1. application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
Tells the delegate when the application has launched and may have additional launch options to handle.
2. applicationWillResignActive:(UIApplication *)application
Tells the delegate that the application is about to become inactive.This method is called to let your application know that it is about to move from the active to inactive state.After calling this method, the application also posts a UIApplicationWillResignActiveNotification notification to give interested objects a chance to respond to the transition.
在应用程序将要由活动状态切换到非活动状态时候，要执行的委托调用，如 按下 home 按钮，返回主屏幕，或全屏之间切换应用程序等。
3. applicationDidEnterBackground:(UIApplication *)application
Tells the delegate that the application is now in the background.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.
4. applicationWillEnterForeground:(UIApplication *)application
Tells the delegate that the application is about to enter the foreground.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.
在应用程序将要进入前台时(被激活)，要执行的委托调用，刚好与 applicationWillResignActive 方法相对应。
5. applicationDidBecomeActive:(UIApplication *)application
Tells the delegate that the application has become active.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.
在应用程序已被激活后，要执行的委托调用，刚好与 applicationDidEnterBackground 方法相对应。
6. applicationWillTerminate:(UIApplication *)application
Tells the delegate when the application is about to terminate.Called when the application is about to terminate. Save data if appropriate.