WIN32：如何告诉所有者绘制的静态控件进行自我刷新？

Question

I have a WIN32 owner-drawn static control that draws a progress bar using two source images (filled and unfilled). Works great on the initial draw:

case WM_DRAWITEM:
    {
        DRAWITEMSTRUCT* draw = (DRAWITEMSTRUCT*)lparam;
        // Manually draw the progress bar.
        if( draw->hwndItem == hwndProgress )
        {
            // Progress bar is 526 pixels wide.
            int left = progressPercent * 526 / 100;
            // Paint sections of window with filled and unfilled bitmaps
            // based on progress bar position.
            HDC hdcMem = ::CreateCompatibleDC(draw->hDC);
            ::SelectObject(hdcMem, hBmpProgressFull);
            ::BitBlt(draw->hDC, 0, 0, left, 36, hdcMem, 0, 0, SRCCOPY);
            ::DeleteDC(hdcMem);
            HDC hdcMem2 = ::CreateCompatibleDC(draw->hDC);
            ::SelectObject(hdcMem2, hBmpProgressEmpty);
            ::BitBlt(draw->hDC, left, 0, 526-left, 36, hdcMem2, left, 0, SRCCOPY);
            ::DeleteDC(hdcMem2);
            return TRUE;
        }
    }
    return 0;

However, I can't seem to get the thing to erase and repaint properly. I've tried SendMessage with WM_PAINT and RedrawWindow and neither one has worked quite right:

bool SetLoginProgressBar(float value)
{
    if( hwndProgress != NULL )
    {
        progressPercent = (int)(value * 100.0);
        //::RedrawWindow(hwndProgress, NULL, NULL, RDW_INVALIDATE|RDW_INTERNALPAINT);
        ::SendMessage(hwndProgress, WM_PAINT, NULL, NULL);
    }
    return true;
}

Instead of redrawing the window with the new values, it just sits there with the initially drawn image and ignores further drawing commands. It draws the progress correctly for the initial value, whether it's 0%, 50%, etc, and I can verify that my WM_DRAWITEM message handler code is being called.

So, what is the correct way to tell this control to erase and redraw in WIN32?

Is is possible that I need to do something like BeginPaint/EndPaint, or delete the hDC in the DRAWITEMSTRUCT that I've been passed?

Answer 1

You aren't deselecting the bitmaps from the memory DC before you destroy the DC. Perhaps the bitmaps are in a state where Windows won't allow you to select them again, so the BitBlts are failing.

PS RedrawWindow is what I use in this situation. InvalidateRect works too, but only if your message loop is running. Which leads to another observation: if you're in the middle of a long running operation, you may not get back to the message loop and your app will appear to be hung, including updates to the progress window.

Answer 2

InvalidateRect() is the function you need to call.

You never send or post WM_PAINT messages—the Window manager does that for you when they are needed (eg windows dragged over your window). If the repaint is due to changes that the Window manager does not know about then you force a repaint cycle by calling InvalidateRect() . Pass NULL for lpRect and the entire client area will be repainted. Pass TRUE for bErase to force the background to be erased when the repaint cycle begins.

When you call InvalidateRect() what happens is that a WM_PAINT message is placed in your message queue and the InvalidateRect() function call returns. When you next clear your message queue you the process the WM_PAINT message.

I suggest that you get hold of a copy of Petzold's Programming Windows book and read all about it.