Question

When I am using AsyncCalls, how do you best get the exception into the main thread? For example:

procedure TUpdater.needsToGoFirst();
begin
  asyncCall := TAsyncCalls.Invoke(procedure 
  begin 
    //some stuff
    if (possiblyTrueCondition = True) then
      raise Exception.Create('Error Message');
    //more stuff
    TAsyncCalls.VCLSync(procedure 
    begin 
      notifyUpdates(); 
    end);
  end);
end;

procedure TUpdater.needsToGoSecond();
begin
  asyncCall2 := TAsyncCalls.Invoke(procedure 
  begin 
    //initial stuff
    asyncCall.Sync();
    //stuff that needs to go second
    TAsyncCalls.VCLSync(procedure 
    begin 
      notifyUpdates(); 
    end);
  end);
end;

I know calling asycCall.Sync will throw the exception for me, but due the the way I’m currently having my thread notify the main thread that updates have been made, I really don’t have anywhere that I’m calling Sync in the main thread. Doing so also proves difficult because I actually have another thread that is calling Sync to make sure some things are set before acquiring resources the first thread should process first.

Do I need to wrap the innards of these functions with a try-catch and use a VCLSync to get the exception to the main thread myself? Is is there a better way to check for exceptions in a main thread idle loop of some kind?

Edit 1

Another thought I had was to create a loops whose only job is to check the IAscynCall references for exceptions and use that to Raise the exception to the main thread, rather than duplicating that code in every post. The needsToGoSecond metod may still get to the exception first, but it will then hold the exception and the loop would catch it there.

Answer 1

You state the following:

I really don’t have anywhere that I’m calling Sync in the main thread. Doing so also proves difficult because I actually have another thread that is calling Sync.

….

Do I need to wrap the innards of these functions with a try-catch and use a VCLSync to get the exception to the main thread myself? Is there a better way to check for exceptions in a main thread idle loop of some kind?

Calling Sync in the other thread will raise the exception there. If you don’t want it raised there, and if it must be raised in the main thread, then there is no option. You simply have to catch any unhandled exceptions yourself, in the async procedure. You can then queue them off to the main thread, perhaps by calling TThread.Queue, posting a Windows message, or some similar queue mechanism. Another option would be to use VCLSync if you don’t mind the synchronisation at that point.

The bottom line is that calling Sync from another thread, and needing the exception to be raised on the main thread, are not compatible goals. Ergo you must catch the exception yourself and stop AsyncCalls dealing with it.

Essentially this is just a broadening of your current approach. At the moment your fire notifications to the main thread, rather than have the main thread sync. After all, I guess you are using an asynchronous approach because you don’t want to sync from the main thread. So, the broadening is that you need to be able to notify the main thread of errors and exceptions, as well as more normal outcomes.