How can a thread notify an object that does not have a window handle?

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I'm new to multithreading, but not a complete novice. I need to perform a call to a webservice in a worker thread.

In the main thread I have a form (TForm) with a private data member (private string) that only the worker thread will write to (I pass the a pointer to it into the thread before it resumes). When the worker thread has finished its webservice call and written the resultant response xml to the private member on the form, the worker thread uses PostMessage to send a message to the form's handle (which I also passed into the thread before it resumed).

interface

const WM_WEBSERVCALL_COMPLETE = WM_USER + 1;

type
  TWebServiceResponseXML = string;
  PWebServiceResponseXML = ^TWebServiceResponseXML;

  TMyForm = class(TForm)
    ...
  private
    ...
    fWorkerThreadID: Cardinal;
    fWebServiceResponseXML: TWebServiceResponseXML;
  public
    ...
    procedure StartWorkerThread;
    procedure OnWebServiceCallComplete(var Message: TMessage); Message WM_WEBSERVCALL_COMPLETE;
  end;

  TMyThread = class(TThread)
  private
  protected
    procedure Execute; override;
  public
    SenderHandle: HWnd;
    RequestXML: string;
    ResponseXML: string;
    IMyService: IService;
    PResponseXML: PWebServiceResponseXML;
  end;

implementation

procedure TMyForm.StartWorkerThread;
var
  MyWorkerThread: TMyThread;
begin
  MyWorkerThread := TMyThread.Create(True);
  MyWorkerThread.FreeOnTerminate := True;
  MyWorkerThread.SenderHandle := self.Handle;
  MyWorkerThread.RequestXML := ComposeRequestXML;
  MyWorkerThread.PResponseXML := ^fWebServiceResponseXML;
  MyWorkerThread.Resume;
end;

procedure TMyForm.OnWebServiceCallComplete(var Message: TMessage);
begin
  // Do what you want with the response xml string in fWebServiceResponseXML
end;

procedure TMyThread.Execute;
begin
  inherited;
  CoInitialize(nil);
  try
    IMyService := IService.GetMyService(URI);
    ResponseXML := IMyService.Search(RequestXML);
    PResponseXML := ResponseXML;
    PostMessage(SenderHandle, WM_WEBSERVCALL_COMPLETE, 0, 0);
  finally
    CoUninitialize;
  end;
end;

It works great, but now I want to do the same thing from a datamodule (which doesn't have a Handle)... so I would really appreciate some useful code to supplement the working model I have.

EDIT

What I really want is the code (if possible) that would allow me to replace the line

MyWorkerThread.SenderHandle := self.Handle;

with

MyWorkerThread.SenderHandle := GetHandleForThisSOAPDataModule;


I have used this technique before with some success: Sending messages to non-windowed applications

Basically, use a second thread as a message pump on a handle obtained via AllocateHWND. This is admittedly irritating, and you would be better off using a library to handle all the details. I prefer OmniThreadLibrary but there are others - see How Do I Choose Between the Various Ways to do Threading in Delphi? and Delphi - Threading frameworks.