How a non-windowed component can receive messages from Windows
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Why do it?
Sometimes we need a non-windowed component (i.e. one that isn‘t derived fromTWinControl) 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 DeallocateHWnddisposes 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 stdcallfunction. We pass a reference to the required method toAllocateHWnd 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 1shows an outline definition for our component class:
type
{ Our class derived from TComponent
or another ancestor class }
TMyClass = class(TComponent)
private
fHWnd: HWND;
{ field to store the window handle }
...
protected
procedure WndMethod(var Msg: TMessage); virtual;
{ window proc - called by Windows to handle
messages passed to our hidden window }
...
public
constructor Create(AOwner: TComponent); override;
{ create hidden window here: store handle in fHWnd}
destructor Destroy; override;
{ free hidden window here }
...
end;
And here are the implementation details:
constructor TMyClass.Create(AOwner: TComponent);
begin
inherited Create(AOwner);
...
// Create hidden window using WndMethod as window proc
fHWnd := AllocateHWnd(WndMethod);
...
end;
destructor TMyClass.Destroy;
begin
...
// Destroy hidden window
DeallocateHWnd(fHWnd);
...
inherited Destroy;
end;
procedure TMyClass.WndMethod(var Msg : TMessage);
var
Handled: Boolean;
begin
// 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
else
// We didn‘t handle message
Handled := False;
end;
if Handled then
// We handled message - record in message result
Msg.Result := 0
else
// We didn‘t handle message
// pass to DefWindowProc and record result
Msg.Result := DefWindowProc(fHWnd, Msg.Msg,
Msg.WParam, Msg.LParam);
end;
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.
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