如何使用GDI在Ca ++中使用GDI绘制窗口（而不是创建图像）？

Question

I managed to find this code snippet and compile it with Cairo:

#define LIBCAIRO_EXPORTS
#include <cairo/cairo.h>
#include <cairo/cairo-win32.h>

int main(int argc, char** argv)
{
    cairo_surface_t *surface;
    cairo_t *cr;
    surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 240, 80);
    cr = cairo_create (surface);

    cairo_select_font_face (cr, "serif", CAIRO_FONT_SLANT_NORMAL,
                                         CAIRO_FONT_WEIGHT_BOLD);
    cairo_set_font_size (cr, 32.0);
    cairo_set_source_rgb (cr, 0.0, 0.0, 1.0);
    cairo_move_to (cr, 10.0, 50.0);
    cairo_show_text (cr, "Hello, World");
    cairo_destroy (cr);
    cairo_surface_write_to_png (surface, "hello.png");
    cairo_surface_destroy (surface);

    return 0;
}

As you can see, it create an image with the text "Hello World" and saved it on the drive. How do I go about creating a win32 surface and draw to a window instead?

I am failing to use: cairo_win32_surface_create

It requires a hdc and I don't know what that is. I tried to read a few tutorials but none seems to walk you through printing to a new window.

I found this link: http://windrealm.org/cairo-gdi/

It has a working demo but it uses int WINAPI WinMain . I don't wish to use that.

Answer 1

There are several ways to get a handle to a device context in Windows. For instance it is typical that the WM_PAINT handler will call BeginPaint to acquire a device context, update the contents to the window and then call EndPaint . In cases where BeginPaint and EndPaint cannot be used (eg outside of a WM_PAINT message) you can use GetDC update the window and then call ReleaseDC .

The device contexts returned by BeginPaint and GetDC allow you to draw directly to a window. Sometimes however you want to draw directly to a bitmap. In this case you would call CreateCompatibleDC , select the bitmap you want to draw to, do your drawing and then call DeleteDC .

In your case what you're looking for is something like the following:

HDC dc = GetDC(windowHandle);
cairo_win32_surface_create(dc);
ReleaseDC(windowHandle, dc);

If you do not want to create a window at this stage (since you are simply saving an image) you can probably get away with using the desktop window to acquire a device context.

HWND windowHandle = GetDesktopWindow();
HDC dc = GetDC(windowHandle);
cairo_win32_surface_create(dc);
ReleaseDC(windowHandle, dc);

There are other calls that can be used to acquire or create a device context. You can find a list of these and related functions here

Creating and using a window is a bit more involved so I'm going to provide you with the basic steps and some sample code that you can use to play around with. To create and use a window...

• Register a window class with RegisterClass
• Create the window with CreateWindow or CreateWindowEx
• Process messages with a message pump by calling GetMessage , TranslateMessage and DispatchMessage

Additionally you will have to implement a function to handle processing of window messages such as WM_PAINT .

NOTE: The following code is UNTESTED but should be correct.

LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    if(message == WM_PAINT)
    {
        HDC dc;
        PAINTSTRUCT ps;
        dc = BeginPaint(hwnd, &ps);

        // do your drawing here

        EndPaint(hwnd, &ps);
    }

    return 0;
}

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

    ///////////////////////////////////////////////////////////
    //  Register a window "class"
    ///////////////////////////////////////////////////////////
    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)COLOR_WINDOW;
    wndclass.lpszMenuName  = NULL;
    wndclass.lpszClassName = szClassName;

    if(!RegisterClass(&wndclass))
    {
        //  error
        return 1;
    }

    ///////////////////////////////////////////////////////////
    //  Create the window and display it (if iCmdShow says so)
    ///////////////////////////////////////////////////////////
    hwnd = CreateWindow(
        szAppName,
        TEXT("Draw Surface"),
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT,
        CW_USEDEFAULT, CW_USEDEFAULT,
        NULL, NULL, hInstance, NULL);

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);


    ///////////////////////////////////////////////////////////
    //  Run the message pump so the window proc recieves events
    ///////////////////////////////////////////////////////////
    while(GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}

Answer 2

I found the easiest way for beginners to open a window and draw some lines into it, is downloading and setting up the gtk+ and using ZetCode's tutorials.

You will probably need to download GTK from here: http://gtk.hexchat.org/

Setting up Gtk+: Using GTK+ in Visual C++

Cairo tutorials: http://zetcode.com/gfx/cairo/

Remember to include all the GTK's dll files in the directory where your compiled executable is.