WINapi. Can't erase background of window with ellipse - c++

Question

I'm just trying to draw an ellipse:
case WM_PAINT:
hdc = BeginPaint(parentWindow, &ps);
Ellipse(hdc, x, y, width, height);
EndPaint(parentWindow, &ps);
, and then erase it with drawing a new ellipse with some new parameters every second using timer:
case WM_CREATE:
SetTimer(hWnd, 1, 1000, NULL);
break;
case WM_TIMER:
x += 5;
InvalidateRect(hWnd, NULL, TRUE);
break;
But ellipses are not erased and layered:
However, i tried to trace WM_ERASEBKGND and it really is sent every InvalidateRect.
Full code:
#include <Windows.h>
#include <windowsx.h>
#include <tchar.h>
#include <iostream>
TCHAR szWindowClass[] = TEXT("CreateThreadWindow");
TCHAR szAppName[] = TEXT("CreateThreadExample");
BOOL InitWindow(HINSTANCE, int);
ATOM MyRegisterClass(HINSTANCE);
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
HWND parentWindow;
MSG msg;
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
MyRegisterClass(hInstance);
if (!InitWindow(hInstance, nCmdShow))
return FALSE;
BOOL bRet;
while ((bRet = GetMessage(&msg, (HWND)NULL, 0, 0)) != 0)
{
if (bRet == -1)
return FALSE;
else
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
return (int)msg.wParam;
}
ATOM MyRegisterClass(HINSTANCE hInstance)
{
WNDCLASS wndClass;
memset(&wndClass, 0, sizeof(wndClass));
wndClass.lpfnWndProc = WndProc;
wndClass.hInstance = hInstance;
wndClass.lpszMenuName = NULL;
wndClass.lpszClassName = szWindowClass;
return RegisterClass(&wndClass);
}
BOOL InitWindow(HINSTANCE hInstance, int nCmdShow)
{
parentWindow = CreateWindow(szWindowClass, szAppName, WS_OVERLAPPEDWINDOW,
300, 0, 600, 600, NULL, NULL, hInstance, NULL);
ShowWindow(parentWindow, nCmdShow);
return TRUE;
}
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wparam, LPARAM lparam)
{
PAINTSTRUCT ps;
HDC hdc;
static int x = 0, y = 0, width = 200, height = 100;
switch (message) {
case WM_ERASEBKGND:
_RPT1(0, "%s\n", "erase");
break;
case WM_PAINT:
hdc = BeginPaint(hWnd, &ps);
Ellipse(hdc, x, y, width, height);
EndPaint(hWnd, &ps);
break;
case WM_CREATE:
SetTimer(hWnd, 1, 1000, NULL);
break;
case WM_TIMER:
x += 5;
InvalidateRect(hWnd, NULL, TRUE);
break;
case WM_DESTROY:
PostQuitMessage(0);
return 0;
default:
return DefWindowProc(hWnd, message, wparam, lparam);
}
}

Answer 1

Your code isn't erasing anything. It's just drawing an ellipse at the specified coordinates. The previously-drawn ellipse is still there.
You mention the WM_ERASEBKGND message, but there are two reasons why that isn't working for you:
In your window procedure (WndProc), you handle the WM_ERASEBKGND message explicitly, which means that it doesn't get passed to the default window procedure (DefWindowProc). A better way to write your window procedure would be the following:
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wparam, LPARAM lparam)
{
static int x = 0, y = 0, width = 200, height = 100;
switch (message) {
case WM_ERASEBKGND:
{
_RPT1(0, "%s\n", "erase");
break;
}
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
Ellipse(hdc, x, y, width, height);
EndPaint(hWnd, &ps);
return 0;
}
case WM_CREATE:
{
SetTimer(hWnd, 1, 1000, NULL);
break;
}
case WM_TIMER:
{
x += 5;
InvalidateRect(hWnd, NULL, TRUE);
return 0;
}
case WM_DESTROY:
{
PostQuitMessage(0);
return 0;
}
default:
break;
}
return DefWindowProc(hWnd, message, wparam, lparam);
}
Now, the default window procedure gets called every time, unless you explicitly return from inside of a case label.
When you register your window class (inside of MyRegisterClass), you zero all fields of the WNDCLASS structure and then explicitly initialize a couple of them. You don't explicitly initialize the hbrBackground field, so it is being set to 0. And when hbrBackground is 0,
When this member is NULL, an application must paint its own background whenever it is requested to paint in its client area. To determine whether the background must be painted, an application can either process the WM_ERASEBKGND message or test the fErase member of the PAINTSTRUCT structure filled by the BeginPaint function.
This means that the default window procedure isn't doing anything in response to the WM_ERASEBKGND message because you didn't give your window a background brush.
You will either need to set hbrBackground to something like COLOR_WINDOW + 1, or you will need to add code to your WM_ERASEBKGND message handler to erase the window's background yourself.
Or, perhaps an even better option would be to forget about the WM_ERASEBKGND message altogether, as many Windows programmers do, because this two-step erase-and-paint approach tends to cause flicker. Leave the hbrBackground field set to NULL, don't do anything in response to the WM_ERASEBKGND message, and do your erasing at the top of the WM_PAINT handler:
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
// Erase background of entire client area.
RECT rcClient;
GetClientRect(hWnd, &rcClient);
FillRect(hdc, &rcClient, reinterpret_cast<HBRUSH>(COLOR_WINDOW+1));
// Do normal drawing.
Ellipse(hdc, x, y, width, height);
EndPaint(hWnd, &ps);
return 0;
}