IOS多任务机制是本文要介绍的内容，对IOS程序持久化很感兴趣，于是首先研究了下iOS的多任务机制.对于大多数的应用，如果不是特别需要，用***的SDK编译出来的程序本身就是支持多任务的—按home键程序进入后台运行(但是注意此时的程序并不是会运行，只是进入后台状态便于其再次进入活动状态，这一点同我们概念中应该有的多任务有区别).而对于一些应用，是需充分运用iOS多任务的特性,如游戏应用和需要网络连接的应用等等。
多任务机制是苹果在iOS 4中引进的，我们首先新建一个工程,查看下appdelegate文件中的内容:
- App cycle
- - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
- { /
- / Override point for customization after application launch.
- // Set the view controller as the window's root view controller and display. 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.
- */
- }
- - (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, called instead of applicationWillTerminate: when the user quits.
- */
- }
- - (void)applicationWillEnterForeground:(UIApplication *)application
- {
- /*
- Called as part of transition from the background to the inactive state: here you can undo many of the changes made on entering the background.
- */
- }
- - (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.
- */
- }
- - (void)applicationWillTerminate:(UIApplication *)application
- {
- /* Called when the application is about to terminate.
- See also applicationDidEnterBackground:.
- */
- }
以上的函数就是ios4中引入的用于支持多任务运行的函数.从函数名我们就可以猜测到程序可能的一些状态:background,active. 图1就是程序的生命周期图.
图1
我们来看一下:程序***次启动,从Not Running状态进入active阶段,程序会调用两个函数:
- didFinishLaunchingWithOptions:和
- applicationDidBecomeActive:
此时点击home键,程序就会进入后台直至进入挂起状态,程序会依次调用下述函数:
- applicationWillResignActive:
- applicationDidEnterBackground:
这时双击home键再进入该程序时,程序会依次调用:
- applicationWillEnterForeground:
- applicationDidBecomeActive:
注意到Background包含running和suspend两种状态.这里的running不是真正意义上的程序运行,而是指的applicationDidEnterBackground:运行部分,通常情况下,系统给此函数执行的时间不会太多,并且执行完后程序就在后台挂起.这就是iOS多任务的绝大部分情形,但是等一下,就三种情况苹果是允许你在后台运行的
音乐
位置
VoIP
你可以在程序中的plist文件中进行设置,如图2:
图2
可以这么说,除了这三种服务允许以我们通常认为的多任务机制在后台运行,其他的程序的多任务,就是上文所指的程序状态.这里不讨论为啥苹果要这样做,只是稍微说明一下,在这种情况下,至少是在表面上实现了多任务:比如说我玩了愤怒的小鸟又想看下天气,然后又想玩小鸟了,这时切换就比较容易,因为程序这时是在后台挂起,还是在内存中运行着的,这样再启动时就会比较快.
程序调用相应函数的时候,系统会发送相应的Notification,这时app就应该适时的保存app状态或是读取app的历史状态,这样才能更好的呈现用户体验，之前说过,程序进入后台的时间很短,有时候app要做的操作还没来得及进行.这时,我们可以使用beginBackgroundTaskWithExpirationHandler:来处理耗时可能比较长的操作.
iOS所谓的多任务并不是我们通常以为的多任务,IOS只允许三种服务在后台运行;其他的只是方便多个app之间的切换.，至于如何实现多任务,比如进入后台程序应该优先进行哪些操作的细节,会在以后结合本人的实际加以说明。
小结：详解iOS多任务机制 初学者必看文档的内容介绍完了，希望本文读你有所帮助！
