The basic idea behind command enabling is that the decision to enable or disable a function should be made by the object that handles the command.

ObjectWindows does this by sending the WM_COMMAND_ENABLE message through the same command chain as a WM_COMMAND event. The event is then received by the window that implements the functionality that you are enabling or disabling. When a WM_COMMAND_ENABLE message is sent depends on the type of command item that is affected. TFrameWindow performs command enabling for menu items when the user clicks a menu, spawning a WM_INITMENUPOPUP message. Gadget windows perform command enabling for control bar buttons during the window's idle processing.

To handle command-enabling messages for a particular function,

Add a member function to the window class to handle the command-enabling message. This function should return void and take a single parameter, a reference to a TCommandEnabler object. The abstract base class TCommandEnabler is declared in the header file window.h.
Place the EV_COMMAND_ENABLE macro in the parent window's response table. This macro takes two parameters, the command identifier and the name of the handler function.

Suppose you have a frame window class that handles a File|Save menu command that uses the command identifier CM_FILESAVE. The class definition would look something like this:

class TMyFrame : public TFrameWindow
{
 public:
  TMyFrame(TWindow* parent = 0, char* title = 0)
   :
TFrameWindow(parent, title), IsDirty(false) {}
 
 protected:
  voidCmFileSave();
 
 DECLARE_RESPONSE_TABLE(TMyFrame);
};
 
DEFINE_RESPONSE_TABLE(TMyFrame)
 EV_COMMAND(CM_FILESAVE,CmFileSave),
END_RESPONSE_TABLE;

If you did not want the user to be able to access the File|Save command if the file had not been modified since it was opened or last saved, you could add a handler function and response table macro to affect the CmFileSave function like this:

class TMyFrame : public
TFrameWindow
{
 public:
  TMyFrame(TWindow *parent = 0, char *title = 0)
   :
TFrameWindow(parent, title), IsDirty(false) {}
 
 protected:
  void CmFileSave();
 
  // This is the command-enabling handler function.
  void
CeFileSave(TCommandEnabler& commandEnabler);
 
 DECLARE_RESPONSE_TABLE(TMyFrame);
};
 
DEFINE_RESPONSE_TABLE(TMyFrame)
 EV_COMMAND(CM_FILESAVE, CmFileSave),
 EV_COMMAND_ENABLE(CM_FILESAVE, CeFileSave),
END_RESPONSE_TABLE;

Notice that the EV_COMMAND macro and the EV_COMMAND_ENABLE macro both use the same command identifier. Often a single function can be accessed through multiple means. For example, many applications let you open a file through a menu item and also through a button on the control bar. Command enabling in ObjectWindows lets you use a single, common identifier to do command enabling for all means of accessing a function, making it unnecessary to write separate command-enabling functions for each different command item.

See Also