AppDelegate中几个常用的回调调用时机

//当应用程序启动时（不包括已在后台的情况下转到前台），调用此回调launchOptions是启动参数，假如用户通过点击push通知启动的应用，这个参数里会存储一些push通知的信息。
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { 
   return YES;
}

//当应用从活动状态主动到非活动状态的应用程序时会调用这个方法。这可导致产生某些类型的临时中断（如传入电话呼叫或SMS消息）。或者当用户退出应用程 序，它开始过渡到的背景状态。使用此方法可以暂停正在进行的任务，禁用定时器，降低OpenGL ES的帧速率。游戏应该使用这种方法来暂停游戏。
//调用时机可能有以下几种：锁屏，按HOME键，下接状态栏，双击HOME键弹出低栏，等情况。
- (void)applicationWillResignActive:(UIApplication *)application {
// Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
// Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
}

//当用户从台前状态转入后台时，调用此方法。使用此方法来释放资源共享，保存用户数据，无效计时器，并储存足够的应用程序状态信息的情况下被终止后，将应用 程序恢复到目前的状态。如果您的应用程序支持后台运行，这种方法被调用，否则调用applicationWillTerminate：用户退出。
- (void)applicationDidEnterBackground:(UIApplication *)application {
// Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
// If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
}


//当应用在后台状态，将要进行动前台运行状态时，会调用此方法。
//如果应用不在后台状态，而是直接启动，则不会回调此方法。
- (void)applicationWillEnterForeground:(UIApplication *)application {
// Called as part of the transition from the background to the inactive state; here you can undo many of the changes made on entering the background.
}

//当应用程序全新启动，或者在后台转到前台，完全激活时，都会调用这个方法。如果应用程序是以前运行在后台，这时可以选择刷新用户界面。
- (void)applicationDidBecomeActive:(UIApplication *)application {
// Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
}

//当应用退出，并且进程即将结束时会调到这个方法，一般很少主动调到，更多是内存不足时是被迫调到的，我们应该在这个方法里做一些数据存储操作。
- (void)applicationWillTerminate:(UIApplication *)application {
// Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
}

//当用户通过其它应用启动本应用时，会回调这个方法，url参数是其它应用调用openURL:方法时传过来的。
- (BOOL)application:(UIApplication *)application handleOpenURL:(NSURL *)url{
return nil;
}