iOS开发 | 修改textField的placeholder的字体颜色、大小

textField.placeholder = @"username is in here!";  
[textField setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];  
[textField setValue:[UIFont boldSystemFontOfSize:16] forKeyPath:@"_placeholderLabel.font"];  

textField的placeholder属于私有接口, 只能通过setValue forKey 的方法来更改