CODEWITHSUNDEEP
c++windowsuser-interfacemfc
Swiss Frank
Swiss Frank

Reputation: 2422

MFC GUI custom control: how to draw cursor updates in response to mouse moves?

I have a custom Windows control subclassed from CButton (no idea why that was selected--this is 17-year-old code; no semblance of button functionality is present).

Its DrawItem( LPDRAWITEMSTRUCT pdis ) method is called by CButton::OnChildNotify in response to WM_DRAWITEM. It renders its scene with the DC CDC::FromHandle( pdis->hDC ).

The mouse event method OnMouseMove() calculates the new cursor position and calls RedrawWindow( NULL, NULL, RDW_INVALIDATE ). A cursor that follows the mouse duly appears at the new mouse position. It works fine, but it's slow. In fact, only the previous and new cursor cells need be redrawn (if that) yet the graphic updates start to lag as the entire scene is rendered many times.

I thought in my OnMouseMove() method, instead of repainting the entire scene, could just paint the cells in question. It already has the exact X and Y coordinates of the cells and pointers to their data. I thought CPaintDC(this) would provide a DC that allowed this, but it doesn't paint. (Doesn't crash either, which is a rare joy.)

My hazy recollection is that the "optimal" way to do this would be to invalidate just the areas of the two cells, and the DrawItem() method would eventually be told these areas were invalidated, and rather than totally repainting it could just work out from the coordinates which cells they were (not an easy operation btw) and repaint them, and that would streamline not only this cursor problem but also ensure only a few cells be painted were the partially-obscured control partially revealed. But time pressure doesn't allow and the use cases don't seem to call for this to be optimized.

So the question is: is there some nice way for OnMouseMove() to re-render a single control immediately, and if so with what DC? (For instance can I cache the DC that I've received in DrawItem() via FromHandle()?

Right now the only idea I have is to have an object member pointing to a single cell to be redrawn, to call RedrawWindow() with this RDW_UPDATENOW flag, and have DrawItem(), if that flag be set, do just that one item. That would result in DrawItem() getting a DC that presumably would work in the way it always has. Seems like a real hack though, is there a better way?

Upvotes: 0

Views: 281

Answers (1)

IInspectable
IInspectable

Reputation: 51479

In a Windows application, it is customary to perform all rendering in response to a WM_PAINT (or WM_NCPAINT) message. Code that needs to trigger a repaint marks part or all of window's client area as dirty, by calling InvalidateRect (and friends). The system is optimized for this approach, coalescing multiple requests into a single update region, and subsequently issuing a WM_PAINT message, when there is no more important work to do (like handling input).

This works reliably, and is usually easier to implement than spreading the rendering across several places. It is, however, perfectly legal to deviate from this, and perform rendering anywhere in your code. While WM_PAINT messages can still arrive at any time, it is desirable to have the out-of-band rendering produce identical visual results as the WM_PAINT handler would, to prevent visual artifacts.

All rendering goes through an abstraction called a device context (DC). When handling a WM_PAINT message in an MFC application, a suitable DC can be obtained by constructing a CPaintDC instance. When rendering anywhere else you cannot use a CPaintDC, but need to use a CClientDC instead (or a CWindowDC, to render the non-client area as well). In general, rendering code need not know, which type of DC it is rendering to, and can usually be reused without change.

Upvotes: 1

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