正常情况下，在AppDelegate中实现下面两个方法，能够监听从后台恢复到前台

- (void)applicationDidEnterBackground:(UIApplication *)application   
{   
    log4info(@"---applicationDidEnterBackground----");   
   //进入后台    
}   
   
- (void)applicationDidBecomeActive:(UIApplication *)application   
{   
   
    NSLog(@"---applicationDidBecomeActive----");   
    //进入前台    
   
       
}  

但是单个UIViewController怎么监听呢

在-viewDidLoad方法中，添加一下代码监听notification

[[[NSNotificationCenter defaultCenter] addObserver:self   
                                         selector:@selector(someMethod:)   
                                             name:UIApplicationDidBecomeActiveNotification object:nil];

别忘了在-dealloc中将监听移除：

[[NSNotificationCenter defaultCenter] removeObserver:self];