是否可以创建诸如DrawFocusRect()的XOR笔?

问题描述

Win32 GDI DrawFocusRect(HDC，const RECT *) 函数在所需devince上下文上绘制矩形的虚线轮廓.关于此函数的最酷的事情是它使用XOR函数绘制点，这样当您在同一设备上下文和矩形上再次调用它时，它将擦除自身:

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

我想获得与 DrawFocusRect()相同的虚线效果，但是我只想要一条线，而不是整个矩形.我尝试通过将高度为1的 RECT 传递给 DrawFocusRect()来执行此操作，但这不起作用，因为它会在矩形顶部对矩形的底线"进行XOR顶线，所以什么都没画.

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.

我可以创建一个普通的HPEN来实现与 DrawFocusRect()相同的效果，以便仅画一条线吗?

Can I create a plain HPEN that achieves the same effect as DrawFocusRect() so I can draw just a single line?

推荐答案

正如@IInspectable所评论，您要使用 SetROP2() .战斗的另一半是创造正确的笔.这是整个过程的震撼:

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);
}

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