Can TOmniEventMonitor be used in a background thread?

Question

Original Question In our Delphi XE4 application we use a TOmniEventMonitor to receive messages from other tasks. As long as this is running in the main thread, it works fine, but once I put the same code in a task, the TOmniEventMonitor stops receiving messages. I have included a simple example of this below -- clicking Button_TestInMainThread results in a file being written as expected, clicking Button_TestInBackgroundThread does not. Is this by design, or is there some way to get this working while still using TOmniEventMonitor? ``` unit mainform; interface uses Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, OtlTask, OtlTaskControl, OtlComm, OtlEventMonitor; const MY_OMNI_MESSAGE = 134; type TOmniEventMonitorTester = class(TObject) fName : string; fOmniEventMonitor : TOmniEventMonitor; fOmniTaskControl : IOmniTaskControl; constructor Create(AName : string); destructor Destroy(); over…

Accepted Answer

There is no problem with TOmniEventMonitor running inside of a thread, provided there is a message pump handling the messages for it. I put this block of code together to demonstrate. This works as expected.

```
procedure TMyThread.Execute;
var
  Message: TMsg;
begin
  FreeOnTerminate := True;
  fOmniEventMonitor := TOmniEventMonitor.Create(nil);
  fOmniEventMonitor.OnTaskMessage := HandleOmniTaskMessage;
  fOmniTaskControl := fOmniEventMonitor.Monitor(CreateTask(OmniTaskProcedure_OneShotTimer)).Run();
  try
    while not Terminated do
    begin
      if MsgWaitForMultipleObjects(0, nil^, False, 1000, QS_ALLINPUT) = WAIT_OBJECT_0 then
      begin
        while PeekMessage(Message, 0, 0, 0, PM_REMOVE) do
        begin
          TranslateMessage(Message);
          DispatchMessage(Message);
        end;
      end;
    end;
  finally
    fOmniTaskControl := nil;
    fOmniEventMonitor.Free;
  end;
end;
```

From what I can see, the TOmniTaskExecutor waits for messages to specific handles. In your code example, it's the terminate event and a couple of comm handles. The messages for the TOmniEventMonitor are never processed.

Changing your `TTestLauncherTask.Initialize` to the following results in it correctly writing out the file. `DoNothingProc` is just an empty method on the class.

```
function TTestLauncherTask.Initialize() : boolean;
begin
  result := inherited Initialize();
  if result then begin
    fOmniTaskMonitorTester := TOmniEventMonitorTester.Create('background');
    // Tell the task about the event monitor
    Task.RegisterWaitObject(fOmniTaskMonitorTester.fOmniEventMonitor.MessageWindow, DoNothingProc);
  end;
end;
```

I am adding the message window for the TOmniEventMonitor to the Task WaitObject list so the handle is then registered with the `MsgWaitForMultipleObjectsEx` call and waiting for Remi and David to tear my message handling to shreds :)

Can TOmniEventMonitor be used in a background thread?

Solution

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