重复长时间运行后台任务在第一次后台任务后没有启动计时器
我通过重复启动计时器来尝试长时间运行后台任务。但是后台任务首先正常运行，下次继续随机连续调用endBackgroundTask方法。
我的背景任务代码：
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { if([self isMultiTaskingSupported]) { NSLog(@"Multitasking Supported"); } else { NSLog(@"Multitasking Not Supported."); } self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]]; // Override point for customization after application launch. if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone) { self.viewController = [[INNOViewController alloc] initWithNibName:@"INNOViewController_iPhone" bundle:nil]; } else { self.viewController = [[INNOViewController alloc] initWithNibName:@"INNOViewController_iPad" bundle:nil]; } self.window.rootViewController = self.viewController; [self.window makeKeyAndVisible]; return YES; } - (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. [self startBackgroundTask:application]; } -(void) startBackgroundTask:(UIApplication *)application { if([self isMultiTaskingSupported]==NO) { return ; } self.myTimer = [NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:@selector(timerMethod:) userInfo:nil repeats:YES]; self.backgroundTaskIdentifier = [application beginBackgroundTaskWithExpirationHandler:^(void){ [self endBackgroundTask:application]; [self startBackgroundTask:application]; }]; } - (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. if (self.backgroundTaskIdentifier != UIBackgroundTaskInvalid) { [self endBackgroundTask:(UIApplication *)application]; } } -(BOOL) isMultiTaskingSupported { BOOL result = NO; //NSLog(@"before %hhd",result); //NSLog(@"method exist or not -> %hhd",[[UIDevice currentDevice] respondsToSelector:@selector(isMultiTaskingSupported)]); if([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)]) { NSLog(@"isMultiTaskingSupported method available in sdk"); result=[[UIDevice currentDevice] isMultitaskingSupported]; //NSLog(@"after %hhd",result); } return result; } -(void) timerMethod:(NSTimer *)paramSender { NSTimeInterval backgroundTimeRemaining = [[UIApplication sharedApplication] backgroundTimeRemaining]; if(backgroundTimeRemaining == DBL_MAX) { NSLog(@"Background Time = undetermined"); } else { NSLog(@"Background Time = %0.02f",backgroundTimeRemaining); } } -(void) endBackgroundTask:(UIApplication *)application { NSLog(@"endBackgroundTask"); dispatch_queue_t mainQueue = dispatch_get_main_queue(); __weak INNOAppDelegate *weakSelf=self; dispatch_async(mainQueue, ^(void){ //Complete a long background task in AppDelegate INNOAppDelegate *strongSelf = weakSelf; if(strongSelf != nil) { [strongSelf.myTimer invalidate]; [[UIApplication sharedApplication]endBackgroundTask:self.backgroundTaskIdentifier]; strongSelf.backgroundTaskIdentifier=UIBackgroundTaskInvalid; NSLog(@"cleaned BackgroundTask"); } }); } 日志：
2013-04-24 08:50:14.510 MultiTasking[651:c07] Background Time = 17.00 2013-04-24 08:50:15.510 MultiTasking[651:c07] Background Time = 16.00 2013-04-24 08:50:16.510 MultiTasking[651:c07] Background Time = 15.00 2013-04-24 08:50:17.510 MultiTasking[651:c07] Background Time = 14.00 2013-04-24 08:50:18.510 MultiTasking[651:c07] Background Time = 13.00 2013-04-24 08:50:19.510 MultiTasking[651:c07] Background Time = 12.00 2013-04-24 08:50:20.510 MultiTasking[651:c07] Background Time = 11.00 2013-04-24 08:50:21.510 MultiTasking[651:c07] Background Time = 10.00 2013-04-24 08:50:22.510 MultiTasking[651:c07] Background Time = 9.00 2013-04-24 08:50:23.510 MultiTasking[651:c07] Background Time = 8.00 2013-04-24 08:50:24.510 MultiTasking[651:c07] Background Time = 7.00 2013-04-24 08:50:25.510 MultiTasking[651:c07] Background Time = 6.00 2013-04-24 08:50:26.510 MultiTasking[651:c07] Background Time = 5.00 2013-04-24 08:50:26.510 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.511 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.512 MultiTasking[651:c07] cleaned BackgroundTask 2013-04-24 08:50:26.512 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.512 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.513 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.514 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.514 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.515 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.515 MultiTasking[651:c07] cleaned BackgroundTask 2013-04-24 08:50:26.516 MultiTasking[651:c07] cleaned BackgroundTask 2013-04-24 08:50:26.516 MultiTasking[651:c07] cleaned BackgroundTask 2013-04-24 08:50:26.516 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.517 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.518 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.518 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.519 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.519 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.520 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.520 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.521 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.522 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.523 MultiTasking[651:c07] endBackgroundTask 2013-04-24 08:50:26.523 MultiTasking[651:c07] isMultiTaskingSupported method available in sdk 2013-04-24 08:50:26.524 MultiTasking[651:c07] endBackgroundTask
这可能是由于您的到期处理程序块结束任务然后再次启动它。
此外，您仍然在endBackgroundTask块中捕获self （使用strongSelf.backgroundTaskIdentifier ）