Am i allowed to use a DC outside of a paint cycle? Is my window's DC guaranteed to be valid forever?
i'm trying to figure out how long my control's Device Context (DC) is valid.
i know that i can call:
GetDC(hWnd);
to get the device context of my control's window, but is that allowed?
When Windows sends me a WM_PAINT message, i am supposed to call BeginPaint/EndPaint to properly acknowledge that i've painted it, and to internally clear the invalid region:
BeginPaint(hWnd, {out}paintStruct);
try
//Do my painting
finally
EndPaint(hWnd, paintStruct);
end;
But calling BeginPaint also returns me a DC inside the PAINTSTRUCT structure. This is the DC that i should be painting on.
i cannot find anything in the documentation that says that the DC returned by BeginPaint() is the same DC that i would get from GetDC().
Especially now, in the days of Desktop Composition, is it valid to paint on a DC that i obtain outside of BeginPaint?
There seem to be 2 ways i can get a DC to paint on during a paint cycle:
dc = GetDC(hWnd);
BeginPaint(&paintStruct);
There is a 3rd way, but it seems to be a bug with the Borland Delphi that i develop with.
During WM_PAINT processing, Delphi believes that the wParam is a DC, and proceeds to paint on it. Whereas the MSDN says that the wParam of a WM_PAINT message is unused.
The Why
My real goal is to try to keep a persistent GDI+ Graphics object against an HDC, so that i can use some better performing features of GDI+ that depend on having a persistent DC.
During the WM_PAINT message handling i want to draw a GDI+ image to the canvas. The following nieve version is very slow:
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 contains a faster performing bitmap, a CachedBitmap. But using it without thinking gives no performance benefit:
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);
}
The performance gain comes from creating the CachedBitmap once, so on program initialization:
m_graphics = new Graphics(GetDC(m_hwnd));
m_cachedBitmap = new CachedBitmap(b_someBitmap, m_graphcis);
And now on the paint cycle:
WM_PAINT:
{
PAINTSTRUCT ps;
BeginPaint(m_hwnd, ps);
m_graphics.DrawCachedBitmap(m_cachedBitmap, 0, 0);
EndPaint(h_hwnd, ps);
}
Except now i'm trusting that the DC i obtained after program initializtion will be the same DC for my window as long as the application is running. This means that it survives through:
- fast user switches
- composition enabled/disabled
- theme switching
- theme disabling
i find nothing in MSDN that guarantees that the same DC will be used for a particular window for as long as the window exists.
Note: i am not using double-buffering, because i want to be a good developer, and do the right thing. * Sometimes that means you double-buffering is bad.