开发应用程序都要了解其生命周期，开始接触android时也是从应用程序生命周期开始的，android的应用程序生命周期更多是其组件的生命周期，例如Activity、Service。今天我们接触一下iOS应用程序的生命周期，
iOS的入口在main.m文件：
int main(int argc, char *argv[]) { @autoreleasepool { return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class])); } }
main函数的两个参数，iOS中没有用到，包括这两个参数是为了与标准ANSI C保持一致。
UIApplicationMain函数，前两个和main函数一样，重点是后两个，官方说明是这样的：
// If nil is specified for principalClassName, the value for NSPrincipalClass from the Info.plist is used. If there is no // NSPrincipalClass key specified, the UIApplication class is used. The delegate class will be instantiated using init. UIKIT_EXTERN int UIApplicationMain(int argc, char *argv[], NSString *principalClassName, NSString *delegateClassName);
后两个参数分别表示程序的主要类(principal class)和代理类(delegate class)。如果主要类(principal class)为nil，将从Info.plist中获取，如果Info.plist中不存在对应的key，则默认为UIApplication；如果代理类(delegate class)将在新建工程时创建。
根据UIApplicationMain函数，程序将进入AppDelegate.m，这个文件是xcode新建工程时自动生成的。下面看一下AppDelegate.m文件，这个关乎着应用程序的生命周期。
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease]; // Override point for customization after application launch. self.window.backgroundColor = [UIColor whiteColor]; [self.window makeKeyAndVisible]; NSLog(@"iOS_didFinishLaunchingWithOptions"); return YES; } - (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. NSLog(@"iOS_applicationWillResignActive"); } - (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. NSLog(@"iOS_applicationDidEnterBackground"); } - (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. NSLog(@"iOS_applicationWillEnterForeground"); } - (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. NSLog(@"iOS_applicationDidBecomeActive"); } - (void)applicationWillTerminate:(UIApplication *)application { // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:. NSLog(@"iOS_applicationWillTerminate"); }
1、application didFinishLaunchingWithOptions：当应用程序启动时执行，应用程序启动入口，只在应用程序启动时执行一次。若用户直接启动，lauchOptions内无数据,若通过其他方式启动应用，lauchOptions包含对应方式的内容。
2、applicationWillResignActive：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：在应用程序已进入后台程序时，要执行的委托调用。
4、applicationWillEnterForeground：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：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：在应用程序要完全推出的时候，要执行的委托调用，这个需要要设置UIApplicationExitsOnSuspend的键值。
初次启动：
2013-05-24 20:20:31.550 LifeIOS[451:c07] iOS_didFinishLaunchingWithOptions
2013-05-24 20:20:31.551 LifeIOS[451:c07] iOS_applicationDidBecomeActive
按下home键：
2013-05-24 20:22:17.349 LifeIOS[451:c07] iOS_applicationWillResignActive
2013-05-24 20:22:17.350 LifeIOS[451:c07] iOS_applicationDidEnterBackground
点击程序图标进入：
2013-05-24 20:22:56.913 LifeIOS[451:c07] iOS_applicationWillEnterForeground
2013-05-24 20:22:56.914 LifeIOS[451:c07] iOS_applicationDidBecomeActive
程序中没有设置UIApplicationExitsOnSuspend的值，程序不会彻底退出。
看上面iOS的生命周期，是不是和Android的Activity生周期也相似，所以说程序是相通的，对比着学习也是收获最大的。
