正常情況下，在AppDelegate中實現下面兩個方法，能夠監聽從後台恢復到前台

[cpp] 
- (void)applicationDidEnterBackground:(UIApplication *)application 
{ 
    log4info(@"---applicationDidEnterBackground----"); 
   //進入後台  
} 
 
- (void)applicationDidBecomeActive:(UIApplication *)application 
{ 
 
    NSLog(@"---applicationDidBecomeActive----"); 
    //進入前台  
 
     
} 

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    log4info(@"---applicationDidEnterBackground----");
   //進入後台
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{

    NSLog(@"---applicationDidBecomeActive----");
    //進入前台

   
}

但是單個UIViewController怎麼監聽呢

在-viewDidLoad方法中，添加一下代碼監聽notification

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

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(someMethod:)
                                             name:UIApplicationDidBecomeActiveNotification object:nil];
別忘了在-dealloc中將監聽移除：

[cpp] 
[[NSNotificationCenter defaultCenter] removeObserver:self]; 

[[NSNotificationCenter defaultCenter] removeObserver:self];