windows - Win32 : Does a window have the same HDC for its entire lifetime?

Question

我可以在油漆周期之外使用 DC 吗？ 我 window 的 DC 是否保证永远有效？

我想弄清楚我的控件的设备上下文 (DC) 的有效期有多长。

我知道我可以打电话:

GetDC(hWnd);

获取控件窗口的设备上下文，但允许这样做吗？

当 Windows 向我发送 WM_PAINT 消息时，我应该调用 BeginPaint/EndPaint正确确认我已经绘制了它，并在内部清除无效区域:

BeginPaint(hWnd, {out}paintStruct);
try
   //Do my painting
finally
   EndPaint(hWnd, paintStruct);
end;

但是调用 BeginPaint 也会在 PAINTSTRUCT 结构中返回一个 DC。这是我应该绘画的 DC。

我在文档中找不到任何内容表明 BeginPaint() 返回的 DC 与我从 GetDC() 获得的 DC 相同。

特别是现在，在 Desktop Composition 时代，在我从 BeginPaint 之外获得的 DC 上绘画是否有效？

似乎有两种方法可以让 DC 在绘制周期中进行绘制:

1. dc = GetDC (hWnd);

2. 开始绘制(&paintStruct);

还有第三种方法，但它似乎是我开发的 Borland Delphi 的一个错误。

期间 WM_PAINT处理，Delphi 认为 wParam 是一个 DC，并继续在其上绘制。而 MSDN 表示 WM_PAINT 消息的 wParam 未使用。

原因

我的真正目标is to try to keep a persistent GDI+ Graphics object针对 HDC，以便我可以使用 GDI+ 的一些性能更好的功能，这些功能依赖于具有持久性 DC。

在 WM_PAINT 消息处理期间，我想在 Canvas 上绘制 GDI+ 图像。以下 nieve 版本非常慢:

WM_PAINT:
{
   PAINTSTRUCT ps;
   BeginPaint(m_hwnd, ps);
   Graphics g = new Graphics(ps.hdc);
   g.DrawImage(m_someBitmap, 0, 0);
   g.Destroy();
   EndPaint(h_hwnd, ps);
}

GDI 包含一个执行速度更快的位图，一个 CachedBitmap。但是不假思索地使用它不会带来任何性能优势:

WM_PAINT:
{
   PAINTSTRUCT ps;
   BeginPaint(m_hwnd, ps);

   Graphics g = new Graphics(ps.hdc);
   CachedBitmap bm = new CachedBitmap(m_someBitmap, g);
   g.DrawCachedBitmap(m_bm, 0, 0);
   bm.Destroy();
   g.Destroy();
   EndPaint(h_hwnd, ps);
}

性能提升来自于创建一次 CachedBitmap，所以在程序初始化时:

m_graphics = new Graphics(GetDC(m_hwnd));
m_cachedBitmap = new CachedBitmap(b_someBitmap, m_graphcis);

现在开始绘制周期:

WM_PAINT:
{
   PAINTSTRUCT ps;
   BeginPaint(m_hwnd, ps);
   m_graphics.DrawCachedBitmap(m_cachedBitmap, 0, 0);
   EndPaint(h_hwnd, ps);
}        

除了现在我相信只要应用程序正在运行，我在程序初始化后获得的 DC 将与我的窗口的 DC 相同。这意味着它通过以下方式存活:

• 快速用户切换
• 组合启用/禁用
• 主题切换
• 主题停用

我在 MSDN 中找不到任何内容可以保证只要窗口存在，相同的 DC 将用于特定窗口。

注意:我没有使用双缓冲，because i want to be a good developer, and do the right thing . * 有时这意味着您的双缓冲很糟糕。

Answer 1

据我所知，唯一可能(或可能不会)做您正在寻找的方法是使用 CS_OWNDC 创建窗口。类(class)作风。

它所做的是为类中的每个窗口分配一个唯一的设备上下文。

编辑

来自链接的 MSDN 文章:

A device context is a special set of values that applications use for drawing in the client area of their windows. The system requires a device context for each window on the display but allows some flexibility in how the system stores and treats that device context.

If no device-context style is explicitly given, the system assumes each window uses a device context retrieved from a pool of contexts maintained by the system. In such cases, each window must retrieve and initialize the device context before painting and free it after painting.

