윈도우 무효화 (InvalidateRect)

#include <windows.h>

LRESULT CALLBACK WndProc(HWND,UINT,WPARAM,LPARAM);
HINSTANCE g_hInst;
LPSTR lpszClass="WmPaint";

int APIENTRY WinMain(HINSTANCE hInstance,HINSTANCE hPrevInstance
         ,LPSTR lpszCmdParam,int nCmdShow)
{
    HWND hWnd;
    MSG Message;
    WNDCLASS WndClass;
    g_hInst=hInstance;
    
    WndClass.cbClsExtra=0;
    WndClass.cbWndExtra=0;
    WndClass.hbrBackground=(HBRUSH)GetStockObject(WHITE_BRUSH);
    WndClass.hCursor=LoadCursor(NULL,IDC_ARROW);
    WndClass.hIcon=LoadIcon(NULL,IDI_APPLICATION);
    WndClass.hInstance=hInstance;
    WndClass.lpfnWndProc=(WNDPROC)WndProc;
    WndClass.lpszClassName=lpszClass;
    WndClass.lpszMenuName=NULL;
    WndClass.style=CS_HREDRAW | CS_VREDRAW;
    RegisterClass(&WndClass);

    hWnd=CreateWindow(lpszClass,lpszClass,WS_OVERLAPPEDWINDOW,
         CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,
         NULL,(HMENU)NULL,hInstance,NULL);
    ShowWindow(hWnd,nCmdShow);
    
    while(GetMessage(&Message,0,0,0)) {
        TranslateMessage(&Message);
        DispatchMessage(&Message);
    }
    return Message.wParam;
}

int Graph=0;
LRESULT CALLBACK WndProc(HWND hWnd,UINT iMessage,WPARAM wParam,LPARAM lParam)
{
    PAINTSTRUCT ps;
    HDC hdc;
    RECT rt;                                    // 무효영역을 설정해주기 위한 것
    int i;
    switch(iMessage) {
    case WM_PAINT:
        hdc=BeginPaint(hWnd, &ps);
        for (i=0;i<600;i+=5) {
            MoveToEx(hdc,0,i,NULL);
            LineTo(hdc,800,i);
        }
        for (i=0;i<800;i+=5) {
            MoveToEx(hdc,i,0,NULL);
            LineTo(hdc,i,600);
        }
        if (Graph==0)
            Rectangle(hdc, 10,10,200,200);
        else
            Ellipse(hdc, 10,10,200,200);
        EndPaint(hWnd, &ps);
        return 0;
    case WM_LBUTTONDOWN:
        if (Graph==0)
            Graph=1;
        else
            Graph=0;

        SetRect(&rt, 10,10,200,200);            // 무효영역 설정
        // SetRect(&rt,10,10,100,100);
        InvalidateRect(hWnd, &rt, TRUE);        // SetRect로 지정한 rt영역만 무효화시킨다.
        //InvalidateRect(hWnd,NULL,TRUE);        // NULL이므로 윈도우전체를 무효화시킨다.
        //SendMessage(hWnd, WM_PAINT, 0,0);        // 이렇게 하면 안된다.
        UpdateWindow(hWnd);
        //Sleep(1000);
        return 0;
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }
    return(DefWindowProc(hWnd,iMessage,wParam,lParam));
}