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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

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.

A hidden window requires a 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:

  2  { Our class derived from TComponent 
  3    or another ancestor class }
  4  TMyClass = class(TComponent)
  5  private
  6    fHWnd: HWND;
  7      { field to store the window handle }
  8    ...
  9  protected
 10    procedure WndMethod(var Msg: TMessage); virtual;
 11      { window proc - called by Windows to handle 
 12        messages passed to our hidden window }
 13    ...
 14  public
 15     constructor Create(AOwner: TComponent); override;
 16       { create hidden window here: store handle in fHWnd}
 17     destructor Destroy; override;
 18       { free hidden window here }
 19     ...
 20  end;
Listing 1

And here are the implementation details:

  1constructor TMyClass.Create(AOwner: TComponent);
  3  inherited Create(AOwner);
  4  ...
  5  // Create hidden window using WndMethod as window proc
  6  fHWnd := AllocateHWnd(WndMethod);
  7  ...
 10destructor TMyClass.Destroy;
 12  ...
 13  // Destroy hidden window
 14  DeallocateHWnd(fHWnd);
 15  ...
 16  inherited Destroy;
 19procedure TMyClass.WndMethod(var Msg : TMessage);
 21  Handled: Boolean;
 23  // Assume we handle message
 24  Handled := True;
 25  case Msg.Msg of
 26    WM_SOMETHING: DoSomething;
 27      // Code to handle a message
 28    WM_SOMETHINGELSE: DoSomethingElse;
 29      // Code to handle another message
 30    // Handle other messages here
 31    else
 32      // We didn't handle message
 33      Handled := False;
 34  end;
 35  if Handled then
 36    // We handled message - record in message result
 37    Msg.Result := 0
 38  else
 39    // We didn't handle message
 40    // pass to DefWindowProc and record result
 41    Msg.Result := DefWindowProc(fHWnd, Msg.Msg,
 42      Msg.WParam, Msg.LParam);
Listing 2

Of course, we could just use the Windows API to create a window the hard way and provide a windows procedure. But it is much more difficult to use a method as a window procedure if we do it this way. The cleverness of AllocateHWnd is 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. Obviously, a method is more useful since it has access to the class' private data.

A Real World Example

While there is no demo code to accompany this article, 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.

Feel free to check it out.


I hope you found this article useful.

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