mail sending canceled");
        break;
    case MFMailComposeResultFailed:
        NSLog(@"Result: Mail sending failed");
        UIAlertView *message = [[UIAlertView alloc] initWithTitle:@"Sending Failed"
                                                          message:@"The email could not be sent."
                                                         delegate:nil
                                                cancelButtonTitle:@"OK"
                                                otherButtonTitles:nil];

        [message show];
        break;
    default:
        NSLog(@"Result: Mail not sent");
        break;
}

当我将代码放入 case 时，为什么会产生错误？

Answer 1

放在括号里:

case MFMailComposeResultFailed: {
    NSLog(@"Result: Mail sending failed");
    UIAlertView *message = [[UIAlertView alloc] initWithTitle:@"Sending Failed"
                                                      message:@"The email could not be sent."
                                                     delegate:nil
                                            cancelButtonTitle:@"OK"
                                            otherButtonTitles:nil];

    [message show];
    break;
  }