How to make a Message Only Window

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Introduction

An article on making a message only window class and why this is a useful thing to do.

Background

According to Microsoft:

A message-only window enables you to send and receive messages. It is not visible, has no z-order, cannot be enumerated, and does not receive broadcast messages. The window simply dispatches messages.

Simpler: You can send messages (PostMessage and SendMessage) to it. If you have a class which is derived from it, you can have it reacting to messages from other areas of code in a similar fashion to a normal window.

Benefits of using a message only window

• Free queue for messages.

  What do you mean?

  If your system is message based from some sort of external IO say, and you are in need of a queuing facility, then your application can use PostMessage to post messages to the hidden window. These messages will be queued in the normal Windows message queue.

• Inter/intra process communication

  What do you mean?

  Another part of your application (or an external application) can handle the arrival of these messages.

How do I make one?

It is pretty easy actually.

In the class that is to deal with the messages, follow this procedure:

1. Make it inherit from CWnd, even if it means you multiply inherit.

   //
   class CMyMessageOnlyWindowClass : public CWnd
   //

2. In the constructor or some other useful initialization type function, use the following code (change where needed).

   //
   CString wnd_class_name = ::AfxRegisterWndClass(NULL);
   BOOL created = this->CreateEx(0,wnd_class_name, 
       "CMyMessageOnlyWindowClass",0 ,0 ,0 ,0 ,0 ,HWND_MESSAGE,0);
   //

3. Either register your own Windows message:

   const CString THIS_IS_MY_MESSAGE_STRING = "THIS_IS_MY_MESSAGE_STRING";
   const UINT MY_REGISTERED_MESSAGE = 
                           ::RegisterWindowMessage(THIS_IS_MY_MESSAGE_STRING);

4. Or use something easier like:

   const UINT MY__MESSAGE = WM_USER+14;

5. Create your message handling function in your .h and .cpp files.

   .h first

   afx_msg LRESULT DealWithMyRegisteredMsg(WPARAM wParam, LPARAM lParam);

   or

   afx_msg LRESULT DealWithMyMsg(WPARAM wParam, LPARAM lParam);

   Now, the cpp file

   LRESULT CMyMessageOnlyWindowClass::DealWithMyRegisteredMsg(WPARAM wParam, 
                                                               LPARAM lParam)
   { 
        AfxMessageBox("Chris is ready for a beer"); 
        return LRESULT(true); 
   }

   or

   LRESULT CMyMessageOnlyWindowClass::DealWithMyMsg(WPARAM wParam, 
                                                         LPARAM lParam)
   { 
        AfxMessageBox("Chris is ready for a beer"); 
        return LRESULT(true); 
   }

6. Add the message map:

   BEGIN_MESSAGE_MAP(CMyMessageOnlyWindowClass, CWnd) 
        ON_REGISTERED_MESSAGE(MY_REGISTERED_MESSAGE, DealWithMyRegisteredMsg) 
        // or ON_MESSAGE(BLAH_MESSAGE,DealWithMyRegisteredMsg)
   END_MESSAGE_MAP()

7. Now, go back to your .h file, and at the very end of your class - after all the functions and variables and so on (but before the “};”), – stick:

   DECLARE_MESSAGE_MAP()

   This says “I will be using a message map in my class” (well, we have just defined it).

8. Now, when you want to post to this window, all you need is the handle to it…or a pointer to it, and you will find it should behave just like a normal window, only you can’t see it etc.

What else do I need to know?

1. FindWindow: If you want to use FindWindow, you need to pay attention to the MSDN which says… "To find message-only windows, specify HWND_MESSAGE in the hwndParent parameter of the FindWindowEx function. In addition, FindWindowEx searches message-only windows as well as top-level windows if both the hwndParent and hwndChildAfter parameters are NULL."
2. Multiple Inheritance: When multiply inheriting, the CWnd needs to come first, for example;

   class CMyMessageOnlyWindowClass : public CWnd, public COtherClass

3. That's about it, remember: you can't view (see) this window!

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