SetWindowRgn

MyWnd.cpp

#include "stdafx.h" 
#include "MyWnd.h" 

static CString s_wndclazz;

IMPLEMENT_DYNAMIC(MyWnd, CWnd)

BEGIN_MESSAGE_MAP(MyWnd, CWnd)
    ON_WM_CREATE()
    ON_WM_PAINT()
END_MESSAGE_MAP()

void MyWnd::RegisterWndClazz()
{
    if (!s_wndclazz.IsEmpty())
        return;
    s_wndclazz = AfxRegisterWndClass(
        CS_VREDRAW | CS_HREDRAW,
        ::LoadCursor(NULL, IDC_ARROW),
        (HBRUSH)::GetStockObject(WHITE_BRUSH),
        ::LoadIcon(NULL, IDI_APPLICATION));
}

bool MyWnd::CreateMe()
{
    RegisterWndClazz();
    CRect r = GetInitialRect();
    return CreateEx(
        0,
        s_wndclazz,
        L"MyWnd",
        WS_POPUP | WS_CLIPSIBLINGS | WS_CLIPCHILDREN,
        { r.left, r.top, r.right, r.bottom }, // {100, 100, 740, 630}
        nullptr,
        0);
}

CSize MyWnd::GetBaseSize(
) {
    return { 640, 480 };
}

CRect MyWnd::GetInitialRect(
) {
    int offset = 100;
    CSize sz = GetBaseSize();
    CRect r(0, 0, sz.cx, sz.cy + m_tri_size);
    r.MoveToXY(100, 100);
    return r;
}

int MyWnd::OnCreate(
    LPCREATESTRUCT createStruct
) {
    int ret = __super::OnCreate(createStruct);
    if (ret == -1)
        return ret;
    GetRgn(m_rgn);
    SetWindowRgn(m_rgn, TRUE);
    return 0;
}

void MyWnd::GetRgn(
    CRgn & rgn
) {
    CSize sz = GetBaseSize();
    CRect r(0, m_tri_size, sz.cx, sz.cy + m_tri_size);
    int xc = (r.left + r.right) / 2;
    CPoint vs[] = {
        { r.left, r.top },          // {x=0 y=50}
        { xc - m_tri_size, r.top }, // {x=270 y=50}
        { xc, r.top - m_tri_size }, // {x=320 y=0}
        { xc + m_tri_size, r.top }, // {x=370 y=50}
        { r.right, r.top },         // {x=640 y=50}
        { r.right, r.bottom },      // {x=640 y=530}
        { r.left, r.bottom }        // {x=0 y=530}
    };
    rgn.CreatePolygonRgn(vs, _countof(vs), ALTERNATE);
}

void MyWnd::OnPaint(
) {
    CPaintDC dc(this);
    CRect r;
    GetClientRect(r);
    dc.FillSolidRect(r, RGB(255, 0, 0));
}

MyWnd.h

#pragma once

class MyWnd : public CWnd
{
    DECLARE_DYNAMIC(MyWnd)
    DECLARE_MESSAGE_MAP()

public:
    MyWnd() {}
    virtual ~MyWnd() {}
    static void RegisterWndClazz();
    virtual bool CreateMe();

protected:
    afx_msg int OnCreate(LPCREATESTRUCT createStruct);
    afx_msg void OnPaint();
    virtual void GetRgn(CRgn & rgn);
    virtual CSize GetBaseSize();
    virtual CRect GetInitialRect();

private:
    int m_tri_size = 50;
    CRgn m_rgn;
};

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