/*
This example uses the MakeObjectInstance function to subclass
a form's WinProc for the purpose of trapping Windows messages.
This needs to be done for components that are not decendents
of TWinControl.  MakeObjectInstance is used here to allocate
a block of memory for the class object.  Then we fill the
memory with the machine code necessary to jump to a unique
class object member function, and then use the address of that
block of memory as the callback address.
This example was furnished by Robert Dunn and nicely described
at: http://home.att.net/~robertdunn/Papers/MethodCallbacks.html
Copyright © 1998-2001 Thin Air Enterprises and Robert Dunn.
All rights reserved.
*/
class TMyComponent : public TComponent
{
protected:
  void* FNewWndProcInst;
  void* FOldWndProcInst;
  virtual void __fastcall MyWndProc(Messages::TMessage& Msg);
public:
  __fastcall TMyComponent(TComponent* Owner);
  __fastcall ~TMyComponent(void);
};

__fastcall TForm1::TForm1(TComponent* Owner): TForm(Owner)
{
}

// This is the constructor. Create the instance callback and
// save it to FNewWndProcInst and save the old window proc to
// FOldWndProcInst.  Subclass the form.
//
__fastcall TMyComponent::TMyComponent(TComponent* Owner)
  : TComponent(Owner), FOldWndProcInst(0), FNewWndProcInst(0)
{
  // if the owner is a form... (your needs may differ)
  TForm* form = dynamic_cast<TForm*>(Owner);
  if (form)
  {
    FNewWndProcInst = MakeObjectInstance(MyWndProc);
    FOldWndProcInst = (void*) ::SetWindowLong(
      form->Handle, GWL_WNDPROC, (LONG) FNewWndProcInst);
    if (!FOldWndProcInst) { /* error */ };
  }
  // other initialization
}

// This is the destructor. Restore the original WndProc and
// free the instance callback.
//
__fastcall TMyComponent::~TMyComponent(void)
{
  if (FNewWndProcInst)
  {
    (void*)SetWindowLong(
      Form1->Handle, GWL_WNDPROC, (LONG) FOldWndProcInst);
    FreeObjectInstance(FNewWndProcInst);
  }
}

// This is the callback method, in this example it is used
// to filter messages to the underlying Windows control.
//
TMessage oldMsg;

void __fastcall TMyComponent::MyWndProc(Messages::TMessage& msg)
{
  // Do whatever you want with the message before calling
  // (or not calling) the the original WndProc() to handle
  // the message call to the original WndProc.
  // Note that, if you have instantiated multiple objects
  // of this class, then you are actually chaining back to
  // MyWndProc() in each earlier instantiation until you get
  // back to the original WndProc.
  msg.Result = CallWindowProc(
    (int(__stdcall*)()) FOldWndProcInst, Form1->Handle,
    msg.Msg, msg.WParam, msg.LParam);
  // do whatever you want after the original WndProc() has handled the message

  if (oldMsg.Msg != msg.Msg)
  {
    Form1->ListBox1->Items->Add(
      "Message recieved: Msg: " + IntToStr((int)msg.Msg) + " WParam: " +
      IntToStr((int)msg.WParam) + " LParam: " + IntToStr((int)msg.LParam));
    oldMsg = msg;
  }
}

TMyComponent *mycomp;

void __fastcall TForm1::FormCreate(TObject *Sender)
{
  mycomp = new TMyComponent(this);
  oldMsg.Msg = 0;
  oldMsg.WParam = 0;
  oldMsg.LParam = 0;
}

void __fastcall TForm1::FormDestroy(TObject *Sender)
{
  delete mycomp;
}