ios - textfield 的rightview 的显示问题

有如下 代码:
给textfield设置了一个rightview
然后设置了 field.rightViewMode = UITextFieldViewModeAlways;
为什么还是只有当这个field成为firsetresponder的时候这个rightview 才回显示出来?
求大虾解答

{
        button *button = [[button alloc] init];

        [button setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
        button.titleLabel.font = [UIFont systemFontOfSize:11];
        button.layer.cornerRadius = 5.f;
        button.layer.masksToBounds = YES;
        [button addTarget:self action:@selector(acquireButtonClick) forControlEvents:UIControlEventTouchUpInside];
        self.acquireButton = button;
        if (_seconds > 0) {
            [_acquireButton setTitle:[NSString stringWithFormat:@"%@秒后重试",@(_seconds)] forState:UIControlStateDisabled];
            _acquireButton.enabled = NO;
            [self validTimer];
        } else {
            _acquireButton.enabled = YES;
             [button setTitle:@"获取验证码" forState:UIControlStateNormal];
            [self invalidTimer];
        }
    }

    {
        UITextField *field = [[UITextField alloc] init];
        field.placeholder = @"请输入验证码";
        field.backgroundColor = [UIColor clearColor];
        field.borderStyle = UITextBorderStyleNone;
        field.font = [UIFont systemFontOfSize:14];
        field.textColor = [UIColor appGray150Color];
        field.rightView = _acquireButton;
        field.rightViewMode = UITextFieldViewModeAlways;
        [self addSubview:field];
        self.codeFieled = field;
    }