ios 监听app从后台恢复到前台

1. APPDelegate中的代理方法可以监听到程序前后台切换的状态

- (void)applicationDidEnterBackground:(UIApplication * )application
{
    NSLog(@"---applicationDidEnterBackground----")
    // 进入后台
}

- (void)applicationDidBecomeActive:(UIApplication * )application
{
    NSLog(@"---applicationDidBecomeActive----")
    // 进入前台
}

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    NSLog(@"---applicationDidEnterBackground----")
    // 进入后台
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    NSLog(@"---applicationDidBecomeActive----")
    // 进入前台
}

2. 单个ViewController通过在ViewDidLoad里注册筛选器通知来监听，在dealloc中移除通知

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(someMethod1:)
                                             name:UIApplicationDidBecomeActiveNotification object:nil];