Property cannot be found in forward class object ViewController

在项目中如下代码：

[[AppDelegate instance].viewController.messageTextField resignFirstResponder];

结果提示如下错误：

Property 'messageTextField' cannot be found in forward class object 'ViewController'

解决办法：

在ViewController类中定义以下方法：

-(void)resignTextFieldFirstResponder{

[self.messageTextField
resignFirstResponder];
}

然后再把：

[[AppDelegate instance].viewController.messageTextField resignFirstResponder];

替换成:

ViewController *viewController = AppDelegate instance].viewController;

[viewController resignTextFieldFirstResponder];

即可。