Win32程序不停发送WM_PAINT消息

Wn32编程中，在响应WM_PAINT消息时，很容易造成处理器使用率过高。造成处理器负载过高的原因是窗口时刻都在检查自己的Update Region，如果有则发出WM_PAINT，直到调用BeginPaint和EndPaint后才会将Update Region置空。如果在消息相应函数中没有加入BeginPaint，EndPaint 则 WM_PAINT 就会不断被发送，造成死循环。

如果不主动响应WM_PAINT，则该消息会进入DefWindowProc，它会调用那两个函数。

在MFC中也一样，一般在OnPaint中会出现CPaintDC dc(this) 该类的构造函数会调用BeginPaint，析构函数会调用EndPaint。也可能出现父类的OnPaint，如果把这些注释掉，运行后cpu占用率依旧会很高。

例如下面的一个Win32的例子，如果我们把WM_PAINT处理部分打开，就会造成负载过高。如果注释掉该代码，就没有问题。但是如果在里面添加BeginPaint以及EndPaint，那么就不会造成CPU负载过高这种问题。

#include <windows.h>
#include <aygshell.h>
#pragma comment(lib, "aygshell.lib")

#define IDC_MY_STATIC 100
#define IDC_MY_BUTTON 101
#define IDC_MY_EDIT   102

LRESULT WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

int _tWinMain(HINSTANCE hInstance, HINSTANCE hPreInstance, LPTSTR lpszCmdLine, int nShowCmd)
{
    WNDCLASS wc = {0};
    HWND hWnd = NULL;
    MSG msg = {0};

    DEBUGMSG(TRUE, (TEXT("hInstance=%p, lpszCmdLine=%s, nShowCmd=%d\n"), hInstance, lpszCmdLine, nShowCmd));

    // 1. 注册窗口类（指定一个函数指针）
    wc.style         = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc   = WndProc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance     = hInstance;
    wc.hIcon         = NULL;
    wc.hCursor       = NULL;
    wc.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = TEXT("GWND");
    if (0 == RegisterClass(&wc))
    {
        return -1;
    }

    // 2. 创建窗口
    hWnd = CreateWindow(TEXT("GWND"), TEXT("掌中"), WS_VISIBLE, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL, hInstance, NULL);
    if (!IsWindow(hWnd))
    {
        return -1;
    }

    // 3. 显示窗口
    ShowWindow(hWnd, nShowCmd);
    UpdateWindow(hWnd);

    // 4. 消息循环
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
    
}

LRESULT WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    HINSTANCE hTemp = NULL;
    SHINITDLGINFO shidi = {0};
    WORD wID = 0;
    WORD wEvent = 0;

    DEBUGMSG(TRUE, (TEXT("hWnd=%p, uMsg=%04X, wParam=%p, lParam=%p\n"), hWnd, uMsg, wParam, lParam));
    switch (uMsg)
    {
    case WM_CREATE:
        {
            hTemp = GetModuleHandle(NULL);
            DEBUGMSG(TRUE, (TEXT("hTemp=%p\n"), hTemp));
            CreateWindow(TEXT("BUTTON"), TEXT("OPEN"), WS_CHILD | WS_VISIBLE, 10, 100, 30, 20, hWnd, (HMENU)IDC_MY_BUTTON, hTemp, NULL);

            DEBUGMSG(TRUE, (TEXT("WM_CREATE\n")));
            shidi.dwMask  = SHIDIM_FLAGS;
            shidi.hDlg    = hWnd;
            shidi.dwFlags = SHIDIF_CANCELBUTTON | SHIDIF_SIZEDLGFULLSCREEN;
            SHInitDialog(&shidi);
        }
        break;

     //case WM_PAINT:
     //    {
     //        DEBUGMSG(TRUE, (TEXT("WM_PAINT\n")));
     //    }
     //    break;

    case WM_COMMAND:
        {
            DEBUGMSG(TRUE, (TEXT("WM_COMMAND\n")));
            wID = LOWORD(wParam);
            wEvent = HIWORD(wParam);
            switch (wID)
            {
            case IDCANCEL:
                {
                    DestroyWindow(hWnd);
                }
                break;

            case IDC_MY_BUTTON:
                {
                    MessageBox(hWnd, TEXT("HI"), TEXT("Tip"), MB_OK);
                }
                break;
            }
        }
        break;

    case WM_DESTROY:
        {
            DEBUGMSG(TRUE, (TEXT("WM_DESTROY\n")));
            PostQuitMessage(0);
        }
        break;

    case WM_KEYDOWN:
        {
            DEBUGMSG(TRUE, (TEXT("WM_KEYDOWN\n")));
        }
        break;

    case WM_KEYUP:
        {
            DEBUGMSG(TRUE, (TEXT("WM_KEYUP\n")));
        }
        break;

    case WM_CHAR:
        {
            DEBUGMSG(TRUE, (TEXT("WM_CHAR\n")));
        }
        break;

    case WM_LBUTTONDOWN:
        {
            DEBUGMSG(TRUE, (TEXT("WM_LBUTTONDOWN\n")));
        }
        break;

    case WM_LBUTTONUP:
        {
            DEBUGMSG(TRUE, (TEXT("WM_LBUTTONUP\n")));
        }
        break;

    case WM_INITDIALOG:
        {
            DEBUGMSG(TRUE, (TEXT("WM_INITDIALOG\n")));
        }
        break;

    case WM_SYSKEYDOWN:
        {
            DEBUGMSG(TRUE, (TEXT("WM_SYSKEYDOWN\n")));
        }
        break;

    default:
        return DefWindowProc(hWnd, uMsg, wParam, lParam);
    }
    return 0;
}