IOS Application生命周期

浏览数：36 / 时间：2015年06月11日

应用程序的状态

Not running（未运行）:程序没启动

Inactive（未激活）:程序在前台运行，不过没有接收到事件。在没有事件处理情况下程序通常停留在这个状态

Active（激活）:程序在前台运行而且接收到了事件。这也是前台的一个正常的模式

Backgroud（后台）:序在后台而且能执行代码，大多数程序进入这个后台后会在在这个状态上停留一会。时间到之后会进入挂起状态(Suspended)。有的程序经过特殊的请求后可以长期处于Backgroud状态

Suspended（挂起）:程序在后台不能执行代码。系统会自动把程序变成这个状态而且不会发出通知。当挂起时，程序还是停留在内存中的，当系统内存低时，系统就把挂起的程序清除掉，为前台程序提供更多的内存

入口函数

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

// 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);

argc和argv是为了与C语言保持一致，在这没用到，不详述。

后面两个参数为principalClassName(主要类名)和delegateClassName(委托类名)。

如果principalClassName是nil，那么它的值将从Info.plist中获取，如果Info.plist中没有，则默认为UIApplication。principalClass这个类除了管理整个程序的生命周期之外什么都不做，它只赋值监听事件然后交给delegateClass去做。

而delegateClass将在工程新建时实例化一个对象。

NSStringFromClass([XYZAppDelegate class])

相当于@"XYZAppDelegate"。

Delegate类实现文件

注意代码中的官方注释。

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    NSLog(@"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(@"WillResignActive");
}

- (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(@"DidEnterBackground");
}

- (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(@"WillEnterForeground");
}

- (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(@"DidBecomeActive");
}

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