UIApplicationDelegate协议
// app接收到内存警告时调用
- (void)applicationDidReceiveMemoryWarning:(UIApplication *)application;
// app进入后台时调用（比如按了home键）
- (void)applicationDidEnterBackground:(UIApplication *)application;
// app启动完毕时调用
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions;
Application系统事件
程序加载完毕 application: didFinishLaunchingWithOptions:
程序获取焦点 applicationDidBecomeActive:
程序进入后台 applicationDidEnterBackground:
程序失去焦点 applicationWillResignActive:
程序从后台回到前台 applicationWillEnterForeground:
内存警告 applicationDidReceiveMemoryWarning:
程序即将退出 applicationWillTerminate:
原文：http://www.cnblogs.com/lianfu/p/5250753.html