beginBackgroundTaskWithExpirationHandler，從服務器下載數據

Question

我做了一些關於beginBackgroundTaskWithExpirationHandler的研究，我不認爲我正確理解它。我的印象是，如果我們的應用程序進入後臺，那麼我可以在其中包含一個長期任務，該任務將繼續運行。因此，當您第一次運行我們的應用時，您可以從我們的服務器下載更多數據，爲用戶提供更多信息。由於這可能需要一段時間，我希望此任務在後臺和睡眠模式下運行（如果可能）。

我沒有beginBackgroundTaskWithExpirationHandler原來的功能，當用戶點擊下載時調用的樣子：

-(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 
     } 
} 

此功能假定應用程序是在前臺。我試圖做有這個任務在後臺是：

-(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"); 
    } 
    } 

我要對這個在試圖總結我的長期運行的任務在beginBackgroundTaskWithExpirationHandler塊的正確方法？它看起來不像我，因爲expiratinHandler塊中的代碼都沒有被調用。提前致謝！

Answer 1

我只研究此功能首次使用它，但我看到您看起來缺少的是平衡調用endBackgroundTask： 您在過期處理程序的頂部有它，但那只是如果達到時間限制，則調用。它也需要添加到您正在嘗試完成的任務的末尾。假設您的流程在到期之前完成，它將被稱爲何處。

-(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]; 
}];