To avoid retrieving a device context each time it needs to paint inside a window, an application can specify the CS_OWNDC style for the window class. This class style directs the system to create a private device context — that is, to allocate a unique device context for each window in the class. The application need only retrieve the context once and then use it for all subsequent painting.

Windows 95/98/Me: Although the CS_OWNDC style is convenient, use it carefully, because each device context uses a significant portion of 64K GDI heap.

也许这个例子会更好地说明 CS_OWNDC 的使用:

#include <windows.h>

static TCHAR ClassName[] = TEXT("BitmapWindow");
static TCHAR WindowTitle[] = TEXT("Bitmap Window");

HDC m_hDC;
HWND m_hWnd;

LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    static PAINTSTRUCT ps;

    switch (msg)
    {
    case WM_PAINT:
        {
            BeginPaint(hWnd, &ps);

            if (ps.hdc == m_hDC)
                MessageBox(NULL, L"ps.hdc == m_hDC", WindowTitle, MB_OK);
            else
                MessageBox(NULL, L"ps.hdc != m_hDC", WindowTitle, MB_OK);

            if (ps.hdc == GetDC(hWnd))
                MessageBox(NULL, L"ps.hdc == GetDC(hWnd)", WindowTitle, MB_OK);
            else
                MessageBox(NULL, L"ps.hdc != GetDC(hWnd)", WindowTitle, MB_OK);

            RECT r;
            SetRect(&r, 10, 10, 50, 50);
            FillRect(m_hDC, &r, (HBRUSH) GetStockObject( BLACK_BRUSH ));

            EndPaint(hWnd, &ps);
            return 0;
        }
    case WM_DESTROY:
        {
            PostQuitMessage(0);
            return 0;
        }
    }
    return DefWindowProc(hWnd, msg, wParam, lParam);
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)
{   
    WNDCLASSEX wcex;

    wcex.cbClsExtra = 0;
    wcex.cbSize = sizeof(WNDCLASSEX);
    wcex.cbWndExtra = 0;
    wcex.hbrBackground = (HBRUSH) GetStockObject( WHITE_BRUSH );
    wcex.hCursor = LoadCursor( NULL, IDC_ARROW );
    wcex.hIcon = LoadIcon( NULL, IDI_APPLICATION );
    wcex.hIconSm = NULL;
    wcex.hInstance = hInstance;
    wcex.lpfnWndProc = WndProc;
    wcex.lpszClassName = ClassName;
    wcex.lpszMenuName = NULL;
    wcex.style = CS_OWNDC;

    if (!RegisterClassEx(&wcex))
        return 0;

    DWORD dwExStyle = 0;
    DWORD dwStyle = WS_OVERLAPPEDWINDOW | WS_VISIBLE;

    m_hWnd = CreateWindowEx(dwExStyle, ClassName, WindowTitle, dwStyle, 0, 0, 300, 300, NULL, NULL, hInstance, NULL);

    if (!m_hWnd)
        return 0;

    m_hDC = GetDC(m_hWnd);

    MSG msg;
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return msg.wParam;
}

CS_OWNDcflags不会与 CS_CLASSDcflags混淆:

Allocates one device context to be shared by all windows in the class. Because window classes are process specific, it is possible for multiple threads of an application to create a window of the same class. It is also possible for the threads to attempt to use the device context simultaneously. When this happens, the system allows only one thread to successfully finish its drawing operation.

如果一切都失败了，那就是 reconstruct CachedBitmap。

When you construct a CachedBitmap object, you must pass the address of a Graphics object to the constructor. If the screen associated with that Graphics object has its bit depth changed after the cached bitmap is constructed, then the DrawCachedBitmap method will fail, and you should reconstruct the cached bitmap. Alternatively, you can hook the display change notification message and reconstruct the cached bitmap at that time.

我并不是说 CS_OWNDC 是完美的解决方案，但它是朝着更好的解决方案迈出的一步。

编辑

示例程序似乎在使用 CS_OWNDcflags的屏幕分辨率/位深度更改测试期间保留相同的 DC，但是，当该标志被删除时，DC 是不同的(Window 7 64 位旗舰版)(应该 在不同的操作系统版本上工作相同......尽管测试不会有什么坏处)。

编辑2

这个例子没有调用 GetUpdateRect 来检查窗口是否需要在 WM_PAINT 期间被绘制。这是一个错误。