Delphi How to wait for socket answer inside procedure?

Question

For some specific needs i need to create procedure that waits for socket request (or answer) in dll:

TForm1 = class(TForm)
  ServerSocket1: TServerSocket;

......

procedure MyWaitProc; stdcall;
begin
  Go := false;
  while not Go do
  begin
    // Wating...
    // Application.ProcessMessages;     // Works with this line
  end;
end;


procedure TForm1.ServerSocket1ClientRead(Sender: TObject;
  Socket: TCustomWinSocket);
begin
  MessageBoxA(0, PAnsiChar('Received: '+Socket.ReceiveText), '', MB_OK);
  Go := true;
end;

exports
  MyWaitProc;

When I call Application.ProcessMessages everything works fine: application waits for request and then continues. But in my case calling Application.ProcessMessages causes to unlocking main form on host application (not dll's one). When I don't call Application.ProcessMessages application just hangs couse it cannot handle message...

So, how to create such a procedure that's wating for socket answer ? Maybe there a way to wait for socket answer without using Application.ProcessMessages ?

EDIT

I also tried to use TIdTCPServer, for some reasons, the result is the same.

TForm1 = class(TForm)
  IdTCPServer1: TIdTCPServer;
.....

procedure MyWaitProc; stdcall;
begin
  Go := false;
  while not Go do
  begin
    // Waiting ...
    // Application.ProcessMessages;
  end;
end;

procedure TForm1.IdTCPServer1Execute(AContext: TIdContext);
var
  s: string;
begin
  s := AContext.Connection.Socket.ReadString(1);
  AllText := AllText + s;
  Go := True;
end;

Answer 1

TServerSocket runs in non-blocking mode by default, which depends on processing window messages. To remove that dependancy, you have to switch it to blocking mode instead.

TIdTCPServer runs in blocking mode exclusively, so no window messages. If you are having a problem with it, then you are misusing it. For example, in your TServerSocket code, you do not set Go = True until after a response has been received, but in your TServerSocket code you are setting Go = True before reading a response instead.

As an alternative, have a look at Indy's TIdSimpleServer component. TIdSimpleServer is synchronous and only accepts 1 connection at a time, whereas TIdTCPServer is asynchronous and accepts many connections at a time. For example:

TForm1 = class(TForm) 
  ServerSocket: TIdSimpleServer; 

procedure MyWaitProc; stdcall; 
var
  s: String;
begin 
  ServerSocket.Listen;
  s := ServerSocket.IOHandler.ReadLn;
  ServerSocket.Disconnect;
  MessageBox(0, PChar('Received: '+s), '', MB_OK); 
end; 

exports 
  MyWaitProc; 

Answer 2

Rather than creating a loop that occasionally calls Application.ProcessMessages you can create a descendant of TThread and move the socket request to the TThread.Execute method. Use TThread.OnTerminate to notify your form(or any other class) when the thread has completed its work.

There is sample code which gives more details about how to use TThread.

There are several other 3rd party threading libraries that either provide more flexibility or are easier to use than TThread and I would highly recommend any of them over TThread if you are new to multi-threading.

Note: There are some serious side-effects to using Application.ProcessMessages. You are seeing one of them in your code with the dll unlocking the application's mainform. It breaks the single-threaded UI model the VCL is build upon. ProcessMessages has its place but using threads is more appropriate for the situation you're describing.

var Slowpoke: TMyLongRunningProcessThread;

procedure MyWaitProc(Completed:TNotifyEvent)
begin
  Slowpoke := TMyLongRunningProcessThread.Create(True);
  Slowpoke.FreeOnTerminate := True;
  Slowpoke.OnTerminate := Completed;
  Slowpoke.Resume;
end;

MyWaitProc returns immediately after starting the thread so the GUI is free to respond to user actions. When the thread terminates it calls the event handler pointed to by Completed.

Obviously if you need to retrieve data from the thread you'll want to either have the thread write to an accessible memory location before it Frees itself or remove the FreeOnTerminate so the data can be retreived from the thread through a property.