应用程序执行的生命周期

#import <UIKit/UIKit.h>

#import "AppDelegate.h"

int main(int argc, char * argv[])
{
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

int main(int argc, char * argv[])
{
    for(int i=0; i<argc; i++)
    {
        NSLog(@"arg %i: %s",i,argv[i]);
    }
    
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

int UIApplicationMain (
   int argc,
   char *argv[],
   NSString *principalClassName,
   NSString *delegateClassName
);

- (BOOL)application:(UIApplication *)application willFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    return YES;
}

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    return YES;
}

- (BOOL)application:(UIApplication *)application willFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [application setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
    
    NSLog(@"current state:%d.(%d,%d,%d)",application.applicationState,
                                         UIApplicationStateActive,
                                         UIApplicationStateInactive,
                                         UIApplicationStateBackground);
    return YES;
}

- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
    //do something...
}

- (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.
}

case 1：当按下home键
function:[applicationWillResignActive], state:[Active]
function:[applicationDidEnterBackground], state:[Background]

case 2：当双击home键，然后选择返回当前app：
function:[applicationWillResignActive], state:[Active]
function:[applicationDidBecomeActive], state:[Active]

case 3：当双击home键，然后选择其它app：
function:[applicationWillResignActive], state:[Active]
function:[applicationDidEnterBackground], state:[Background]

case 4：当有来电，拒绝接听：
function:[applicationWillResignActive], state:[Active]
function:[applicationDidBecomeActive], state:[Active]

case 5：当有来电，接听后并挂断：
function:[applicationWillResignActive], state:[Active]
function:[applicationDidEnterBackground], state:[Background]
function:[applicationWillEnterForeground], state:[Background]
function:[applicationDidBecomeActive], state:[Active]

case 6：当有短信或顶部的推送消息，点击查看：
function:[applicationWillResignActive], state:[Active]
function:[applicationDidEnterBackground], state:[Background]

- (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.
}

- (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.
}

- (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.
}

- (void)applicationWillTerminate:(UIApplication *)application
{
    // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
}