objective-c弹出一个带文本输入框的alterview,文本框不显示

[qq_28232189]

UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"类别修改" message:@" " delegate:nil cancelButtonTitle:@"取消" otherButtonTitles:@"修改",nil];
UITextField * txt = [[UITextField alloc] init];
txt.backgroundColor = [UIColor whiteColor];
txt.frame = CGRectMake(alert.center.x+65,alert.center.y+48, 150,23);
[alert addSubview:txt];
[alert show];
return;
为啥呀，弹出来的alter什么都有，就是没有文本输入框，大神解释下