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Message handler

Message handlers are methods that implement responses to dynamically dispatched messages. Delphi’s VCL uses message handlers to respond to Windows messages.

In Delphi a message handler is created by including the message directive in a method declaration, followed by an integer constant between 1 and 49151 which specifies the message ID.

In CBuilder the routine for handling the message can be declared as by the VCL_MESSAGE_HANDLER macro.

For example the two message handlers:

  procedure WMVScroll(var Message: TWMVScroll); 
                      Message WM_VSCROLL;
  procedure WMHScroll(var Message: TWMHScroll); 
                      Message WM_HSCROLL;

are translated to CBuilder C++:

  MESSAGE void __fastcall WMVScroll( TWMVScroll& Message ) 
               /*# WM_VSCROLL */;
  MESSAGE void __fastcall WMHScroll( TWMHScroll& Message ) 
               /*# WM_HSCROLL */;

  BEGIN_MESSAGE_MAP
    VCL_MESSAGE_HANDLER(WM_VSCROLL, TWMVScroll, WMVScroll)
    VCL_MESSAGE_HANDLER(WM_HSCROLL, TWMHScroll, WMHScroll)
  END_MESSAGE_MAP( TPanel )