MFC CScrollView does not clear background - c++

Question

My English is not perfect. I am using Visual C++ 2019 and MFC. Example program: an SDI program, the base of the view is CScrollView, draws 128*128 0s in a matrix. But MFC does not clear the background at scrolling with the scrollbar. Have you an idea? Thank you.
In settings of Windows, I am using 96 dpi * 3 = 288 dpi.
I tried: 96 dpi mode is affected so.
How can I upload the example program to this?
void CsdView::OnDraw(CDC *pDC) {
CsdDoc *pDoc = GetDocument();
ASSERT_VALID(pDoc);
if (!pDoc)
return;
CRect rect;
GetClientRect(&rect);
pDC->FillSolidRect(rect, 0xFFFFFF);
CPoint pos = GetDeviceScrollPosition();
TRACE(L"OnDraw: %4u.%4u - %4u.%4u, %4u.%4u\n", rect.right, rect.bottom, pos.x, pos.y, rect.right + pos.x, rect.bottom+pos.y);
for (int i = 0; i < 128; ++ i)
for (int j = 0; j < 128; ++ j)
pDC->TextOutW(j*20 - pos.x, i*54 - pos.y, L"0", 1);
}
void CsdView::OnInitialUpdate() {
CScrollView::OnInitialUpdate();
CSize sizeTotal;
sizeTotal.cx = 20*128;
sizeTotal.cy = 54*128;
SetScrollSizes(MM_TEXT, sizeTotal);
}
BOOL CsdView::OnEraseBkgnd(CDC *pDC) {
CBrush brush(0xFFFFFF);
FillOutsideRect(pDC, &brush);
return TRUE;
// return CScrollView::OnEraseBkgnd(pDC);
}
I can not upload picture and code as a comment, so I must edit the original question.
A little bug is remained. The orginal code (MDI MFC):
void CIDEView::OnDraw(CDC *pDC) {
CIDEDoc *const d = GetDocument();
ASSERT_VALID(d);
if (! d)
return;
CPoint const pos = GetDeviceScrollPosition();
CRect rect;
GetClientRect(&rect);
OffsetRect(&rect, pos.x, pos.y);
pDC->FillSolidRect(rect, bkcolor);
auto oldfont = pDC->SelectObject(&font);
pDC->SetBkColor(bkcolor);
pDC->SetTextColor(textcolor);
pDC->SetBkMode(TRANSPARENT);
const int cxs = pos.x / mincw, cys = pos.y / lineheight;
const int cxe = (rect.right + mincw-1) / mincw,
cye = (rect.bottom + 41) / lineheight;
for (int i = cys; i <= cye; ++ i)
for (int j = cxs; j <= cxe; ++ j)
pDC->TextOutW(textmargin+j*mincw, i*lineheight, L"0", 1);
pDC->SelectObject(oldfont);
}
void CIDEView::OnInitialUpdate() {
CScrollView::OnInitialUpdate();
SetScrollSizes(MM_TEXT, {linewidth, 128*lineheight});
}
BOOL CIDEView::OnEraseBkgnd(CDC *pDC) {
return TRUE;
}

Answer 1

The CScrollView class is a view with scrolling capabilities. You use it almost like a CView (ie drawing in the OnDraw() member), only you have to take into account the possible scrolling.
The GetClientRect() function returns the visible client area, and the coordinates returned are not relative to the view origin, but to the window origin, ie the left and top members are always 0. The CDC parameter in the OnDraw() function though are relative to the logical view origin, so an adjustment is needed.
As for your code, you don't need to use the OnEraseBkgnd() function, because you do so in OnDraw(). You fill only the visible part of the window, but that's very much OK. So it would best to remove it. Also, the coordinates passed to the TextOutW() function must be relative to the view origin, so the -pos.x and -pos.y adjustments are wrong. Instead, it's the rectanlge passed to the FillSolidRect() function that needs to be adjusted. So, your code would become:
void CsdView::OnDraw(CDC *pDC)
{
CsdDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
if (!pDoc)
return;
CPoint pos = GetScrollPosition();
CRect rect;
GetClientRect(&rect);
// Adjust client rect to device coordinates
OffsetRect(&rect, pos.x, pos.y);
pDC->FillSolidRect(rect, 0xFFFFFF);
for (int i = 0; i < 128; ++i)
for (int j = 0; j < 128; ++j)
pDC->TextOutW(j * 20, i * 54, L"0", 1);
}
However this code is "wasteful", as it paints the whole view. It can be optimized to paint only the visible part. It will draw only the 0s for which even one pixel lies in the visible part (didn't #define anything, just used your hard-coded 20 and 54 values). Also changed the color to yellow, so you can test it better (white is the default background color).
void CsdView::OnDraw(CDC *pDC)
{
CsdDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
if (!pDoc)
return;
CPoint pos = GetScrollPosition();
CRect rect;
GetClientRect(&rect);
// Adjust client rect to device coordinates
OffsetRect(&rect, pos.x, pos.y);
pDC->FillSolidRect(rect, 0x00FFFF);
// Paint only the items in the visible part
int xL = rect.left / 20,
xR = (rect.right + 19) / 20,
yT = rect.top / 54,
yB = (rect.bottom + 53) / 54;
for (int i = yT; i < yB; ++i)
for (int j = xL; j < xR; ++j)
pDC->TextOutW(j * 20, i * 54, L"0", 1);
}
EDIT:
In the revised code, why are the doc, pos, cxs etc variables const? There are also quite a few bugs in your logic:
You set your view size in OnInitialUpdate(), rather assuming that linewidth equals to textmargin + 128*mincw. If not, revise your code again.
The rect is already adjusted (using OffsetRect()), so it is wrong to add pos.x again.
Since you have the cell sizes in variables, don't use hard-coded numbers. For example the code for cxe should become cxe = (rect.right + mincw - 1) / mincw; Update the cye code similarly.
Also you paint at an offset of textmargin. The code should then become cxe = (rect.right - textmargin + mincw - 1) / mincw;
The code I posted works OK with the < condition in the loops, you don't need <=. Do the math and you will find that this is the correct one.