重复长时间运行的后台任务在第一次后台任务后没有启动计时器

Question

我通过重复启动计时器来尝试长时间运行的后台任务。但后台任务只是第一次运行正常，下次再随机地连续调用endBackgroundTask方法。

我的后台任务代码：

- (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"); 


     } 
    }); 


} 

登录：

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 

Answer 1

这可能是由于结束任务的到期处理程序块，然后再次启动。

此外，你还捕捉self在endBackgroundTask块（使用strongSelf.backgroundTaskIdentifier）