开发应用程序都要了解其生命周期。
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 * __nullable principalClassName, NSString * __nullable delegateClassName);
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。
应用程序的生命周期（AppDelegate.m）：
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
// Override point for customization after application launch.
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");
}
