IOS应用生命周期

应用的生命周期主要涉及到的是UIApplicationDelegate的一些方法，当应用处于不同状态就会调用不同的方法：

启动应用时调用顺序：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions 

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

从前台进入后台时：

- (void)applicationWillResignActive:(UIApplication *)application;

- (void)applicationDidEnterBackground:(UIApplication *)application 

从后台进入前台：

- (void)applicationWillEnterForeground:(UIApplication *)application 

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

关闭应用时：

- (void)applicationWillResignActive:(UIApplication *)application;

- (void)applicationDidEnterBackground:(UIApplication *)application 

- (void)applicationWillTerminate:(UIApplication *)application;