--------》不说废话
在返回页面实现下面方法：
- (void)navigationController:(UINavigationController*)navigationController willShowViewController:(UIViewController*)viewController animated:(BOOL)animated；
注意：需要遵循UINavigationControllerDelegate，并使本类成为代理
用法：
- (void)navigationController:(UINavigationController*)navigationController willShowViewController:(UIViewController*)viewController animated:(BOOL)animated{
if([[viewController class]isSubclassOfClass:[目标类 class]]) {
//在此执行刷新操作
}
//删除代理，防止该controller销毁后引起navigationController.delegate指向野指针造成崩溃
if(![[viewController class]isSubclassOfClass:[self class]]) {
self.navigationController.delegate=nil;
}
}