怎么使用双缓冲技术，写一个示例

双缓冲技术是一种常见的图形处理技术，用于解决图形界面中的闪烁问题。下面是一个使用双缓冲技术的示例代码：

#include <Windows.h>

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

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    WNDCLASSEX wcex;
    HWND hWnd;
    MSG msg;

    // 注册窗口类
    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(NULL, IDI_APPLICATION);
    wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wcex.lpszMenuName = NULL;
    wcex.lpszClassName = "DoubleBufferDemo";
    wcex.hIconSm = LoadIcon(NULL, IDI_APPLICATION);
    RegisterClassEx(&wcex);

    // 创建窗口
    hWnd = CreateWindow("DoubleBufferDemo", "Double Buffer Demo",
        WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, 800, 600,
        NULL, NULL, hInstance, NULL);
    if (!hWnd)
    {
        return FALSE;
    }

    // 显示窗口
    ShowWindow(hWnd, nCmdShow);
    UpdateWindow(hWnd);

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

    return (int)msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static HDC hdcMem;
    static HBITMAP hBitmap, hOldBitmap;
    static RECT rect;
    switch (message)
    {
    case WM_CREATE:
        hdcMem = CreateCompatibleDC(NULL);
        GetClientRect(hWnd, &rect);
        hBitmap = CreateCompatibleBitmap(GetDC(hWnd), rect.right, rect.bottom);
        hOldBitmap = (HBITMAP)SelectObject(hdcMem, hBitmap);
        break;
    case WM_PAINT:
        {
            PAINTSTRUCT ps;
            HDC hdc = BeginPaint(hWnd, &ps);

            // 双缓冲绘制
            BitBlt(hdcMem, 0, 0, rect.right, rect.bottom, NULL, NULL, NULL, WHITENESS);
            Ellipse(hdcMem, 100, 100, 300, 300);
            BitBlt(hdc, 0, 0, rect.right, rect.bottom, hdcMem, 0, 0, SRCCOPY);

            EndPaint(hWnd, &ps);
        }
        break;
    case WM_DESTROY:
        // 释放资源
        SelectObject(hdcMem, hOldBitmap);
        DeleteObject(hBitmap);
        DeleteDC(hdcMem);
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

该示例程序创建了一个窗口，并使用双缓冲技术绘制一个椭圆形，以避免屏幕闪烁。具体实现方法是在窗口的 WM_CREATE 和 WM_DESTROY 消息中创建和释放缓冲区，而在 WM_PAINT 消息中进行绘制操作。其中，双缓冲绘制的核心代码是：

BitBlt(hdcMem, 0, 0, rect.right, rect.bottom, NULL, NULL, NULL, WHITENESS);
Ellipse(hdcMem, 100, 100, 300, 300);
BitBlt(hdc, 0, 0, rect.right, rect.bottom, hdcMem, 0, 0, SRCCOPY);

它先在缓冲区上绘制图形，再将缓冲区的内容复制到窗口的 DC 上，这样可以保证绘制的图形一次性显示，从而避免屏幕闪烁的问题。