beginBackgroundTaskWithExpirationHandler，从服务器下载数据

Question

我做了一些关于beginBackgroundTaskWithExpirationHandler的研究，我不认为我正确理解它。我的印象是，如果我们的应用程序进入后台，那么我可以在其中包含一个长期任务，该任务将继续运行。因此，当您第一次运行我们的应用时，您可以从我们的服务器下载更多数据，为用户提供更多信息。由于这可能需要一段时间，我希望此任务在后台和睡眠模式下运行（如果可能）。

我没有beginBackgroundTaskWithExpirationHandler原来的功能，当用户点击下载时调用的样子：

-(void) HandleDownload:(NSNotification *)notification 
{ 
     DataManager *dmgr = [TestApp sharedDelegate].AppDataManager; 
     NSString *nextPacketName = [dmgr GetNextPacketName]; 
     int totalNumberPackets = [dmgr GetRowCountForTable:@"Manifest"]; 
     if (nextPacketName == nil) 
     { 
      [[NSNotificationCenter defaultCenter] removeObserver:self name:DownloadReferenceSequenceNotification object:nil]; 


      self.ProgressLabel.hidden = YES; 
      self.ProgressBar.hidden = YES; 
     } 
     else 
     { 
      self.ProgressLabel.hidden = NO; 
      self.ProgressLabel.text = [NSString stringWithFormat:@"Downloading %@", nextPacketName]; 
      self.ProgressBar.hidden = NO; 
      self.ProgressBar.progress += 1./(totalNumberPackets); 

      WebServiceManager *wmgr = [WebServiceManager sharedInstance]; 
      [wmgr GetPacket:nextPacketName]; // This function creates the NSURL and starts the request 
     } 
} 

此功能假定应用程序是在前台。我试图做有这个任务在后台是：

-(void) HandleDownload:(NSNotification *)notification 
    { 
UIApplication *app = [UIApplication sharedApplication]; 
    __block UIBackgroundTaskIdentifier taskID; 
    taskID = [app beginBackgroundTaskWithExpirationHandler:^{ 
     NSLog(@"Background task not completed"); 
     [app endBackgroundTask:taskID]; 

      DataManager *dmgr = [TestApp sharedDelegate].AppDataManager; 
      NSString *nextPacketName = [dmgr GetNextPacketName]; 
      int totalNumberPackets = [dmgr GetRowCountForTable:@"Manifest"]; 
      if (nextPacketName == nil) 
      { 
       [[NSNotificationCenter defaultCenter] removeObserver:self name:DownloadReferenceSequenceNotification object:nil]; 


       self.ProgressLabel.hidden = YES; 
       self.ProgressBar.hidden = YES; 
      } 
      else 
      { 
       self.ProgressLabel.hidden = NO; 
       self.ProgressLabel.text = [NSString stringWithFormat:@"Downloading %@", nextPacketName]; 
       self.ProgressBar.hidden = NO; 
       self.ProgressBar.progress += 1./(totalNumberPackets); 

       WebServiceManager *wmgr = [WebServiceManager sharedInstance]; 
       [wmgr GetPacket:nextPacketName]; // This function creates the NSURL and starts the request 
      } 
       NSLog(@"Time remaining: %f", app.backgroundTimeRemaining); 
    }]; 

    if (taskID == UIBackgroundTaskInvalid) { 
     NSLog(@"Failed to create background task identifier"); 
    } 
    } 

我要对这个在试图总结我的长期运行的任务在beginBackgroundTaskWithExpirationHandler块的正确方法？它看起来不像我，因为expiratinHandler块中的代码都没有被调用。提前致谢！

Answer 1

我只研究此功能首次使用它，但我看到您看起来缺少的是平衡调用endBackgroundTask： 您在过期处理程序的顶部有它，但那只是如果达到时间限制，则调用。它也需要添加到您正在尝试完成的任务的末尾。假设您的流程在到期之前完成，它将被称为何处。

-(void) HandleDownload:(NSNotification *)notification 
    { 
UIApplication *app = [UIApplication sharedApplication]; 
    __block UIBackgroundTaskIdentifier taskID; 
    taskID = [app beginBackgroundTaskWithExpirationHandler:^{ 
     NSLog(@"Background task not completed"); 
     [app endBackgroundTask:taskID]; 

     DataManager *dmgr = [TestApp sharedDelegate].AppDataManager; 
     NSString *nextPacketName = [dmgr GetNextPacketName]; 
     int totalNumberPackets = [dmgr GetRowCountForTable:@"Manifest"]; 
     if (nextPacketName == nil) 
     { 
      [[NSNotificationCenter defaultCenter] removeObserver:self name:DownloadReferenceSequenceNotification object:nil]; 


      self.ProgressLabel.hidden = YES; 
      self.ProgressBar.hidden = YES; 
     } 
     else 
     { 
      self.ProgressLabel.hidden = NO; 
      self.ProgressLabel.text = [NSString stringWithFormat:@"Downloading %@", nextPacketName]; 
      self.ProgressBar.hidden = NO; 
      self.ProgressBar.progress += 1./(totalNumberPackets); 

      WebServiceManager *wmgr = [WebServiceManager sharedInstance]; 
      [wmgr GetPacket:nextPacketName]; // This function creates the NSURL and starts the request 
     } 
      NSLog(@"Time remaining: %f", app.backgroundTimeRemaining); 
      [app endBackgroundTask:taskID]; 
}];