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Multithreading: Creating User-Interface Threads

A user-interface thread is commonly used to handle user input and respond to user events independently of threads executing other portions of the application. The main application thread (provided in your CWinApp-derived class) is already created and started for you. This topic describes the steps necessary to create additional user-interface threads.

The first thing you must do when creating a user-interface thread is derive a class from CWinThread. You must declare and implement this class, using the DECLARE_DYNCREATE and IMPLEMENT_DYNCREATE macros. This class must override some functions and can override others. These functions and what they should do are presented in the following table.

Functions to Override When Creating a User-Interface Thread

Function

Purpose

ExitInstance

Perform cleanup when thread terminates. Usually overridden.

InitInstance

Perform thread instance initialization. Must be overridden.

OnIdle

Perform thread-specific idle-time processing. Not usually overridden.

PreTranslateMessage

Filter messages before they are dispatched to TranslateMessage and DispatchMessage. Not usually overridden.

ProcessWndProcException

Intercept unhandled exceptions thrown by the thread's message and command handlers. Not usually overridden.

Run

Controlling function for the thread. Contains the message pump. Rarely overridden.

MFC provides two versions of AfxBeginThread through parameter overloading: one that can only create worker threads and one that can create user-interface threads or worker threads. To start your user-interface thread, call the second overload of AfxBeginThread, providing the following information:

  • The RUNTIME_CLASS of the class you derived from CWinThread.

  • (Optional) The desired priority level. The default is normal priority. For more information about the available priority levels, see SetThreadPriority in the Windows SDK.

  • (Optional) The desired stack size for the thread. The default is the same size stack as the creating thread.

  • (Optional) CREATE_SUSPENDED if you want the thread to be created in a suspended state. The default is 0, or start the thread normally.

  • (Optional) The desired security attributes. The default is the same access as the parent thread. For more information about the format of this security information, see SECURITY_ATTRIBUTES in the Windows SDK.

AfxBeginThread does most of the work for you. It creates a new object of your class, initializes it with the information you supply, and calls CWinThread::CreateThread to start executing the thread. Checks are made throughout the procedure to make sure all objects are deallocated properly should any part of the creation fail.

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