I'm developing an app using MP as a TOleContainer in Delphi 5. This works great--I can manipulate just about everything that I need to, programatically. I'm trying to receive parms (Lat & Lon) from a separate application using a WM_COPYDATA handler. This works great, too--I can receive the parms just fine. The problem occurs when I make a call to any MP functions from the message handler itself.

Apparently, an external call from WM_COPYDATA handers are a no-no. I get a cryptic error (I think from MP) that essentially declares that it has no idea what happened.

My (inelegant) workaround has been to use a TTimer and a 'message received' flag within the message handler in order to periodically check for new data (which I load from the message handler). Is there a better way to do this? Is there an "after WM_COPYDATA" kind of handler or other notification that someone could recommend? Thanks.

Hi,

Should word if the custom message handler for WM_COPYDATA is executing in same thread context as where you have created MapPoint class.

However a more elegant function instead of a Timer wich does the same thing is to use a second custom message handler and post a message to it. The custom message handler will have same effect as the TTimer event.

Thanks, that is the kind of solution that I was hoping for. Could you advise with some specifics on how to do it? I'm guessing that I should place a SendMessage (or PostMessage) within the existing WMCOPYDATA handler, but how do I create the handler for that? Will it not call the same handler recursively--or is there a way to differentiate the senders?

This is a bit new to me, so I apologize for asking what are probably some fundamental questions.

Thanks again.

Hi,

Not SendMessage but PostMessage. The difference is that SendMessage will block until the message handler is done and we don't want that. We want it to execute a while later outside the message handler for the copydata as you do with the Timer. Typically it will execute 1 time slice later (10 millisec).

Here is little example. Compiled but not tested:

Code:

const
   WM_MYMESSAGE = WM_USER + 1;

type
  TForm1 = class(TForm)
  private
    procedure WMCopyData(var Msg: TMessage); message WM_COPYDATA;
    procedure WMMyMessage(var Msg: TMessage); message WM_MYMESSAGE;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TForm1 }

procedure TForm1.WMCopyData(var Msg: TMessage);
var
   CopyData: PCopyDataStruct;
begin
   // Allocate memory for the struct and make a copy of it
   New(CopyData);
   Move(PCopyDataStruct(Msg.LParam)^, CopyData^, SizeOf(tagCOPYDATASTRUCT));
   PostMessage(Handle, WM_MYMESSAGE, 0, integer(CopyData));
end;

// This procedure is executed a little later when
// the message pump for this window is entered
procedure TForm1.WMMyMessage(var Msg: TMessage);
var
   CopyData: PCopyDataStruct;
begin
   CopyData := PCopyDataStruct(Msg.LParam);
   // Here you do the calls to mappoint or whatever
   // Free the allocated memory for the struct
   Dispose(CopyData);
end;

Perfect. Exactly what I needed--Thanks.