Question

我实施performFetchWithCompletionHandler（即：后台获取）从服务器下载一些数据。

要做到这一点，在performFetchWithCompletionHandler内，为了不阻止主线程，我创建并启动一个新任务（所以在后台线程中），当任务完成时我拨打CompletionHandler

给出的performFetchWithCompletionHandler

但无论如何，只要performFetchWithCompletionHandler被调用，我就会在日志中（以及下载任务完成之前）立即收到

警告：应用程序代表收到了来电 -application：performFetchWithCompletionHandler：但是从未调用完成处理程序。

所以我无法理解怎么办？这是否意味着所有工作必须在程序performFetchWithCompletionHandler 内完成（所以在主线程中）并且performFetchWithCompletionHandler必须仅在所有工作完成时返回，阻止这种方式主线程？

请帮助我了解如何正确使用

这是我的代码（它在delphi中，但它完全按照我之前所描述的，没有我用sleep替换的网络连接（15000））：

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.");
    }];
}

警告：应用程序委托收到对-application的调用：performFetchWithCompletionHandler：但是从未调用完成处理程序

1 个答案: