ios - 警告 : Application delegate received call to -application:performFetchWithCompletionHandler: but the completion handler was never called

Question

我实现了 performFetchWithCompletionHandler(即:background fetch)从服务器下载一些数据。

为此，在 performFetchWithCompletionHandler 中，为了不阻塞主线程，我创建并启动了一个新任务(因此在后台线程中)，当任务完成后，我调用了 performFetchWithCompletionHandler

给出的 CompletionHandler

但没关系，一旦调用 performFetchWithCompletionHandler，我就会在日志中立即收到(在下载任务完成之前)

Warning: Application delegate received call to -application:performFetchWithCompletionHandler: but the completion handler was never called.

所以我不明白该怎么做？这是否意味着所有的工作都必须在 performFetchWithCompletionHandler (所以在主线程中) 过程中完成并且 performFetchWithCompletionHandler 必须仅在所有作业完成，以这种方式阻塞主线程？

请帮助我理解如何正确地做

这是我的代码(它在 delphi 中，但它完全符合我之前描述的内容，没有我用 sleep(15000) 替换的网络连接):

class procedure TMainForm.performFetchWithCompletionHandler(self: id; _cmd: SEL; application: PUIApplication; completionHandler: id);
begin

  aTaskID := SharedApplication.beginBackgroundTaskWithExpirationHandler(SynchLocationsTaskExpired);

  //load the Profiles in other thread to not block the calling thread
  fSynchLocationsTask := TThread.CreateAnonymousThread(
    procedure
    begin

      Sleep(15000);


      TThread.Queue(nil,
        procedure
        begin
          ExecutePerformFetchCompletionHandler(completionHandler, UIBackgroundFetchResultNewData); // ==> here the app crash like completionHandler is not valid anymore 
          SharedApplication.endBackgroundTask(aTaskID);  
        end);

    end);
  fSynchLocationsTask.FreeOnTerminate := False;
  fSynchLocationsTask.Start;

  // => here i receive in log Warning: Application delegate received call to -application:performFetchWithCompletionHandler: but the completion handler was never called

end;

Answer 1

- (void)application:(UIApplication*)application performFetchWithCompletionHandler:(nonnull void (^)(UIBackgroundFetchResult))completionHandler {
    NSLog(@"Started background fetch");

    // Create URL session based on NSURLSession object which is required for all background fetch operations.
    MyUrlSession* urlSession = MyUrlSession.new;

    // Update is asynchronous so we have to tell to application that
    // we will call completion handler asynchronously. It prevents showing warning
    // that completion handler was never called.
    UIBackgroundTaskIdentifier backgroundTaskIdentifier = [application beginBackgroundTaskWithName:@"MyBgTask" expirationHandler:^{
        // This will cancel update and will call completion handler with error.
        NSLog(@"Expiration handler for background fetch");
        [urlSession cancel];
    }];

    // Start URL session with completion handler.
    [urlSession startWithCompletionHandler:^(BOOL successful, NSString *errorString) {
        NSLog(@"Background fetch completed with success = %d, error = %@", successful, errorString);
        [application endBackgroundTask:backgroundTaskIdentifier];
        completionHandler(successful ? UIBackgroundFetchResultNewData : UIBackgroundFetchResultFailed);
        NSLog(@"Background fetch has ended.");
    }];
}