在iOS的textfield目标中为文本添加前缀

Question

如何在iOS的文本字段目标c中设置@ test.com（固定字符串，不能更改）这样的前缀字符串，并且用户可以填写userid？ 结果是：userid@test.com

-(void)viewDidLoad {
[super viewDidLoad];
[self.emailField setDelegate:self];
    [self.emailField setSpellCheckingType:UITextSpellCheckingTypeNo];
    [self setShouldUpdateAttributes:YES];

    NSString *fixedString = @"@test.com";
    [self.emailField setText:fixedString];
    [self.emailField addTarget:self action:@selector(textFieldDidChangeText:) forControlEvents:UIControlEventEditingChanged];
    [self textFieldDidChangeText:self.emailField];
}
-(void)textFieldDidChangeText:(UITextField *)sender
{
    if (self.shouldUpdateAttributes) {
        [self setShouldUpdateAttributes:NO];

        NSString *fixedString = @"@test.com";

        NSMutableAttributedString *attributedString = [[NSMutableAttributedString alloc] initWithString:sender.text];

        [attributedString setAttributes:@{NSForegroundColorAttributeName: [UIColor blackColor]} range:NSMakeRange(0, fixedString.length)];

        [attributedString setAttributes:@{NSForegroundColorAttributeName: [UIColor darkGrayColor]} range:NSMakeRange(fixedString.length, sender.text.length - fixedString.length)];

        [sender setAttributedText:attributedString];
        [sender resignFirstResponder];
        [sender becomeFirstResponder];
    }
}

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    [self setShouldUpdateAttributes:YES];
    NSString *fixedString = @"@test.com";
    NSRange substringRange = [textField.text rangeOfString:fixedString];

    if (range.location <= substringRange.location && range.location substringRange.location + substringRange.length) {
        return NO;
    }

    return YES;
}