怎么给BaseTableViewCell设置头像

- (void)setUsername:(NSString *)username

{
    _username = username;

    [self.textLabel setTextWithUsername:_username];

    [self.imageView imageWithUsername:_username placeholderImage:self.imageView.image];
}
怎么 通过 [self.imageView imageWithUsername:_username placeholderImage:self.imageView.image];传头像