BeginPaint

[Mike Stefanik]

I'm having a problem using BeginPaint/EndPaint in an OnPaint handler. I was porting some C++ code, and found that it wasn't drawing correctly. However, when I switched to using GetDC/ReleaseDC, it worked. Here's an example:

Code Select Expand


#autodefine "off"
#define BEGINPAINT

struct PAINTSTRUCT
{
    unsigned int hdc;
    int fErase;
    RECT rcPaint;
    int fRestore;
    int fIncUpdate;
    unsigned byte rgbReserved[32];
}

declare import,BeginPaint(unsigned int hWnd, PAINTSTRUCT *ps), unsigned int;
declare import,GetDC(unsigned int hWnd), unsigned int;
declare import,GetUpdateRect(unsigned int hWnd, RECT *lprc, int bErase), int;
declare import,EndPaint(unsigned int hWnd, PAINTSTRUCT *ps), int;
declare import,ReleaseDC(unsigned int hWnd, unsigned int hdc), int;
declare import,TextOut alias "TextOutA" (unsigned int hdc, int x, int y, pointer lpString, int cbString), int;

class CMainWnd:CWindow
{
    declare OnClose(), int;
    declare OnPaint(), int;
}

global sub main()
{
    CMainWnd mainWnd;

    mainWnd.Create(100, 100, 800, 600,
                   AWS_OVERLAPPEDWINDOW | AWS_HSCROLL | AWS_VSCROLL | AWS_VISIBLE,
                   0,
                   "Test Window",
                   null);

    do
    {
        Wait();
    }
    until mainWnd.m_hwnd = 0;
    return 0;
}

CMainWnd::OnClose(), int
{
    Destroy();
    return 0;
}

#ifdef BEGINPAINT
CMainWnd::OnPaint(), int
{
    PAINTSTRUCT ps;
    unsigned int hdc;

    hdc = BeginPaint(m_hWnd, &ps);
    TextOut(hdc, 10, 10, "Hello, Windows!", 15);
    EndPaint(m_hWnd, &ps);

    return true;
}
#else
CMainWnd::OnPaint(), int
{
    unsigned int hdc;

    hdc = GetDC(m_hWnd);
    TextOut(hdc, 10, 10, "Hello, Windows!", 15);
    ReleaseDC(m_hWnd, hdc);

    return true;
}
#endif


I get the feeling I'm missing something obvious, but I'm not getting what the difference is there. If I use BeginPaint/EndPaint, it's almost as if the window is being erased after the OnPaint handler is called.

[Ionic Wind Support Team]

You don't need BeginPaint as it is called for you before OnPaint is called. 

Look in the source archive for the HandlePaint() function (instance.src).  You only need to use GetHDC();  If you want to use text drawing functions directly from the API then you would need to turn off the TA_UPDATECP style of the device context.

HandlePaint, besides taking care of the autodraw window style, masks out any controls you have in your window.

Paul.

[Mike Stefanik]

Thanks!

[Ionic Wind Support Team]

Just hope I was clear enough

[Mike Stefanik]

I dug around in there and saw how things were being setup, so tweaked my OnPaint handler accordingly. Probably would be a good idea for the docs to mention the caveats if you're trying to use the API directly (or in my case, calling functions in a DLL that were doing the same).

[Ionic Wind Support Team]

Yep.

Aurora uses a saved methodology for the device context of a window.  When you use SetFont, or change the background/foreground colors, or change a line style, etc the current state of the device context is stored.  When you call GetHDC everything is restored for you so it's not necessary to reset a font, pens, brushes, etc.

It makes it a lot more convenient for average GDI tasks.  The only caveat is that text alignment is set to TA_UPDATECP which causes some text drawing functions to ignore the x and y positions.  But that is easily solved with one call to SetTextAlign

Paul.