在非主线程中创建窗口
很多朋友都会有过这样的经历,为什么在主线程中创建窗口且窗口工作很正常,但一移到非主线程(有的朋友喜欢叫它为工作线程),却无法正常工作.本文就这个问题和各位探讨,可能无法做到尽善尽美,但能抛砖引玉也算是欣慰了.
在主线程中创建一个能够正常工作的窗口,估计地球人都知道.
这是一段工作正常的代码:
#include "windows.h" HWND g_hWnd = NULL; HINSTANCE g_hInst; LRESULT WndProc(HWND hWnd,UINT wMsg,WPARAM wParam,LPARAM lParam) { return DefWindowProc(hWnd,wMsg,wParam,lParam); } void CreateWnd(void) { WNDCLASS wc = {0}; wc.style = 0; wc.lpfnWndProc = WndProc; wc.cbClsExtra = 0; wc.cbWndExtra = 0; wc.hInstance = g_hInst; wc.hIcon = NULL; wc.hCursor = LoadCursor(NULL, IDC_ARROW); wc.hbrBackground = (HBRUSH)GetSysColorBrush(COLOR_WINDOW); wc.lpszMenuName = NULL; wc.lpszClassName = TEXT("SimpleWindow"); RegisterClass(&wc); g_hWnd = CreateWindowEx(0, TEXT("SimpleWindow"), TEXT("SimpleWindow"), WS_VISIBLE, 0, 0, 200, 200, NULL, NULL, g_hInst, 0); } int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow) { // TODO: Place code here. g_hInst = hInstance; CreateWnd(); //The message loop MSG msg; while(GetMessage(&msg,NULL,0,0)) { TranslateMessage(&msg); DispatchMessage(&msg); } return 0; }
如果我们创建一个线程,然后在这个线程中创建窗口,看看带给我们的是什么:
#include "windows.h" HWND g_hWnd = NULL; HINSTANCE g_hInst; LRESULT WndProc(HWND hWnd,UINT wMsg,WPARAM wParam,LPARAM lParam) { return DefWindowProc(hWnd,wMsg,wParam,lParam); } void CreateWnd(void) { WNDCLASS wc = {0}; wc.style = 0; wc.lpfnWndProc = WndProc; wc.cbClsExtra = 0; wc.cbWndExtra = 0; wc.hInstance = g_hInst; wc.hIcon = NULL; wc.hCursor = LoadCursor(NULL, IDC_ARROW); wc.hbrBackground = (HBRUSH)GetSysColorBrush(COLOR_WINDOW); wc.lpszMenuName = NULL; wc.lpszClassName = TEXT("SimpleWindow"); RegisterClass(&wc); g_hWnd = CreateWindowEx(0, TEXT("SimpleWindow"), TEXT("SimpleWindow"), WS_VISIBLE, 0, 0, 200, 200, NULL, NULL, g_hInst, 0); } DWORD CreateThread(PVOID pArg) { CreateWnd(); return 0; } int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow) { // TODO: Place code here. g_hInst = hInstance; HANDLE hThrd = CreateThread(NULL,0,CreateThread,NULL,0,NULL); CloseHandle(hThrd); //The message loop MSG msg; while(GetMessage(&msg,NULL,0,0)) { TranslateMessage(&msg); DispatchMessage(&msg); } return 0; }
我们似乎什么都没见到,只是窗口一闪,啥都没了.因为g_hWnd为全局变量,我们的理智告诉我们,在主线程没有退出之前,g_hWnd是不会销毁的.而用断点调试,将会发现在WndProc函数中只能接收WM_CREATE及以后一些消息,之后的再也收不到了,特别是WM_PAINT似乎就凭空消失了!那么,代码什么都没变更,只是移动到了分线程中,为何会出现这个问题呢?
一切似乎很简单,在MSDN中我们找到了答案(原文见:http://support.microsoft.com/kb/90975/en-us):
In a multithreaded application, any thread can call the CreateWindow() API to create a window. There are no restrictions on which thread(s) can create windows.
It is important to note that the message loop and window procedure for the window must be in the thread that created the window. If a different thread creates the window, the window won't get messages from DispatchMessage(), but will get messages from other sources. Therefore, the window will appear but won't show activation or repaint, cannot be moved, won't receive mouse messages, and so on.
该段话大意是:窗口在任何线程中都可以创建,但消息循环必须要和创建窗口在同一线程,否则窗口将无法从DispatchMessage()获取任何消息!
原来如此,最重要是这么一句:It is important to note that the message loop and window procedure for the window must be in the thread that created the window.
好吧,那么我们在支线程中放置消息循环代码,看看是什么结果吧:
#include "windows.h" HWND g_hWnd = NULL; HINSTANCE g_hInst; LRESULT WndProc(HWND hWnd,UINT wMsg,WPARAM wParam,LPARAM lParam) { return DefWindowProc(hWnd,wMsg,wParam,lParam); } void CreateWnd(void) { WNDCLASS wc = {0}; wc.style = 0; wc.lpfnWndProc = WndProc; wc.cbClsExtra = 0; wc.cbWndExtra = 0; wc.hInstance = g_hInst; wc.hIcon = NULL; wc.hCursor = LoadCursor(NULL, IDC_ARROW); wc.hbrBackground = (HBRUSH)GetSysColorBrush(COLOR_WINDOW); wc.lpszMenuName = NULL; wc.lpszClassName = TEXT("SimpleWindow"); RegisterClass(&wc); g_hWnd = CreateWindowEx(0, TEXT("SimpleWindow"), TEXT("SimpleWindow"), WS_VISIBLE, 0, 0, 200, 200, NULL, NULL, g_hInst, 0); } DWORD CreateThread(PVOID pArg) { CreateWnd(); //The message loop MSG msg; while(GetMessage(&msg,NULL,0,0)) { TranslateMessage(&msg); DispatchMessage(&msg); } return 0; } int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow) { // TODO: Place code here. g_hInst = hInstance; HANDLE hThrd = CreateThread(NULL,0,CreateThread,NULL,0,NULL); CloseHandle(hThrd); MSG msg; while(GetMessage(&msg,NULL,0,0)) { TranslateMessage(&msg); DispatchMessage(&msg); } return 0; }
一切正常,如同在主线程创建一样!
当然了,还有点需要注意的,在这个例子中,由于消息循环在主线程和分线程都分别存在,如果在WndProc()调用PostQuitMessage(),那么退出的也仅仅是分线程,而主线程还是会不停地在等待消息,从而导致程序无法正常退出.不过倒不用过分担心,和这个示例代码不同,在实际代码编写中,在主线程往往都会创建主窗口,而在这个主窗口消息处理函数调用PostQuitMessage()则完全可以让主线程正常退出.
事实告诉我们,非主线程创建窗口也能工作正常,只要我们注意一点:消息循环必须要和创建窗口在同一线程!
