Visual C++ 6 Unleashed

By MICKEY WILLIAMS and David Bennett

• Table of Contents
  • Copyright
  • About the Authors
  • About the Contributors
  • Acknowledgments
  • Tell Us What You Think!
  • Introduction
  • How to Use This Book
  • What You Need to Use This Book
  • What's New in Visual C++ 6.0
  • Contacting the Main Author
  • Part I: Introduction
  • Chapter 1. The Visual C++ 6.0 Environment
  • Part II: MFC Programming
  • Chapter 2. MFC Class Library Overview
    • The Application Framework
    • Creating an MFC Application with MFC AppWizard
    • The CWinApp Class
    • The CWinThread Class
    • The CCmdTarget Class
    • The CObject Class
    • Runtime Type Information
    • Debugging Support
    • Putting It All Together
    • The Message Pump
    • Summary
  • Chapter 3. MFC Message Handling Mechanism
  • Chapter 4. The Document View Architecture
  • Chapter 5. Creating and Using Dialog Boxes
  • Chapter 6. Working with Device Contexts and GDI Objects
  • Chapter 7. Creating and Using Property Sheets
  • Chapter 8. Working with the File System
  • Chapter 9. Using Serialization with File and Archive Objects
  • Part III: Internet Programming with MFC
  • Chapter 10. MFC and the Internet Server API (ISAPI)
  • Chapter 11. The WinInet API
  • Chapter 12. MFC HTML Support
  • Part IV: Advanced Programming Topics
  • Chapter 13. Using the Standard C++ Library
  • Chapter 14. Error Detection and Exception Handling Techniques
  • Chapter 15. Debugging and Profiling Strategies
  • Chapter 16. Multithreading
  • Chapter 17. Using Scripting and Other Tools to Automate the Visual C++ IDE
  • Part V: Database Programming
  • Chapter 18. Creating Custom AppWizards
  • Chapter 19. Database Overview
  • Chapter 20. ODBC Programming
  • Chapter 21. MFC Database Classes
  • Chapter 22. Using OLE DB
  • Chapter 23. Programming with ADO
  • Part VI: MFC Support for COM and ActiveX
  • Chapter 24. Overview of COM and Active Technologies
  • Chapter 25. Active Documents
  • Chapter 26. Active Containers
  • Chapter 27. Active Servers
  • Chapter 28. ActiveX Controls
  • Part VII: Using the Active Template Library
  • Chapter 29. ATL Architecture
  • Chapter 30. Creating COM Objects Using ATL
  • Chapter 31. Creating ActiveX Controls Using ATL
  • Chapter 32. Using ATL to Create MTS and COM+ Components
  • Part VIII: Finishing Touches
  • Chapter 33. Adding Windows Help
  • Part IX: Appendix

The Message Pump

Now that all the initialization for your program has executed, the CWinThread::Run() function will start up the message pump, or message loop, of your application. The message pump will do nothing but wait until it receives a message. At this point, the message is dispatched to a message-handler function, provided by your application or by MFC, that will react to the message. The details of message dispatching are covered in the next chapter.

OnIdle()

When I said that the message pump does nothing but wait around between messages, I stretched the truth just a little. When the message pump finds that the message queue is empty, it will call OnIdle(), which may allow you to do some background processing or update the status of your user interface objects, such as disabling toolbars if needed.

The OnIdle() function will be called repeatedly until it returns FALSE, or until a message is received—at which point, the message is dispatched. When OnIdle() returns FALSE, the message pump will sleep until the next message arrives.

If you choose to implement your own OnIdle() function, you should keep in mind that the message queue for your application (or at least this thread) will not be able to process any messages until OnIdle() returns, because this is a non-preemptive method of multitasking. If you want to do preemptive multitasking, see Chapter 16, "Multithreading." Also, your implementation of OnIdle() should call the base class implementation, which updates user interface objects and cleans up some internal data structures. With this in mind, the declaration of OnIdle() looks like this:

virtual BOOL OnIdle(LONG lCount);

The lCount parameter is incremented each time OnIdle() is called and it is set to 0 when new messages are processed.

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