编写一个基于Win32 API 的时钟程序，显示当前系统日期时间

下面是一个基于Win32 API的时钟程序的代码，它可以显示当前系统日期和时间：

#include <windows.h>

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                   PSTR szCmdLine, int iCmdShow)
{
    static TCHAR szAppName[] = TEXT("Clock");
    HWND hwnd;
    MSG msg;
    WNDCLASS wndclass;

    // 初始化窗口类
    wndclass.style         = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc   = WndProc;
    wndclass.cbClsExtra    = 0;
    wndclass.cbWndExtra    = 0;
    wndclass.hInstance     = hInstance;
    wndclass.hIcon         = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor       = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH) GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName  = NULL;
    wndclass.lpszClassName = szAppName;

    // 注册窗口类
    if (!RegisterClass(&wndclass))
    {
        MessageBox(NULL, TEXT("This program requires Windows NT!"),
                   szAppName, MB_ICONERROR);
        return 0;
    }

    // 创建窗口
    hwnd = CreateWindow(szAppName, TEXT("Clock"),
                        WS_OVERLAPPEDWINDOW,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        NULL, NULL, hInstance, NULL);

    // 显示窗口
    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

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

    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static int cxClient, cyClient;
    static HFONT hFont;
    static TCHAR szTime[9], szDate[13];
    HDC hdc;
    PAINTSTRUCT ps;
    RECT rect;
    SYSTEMTIME st;

    switch (message)
    {
    case WM_CREATE:
        // 创建字体
        hFont = CreateFont(48, 0, 0, 0, FW_NORMAL, FALSE, FALSE, FALSE,
                           ANSI_CHARSET, OUT_DEFAULT_PRECIS,
                           CLIP_DEFAULT_PRECIS, DEFAULT_QUALITY,
                           DEFAULT_PITCH | FF_DONTCARE,
                           TEXT("Arial"));
        SetTimer(hwnd, 1, 1000, NULL); // 创建计时器
        return 0;

    case WM_SIZE:
        // 更新窗口大小
        cxClient = LOWORD(lParam);
        cyClient = HIWORD(lParam);
        return 0;

    case WM_TIMER:
        // 更新时间
        GetLocalTime(&st);
        wsprintf(szTime, TEXT("%02d:%02d:%02d"), st.wHour, st.wMinute, st.wSecond);
        wsprintf(szDate, TEXT("%04d/%02d/%02d"), st.wYear, st.wMonth, st.wDay);
        InvalidateRect(hwnd, NULL, TRUE);
        return 0;

    case WM_PAINT:
        // 绘制时钟
        hdc = BeginPaint(hwnd, &ps);
        SelectObject(hdc, hFont);
        SetTextAlign(hdc, TA_CENTER | TA_TOP);
        SetBkMode(hdc, TRANSPARENT);

        // 绘制日期和时间
        rect.left = 0;
        rect.top = cyClient / 2;
        rect.right = cxClient;
        rect.bottom = cyClient / 2 + 60;
        DrawText(hdc, szTime, -1, &rect, DT_CENTER | DT_SINGLELINE | DT_VCENTER);

        rect.left = 0;
        rect.top = cyClient / 2 + 60;
        rect.right = cxClient;
        rect.bottom = cyClient / 2 + 120;
        DrawText(hdc, szDate, -1, &rect, DT_CENTER | DT_SINGLELINE | DT_VCENTER);

        EndPaint(hwnd, &ps);
        return 0;

    case WM_DESTROY:
        // 销毁字体和计时器
        KillTimer(hwnd, 1);
        DeleteObject(hFont);
        PostQuitMessage(0);
        return 0;
    }

    return DefWindowProc(hwnd, message, wParam, lParam);
}

该程序使用了`CreateWindow`函数创建一个名为"Clock"的窗口，并在`WM_CREATE`消息处理程序中创建了一个计时器以每秒更新时间。在`WM_TIMER`处理程序中，程序获取当前系统时间并将其格式化为字符串形式，然后调用`InvalidateRect`函数以触发窗口重绘。在`WM_PAINT`处理程序中，程序绘制日期和时间字符串。在窗口关闭时，程序销毁计时器和字体对象。