Is it possible to create an XOR pen like DrawFocusRect()? - c

Question

The Win32 GDI DrawFocusRect(HDC, const RECT*) function draws the dotted outline of a rectangle on the desired devince context. The cool thing about this function is it draws the dots using an XOR function so that when you call it a second time on the same device context and rectangle, it erases itself:
RECT rc = { 0, 0, 100, 100 };
DrawFocusRect(hdc, &rc); // draw rectangle
DrawFocusRect(hdc, &rc); // erase the rectangle we just drew
I want to achieve the same dotted line effect as DrawFocusRect() but I just want a line, not a whole rectangle. I tried doing this by passing a RECT of height 1 to DrawFocusRect() but this doesn't work because it XORs the "bottom line" of the rectange on top of the top line so nothing gets painted.
Can I create a plain HPEN that achieves the same effect as DrawFocusRect() so I can draw just a single line?

Answer 1

As #IInspectable commented, you want to use SetROP2(). The other half of the battle is creating the correct pen. Here is how the whole thing shakes out:
HPEN create_focus_pen()
{
LONG width(1);
SystemParametersInfo(SPI_GETFOCUSBORDERHEIGHT, 0, &width, 0);
LOGBRUSH lb = { }; // initialize to zero
lb.lbColor = 0xffffff; // white
lb.lbStyle = BS_SOLID;
return ExtCreatePen(PS.GEOMETRIC | PS.DOT, width, &lb, 0, 0);
}
void draw_focus_line(HDC hdc, HPEN hpen, POINT from, POINT to)
{
HPEN old_pen = SelectObject(hdc, hpen);
int old_rop = SetROP2(R2_XORPEN);
MoveToEx(hdc, from.x, from.y, nullptr);
LineTo(hdc, to.x, to.y);
SelectObject(hdc, old_pen);
SetROP2(old_rop);
}