開發(fā)應(yīng)用程序都要了解其生命周期，開始接觸android時也是從應(yīng)用程序生命周期開始的，android的應(yīng)用程序生命周期更多是其組件的生命周期，例如Activity、Service。今天我們接觸一下iOS應(yīng)用程序的生命周期，

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    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函數(shù)的兩個參數(shù)，iOS中沒有用到，包括這兩個參數(shù)是為了與標(biāo)準(zhǔn)ANSI C保持一致。

    UIApplicationMain函數(shù)，前兩個和main函數(shù)一樣，重點是后兩個，官方說明是這樣的：

// 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);

    后兩個參數(shù)分別表示程序的主要類(principal class)和代理類(delegate class)。如果主要類(principal class)為nil，將從Info.plist中獲取，如果Info.plist中不存在對應(yīng)的key，則默認為UIApplication；如果代理類(delegate class)將在新建工程時創(chuàng)建。

    根據(jù)UIApplicationMain函數(shù)，程序?qū)⑦M入AppDelegate.m，這個文件是xcode新建工程時自動生成的。下面看一下AppDelegate.m文件，這個關(guān)乎著應(yīng)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：當(dāng)應(yīng)用程序啟動時執(zhí)行，應(yīng)用程序啟動入口，只在應(yīng)用程序啟動時執(zhí)行一次。若用戶直接啟動，lauchOptions內(nèi)無數(shù)據(jù),若通過其他方式啟動應(yīng)用，lauchOptions包含對應(yīng)方式的內(nèi)容。

   2、applicationWillResignActive：在應(yīng)用程序?qū)⒁苫顒訝顟B(tài)切換到非活動狀態(tài)時候，要執(zhí)行的委托調(diào)用，如 按下 home 按鈕，返回主屏幕，或全屏之間切換應(yīng)用程序等。

   3、applicationDidEnterBackground：在應(yīng)用程序已進入后臺程序時，要執(zhí)行的委托調(diào)用。

   4、applicationWillEnterForeground：在應(yīng)用程序?qū)⒁M入前臺時(被激活)，要執(zhí)行的委托調(diào)用，剛好與applicationWillResignActive 方法相對應(yīng)。

   5、applicationDidBecomeActive：在應(yīng)用程序已被激活后，要執(zhí)行的委托調(diào)用，剛好與applicationDidEnterBackground 方法相對應(yīng)。

   6、applicationWillTerminate：在應(yīng)用程序要完全推出的時候，要執(zhí)行的委托調(diào)用，這個需要要設(shè)置UIApplicationExitsOnSuspend的鍵值。

初次啟動：

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鍵：

2013-05-24 20:22:17.349 LifeIOS[451:c07] iOS_applicationWillResignActive

2013-05-24 20:22:17.350 LifeIOS[451:c07] iOS_applicationDidEnterBackground

點擊程序圖標(biāo)進入：

2013-05-24 20:22:56.913 LifeIOS[451:c07] iOS_applicationWillEnterForeground

2013-05-24 20:22:56.914 LifeIOS[451:c07] iOS_applicationDidBecomeActive

程序中沒有設(shè)置UIApplicationExitsOnSuspend的值，程序不會徹底退出。

   看上面iOS的生命周期，是不是和Android的Activity生周期也相似，所以說程序是相通的，對比著學(xué)習(xí)也是收獲大的。

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網(wǎng)站欄目：iOS學(xué)習(xí)——iOS應(yīng)用程序生命周期(四)-創(chuàng)新互聯(lián)
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