c++ - WinApi 消息循环，Postmessage 与 SendMessage 类似

Question

你好有人可以回答我为什么当我运行这个程序时 MessageBoxes 的顺序是 1,2,4,3 而不是 1,2,3,4。在我看来，程序应该在启动 WM_USER+11 之前结束执行 WM_PAINT 过程，为什么不呢？

// Win32Project6.cpp : Defines the entry point for the application.
//

#include "stdafx.h"
#include "Win32Project6.h"

#define MAX_LOADSTRING 100

// Global Variables:
HINSTANCE hInst;                                // current instance
TCHAR szTitle[MAX_LOADSTRING];                  // The title bar text
TCHAR szWindowClass[MAX_LOADSTRING];            // the main window class name

// Forward declarations of functions included in this code module:
ATOM                MyRegisterClass(HINSTANCE hInstance);
BOOL                InitInstance(HINSTANCE, int);
LRESULT CALLBACK    WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK    About(HWND, UINT, WPARAM, LPARAM);


DWORD thread(LPVOID lpdwThreadParam);

int APIENTRY _tWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPTSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    // TODO: Place code here.
    MSG msg;
    HACCEL hAccelTable;

    // Initialize global strings
    LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadString(hInstance, IDC_WIN32PROJECT6, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);

    // Perform application initialization:
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }

    hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_WIN32PROJECT6));

    // Main message loop:
    while (GetMessage(&msg, NULL, 0, 0))
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}



//
//  FUNCTION: MyRegisterClass()
//
//  PURPOSE: Registers the window class.
//
ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEX wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);

    wcex.style          = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra     = 0;
    wcex.cbWndExtra     = 0;
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_WIN32PROJECT6));
    wcex.hCursor        = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName   = MAKEINTRESOURCE(IDC_WIN32PROJECT6);
    wcex.lpszClassName  = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

    return RegisterClassEx(&wcex);
}

//
//   FUNCTION: InitInstance(HINSTANCE, int)
//
//   PURPOSE: Saves instance handle and creates main window
//
//   COMMENTS:
//
//        In this function, we save the instance handle in a global variable and
//        create and display the main program window.
//
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   HWND hWnd;

   hInst = hInstance; // Store instance handle in our global variable

   hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);

   if (!hWnd)
   {
      return FALSE;
   }
//   PostMessage(hWnd, WM_USER + 11, 0, 0);
   MessageBox(0,"1","Message",0);
   MessageBox(0, "2", "Message", 0);
   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}

//
//  FUNCTION: WndProc(HWND, UINT, WPARAM, LPARAM)
//
//  PURPOSE:  Processes messages for the main window.
//
//  WM_COMMAND  - process the application menu
//  WM_PAINT    - Paint the main window
//  WM_DESTROY  - post a quit message and return
//
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    int wmId, wmEvent;
    PAINTSTRUCT ps;
    HDC hdc;

    switch (message)
    {
    case WM_COMMAND:
        wmId    = LOWORD(wParam);
        wmEvent = HIWORD(wParam);
        // Parse the menu selections:
        switch (wmId)
        {
        case IDM_ABOUT:
            DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
            break;
        case IDM_EXIT:
            DestroyWindow(hWnd);
            break;
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
        }
        break;
    case WM_USER+11:
        MessageBox(hWnd,"4","Message",0);
        break;
    case WM_PAINT:
        hdc = BeginPaint(hWnd, &ps);
        CreateThread(0, 0, (LPTHREAD_START_ROUTINE)&thread, &hWnd, 0, 0);
        MessageBox(hWnd, "3", "Message", 0);
        // TODO: Add any drawing code here...
        EndPaint(hWnd, &ps);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
    UNREFERENCED_PARAMETER(lParam);
    switch (message)
    {
    case WM_INITDIALOG:
        return (INT_PTR)TRUE;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
        {
            EndDialog(hDlg, LOWORD(wParam));
            return (INT_PTR)TRUE;
        }
        break;
    }
    return (INT_PTR)FALSE;
}


DWORD thread(LPVOID lpdwThreadParam)
{

    PostMessage(*(HWND*)(lpdwThreadParam), WM_USER + 11, 0, 0);

    return 0;
}

Answer 1

关于这段代码的一些说明:

• 您不应在 WM_PAINT 中创建线程。您也不应该在 WM_PAINT 中调用 MessageBox。 WM_PAINT 仅用于绘制窗口；这里不应执行其他逻辑。系统优化了 WM_PAINT 调用，当您在此处理程序中开始逾期欢迎时，行为会变得非常棘手。
• 您也不应该像使用 (LPTHREAD_START_ROUTINE)&thread 那样转换函数调用。如果您的函数类型不正确，请更改函数原型(prototype)；不要只是试图隐藏编译器的警告。
• 您可能应该使用 _beginthread，因为 CRT 需要执行一些初始化。
• 您应该将hWnd 作为线程参数传递，而不是&hWnd。指针可能超出范围并变得无效。这是一个严重的错误。

要真正了解发生了什么，您应该先修复这些错误。

回答你的问题，不同的线程没有速度保证。拆分线程后，现在不知道接下来会发生什么。它们是异步的。也许线程代码比您的主线程运行得更快，反之亦然。

请记住，MessageBox 有其自己的内部消息循环。因此 WM_USER+11 (MessageBox4) 处的调用堆栈将如下所示:

MessageBox (4)
WndProc (WM_USER+11)
DispatchMessage
MessageBox (3), before it's actually shown
WndProc (WM_PAINT)
DispatchMessage
WinMain

因此您可以看到，如果线程发布消息的速度足够快，它会在显示 MessageBox(3) 之前得到处理。

我的猜测是，如果您使用更简单的调试技术(例如 OutputDebugString)，您将观察到更可预测的行为。