Windows program----move a rect - windows

Question

I try to move a rect, but I'm puzzled. I just creat a rect, but after I move it 2 steps, the result is it have two copies. I used to wrote similar code, but this time something goes wrong.
just like this
begin:
b
move two steps:
b
b
b
LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
HDC hdc ;
static int x_left,y_top,x_right,y_bottom;
PAINTSTRUCT ps ;
switch (message)
{
case WM_CREATE:
hdc = GetDC (hwnd) ;
x_left=100;
x_right=300;
y_top=50;
y_bottom=200;
Rectangle(hdc,x_left,y_top,x_right,y_bottom);
ReleaseDC (hwnd, hdc);
return 0;
case WM_KEYDOWN:
switch(wParam){
case VK_DOWN:
y_top+=160;
y_bottom+=160;
break;
default:break;
}
InvalidateRect(hwnd,NULL, FALSE);
break;
case WM_PAINT:
hdc=BeginPaint(hwnd,&ps);
Rectangle(hdc,x_left,y_top,x_right,y_bottom);
EndPaint(hwnd,&ps);
return 0;
case WM_DESTROY :
PostQuitMessage (0) ;
return 0 ;
}
return DefWindowProc (hwnd, message, wParam, lParam) ;
}

Answer 1

Replace the InvalidateRect line with:
InvalidateRect(hwnd,NULL, TRUE);
Your WM_PAINT doesn't explicitly paint the background. When you call InvalidateRect, you're not telling the function to paint the background with the window color. Ergo, the drawing (the rectangle) that was already there remains drawn.
Also, don't draw in WM_CREATE. The window isn't visible yet. The moment it appears, WM_PAINT will fire. You can draw with GetDC/ReleaseDC in the WM_KEYDOWN handler if you wish, but the InvalidateRect/WM_PAINT sequence won't be noticeably slower, and the code is cleaner that way.