UIAlertView输入提示

我在我的应用程序中需要输入提示,我已经尝试过了

// Create a new item
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"New item" message:@"Enter a name for the item" delegate:nil cancelButtonTitle:@"Cancel" otherButtonTitles:@"Add", nil];
alert.alertViewStyle = UIAlertViewStylePlainTextInput;
[alert show];

然后像这样处理它:

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex{
// The user created a new item, add it
if (buttonIndex == 1) {
    // Get the input text
    NSString *newItem = [[alertView textFieldAtIndex:0] text];
}
}

但它看起来不像clickedButtonAtIndex被调用,为什么？

亲切的问候,Erik