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How a non-windowed component can receive messages from Windows


Why do it?

Sometimes we need a non-windowed component (i.e. one that isn't derived from TWinControl) to receive Windows messages. To receive messages the component needs a window handle, but a non-windowed component hasn't got one! This article is about how to enable such a component to use a hidden window to receive messages.

How it's done

My Clipboard Viewer Component is a non-visual component that uses the hidden window techniques described here. The window receives Windows messages that provide information about changes to the clipboard.

The Delphi library function AllocateHWnd is used to create a hidden window for us and the related DeallocateHWnd disposes of the window when we've finished with it.

The hidden window requires window procedure. AllocateHWnd enables us to use a method as a window procedure where Windows normally requires a stdcall function. We pass a reference to the required method to AllocateHWnd and it takes care of the problem of registering the method as a window procedure for us. Inside the registered method we handle the messages we are interested in and hand the rest off to Windows using the DefWindowProc API call.

Listing 2 below provides a skeleton of how to use AllocateHWnd. First though, Listing 1 shows an outline definition for our component class:

  { Our class derived from TComponent 
    or another ancestor class }
  TMyClass = class(TComponent)
    fHWnd: HWND;
      { field to store the window handle }
    procedure WndMethod(var Msg: TMessage); virtual;
      { window proc - called by Windows to handle 
        messages passed to our hidden window }
     constructor Create(AOwner: TComponent); override;
       { create hidden window here: store handle in fHWnd}
     destructor Destroy; override;
       { free hidden window here }

And here are the implementation details:

constructor TMyClass.Create(AOwner: TComponent);
  inherited Create(AOwner);
  // Create hidden window using WndMethod as window proc
  fHWnd := AllocateHWnd(WndMethod);

destructor TMyClass.Destroy;
  // Destroy hidden window
  inherited Destroy;

procedure TMyClass.WndMethod(var Msg : TMessage);
  Handled: Boolean;
  // Assume we handle message
  Handled := True;
  case Msg.Msg of
    WM_SOMETHING: DoSomething;
      // Code to handle a message
    WM_SOMETHINGELSE: DoSomethingElse;
      // Code to handle another message
    // Handle other messages here
      // We didn't handle message
      Handled := False;
  if Handled then
    // We handled message - record in message result
    Msg.Result := 0
    // We didn't handle message
    // pass to DefWindowProc and record result
    Msg.Result := DefWindowProc(fHWnd, Msg.Msg,
      Msg.WParam, Msg.LParam);

Of course, we could just use the Windows API to create a window the hard way and provide a windows procedure. But it is more difficult to use a method as a window procedure if we do it this way. The clever features about AllocateHWnd are that (a) it creates the hidden window for us and (b) it allows us to use a method, rather than a simple procedure, as the window procedure – and a method is more useful since it has access to the class's private data.