How to change an image in a window MFC application - c++

Question

Let me start off by saying that I am very new to MFC applications, so please bear with me...
I would like to create a window with an image (lets say a green light). When necessary, I want the application to change the image (to a yellow or red light). I have code that will create the window and display the image (green light), but it will not update the image (when i == 15 or i == 50)
#include "resource.h"
using namespace std;
const char g_szClassName[] = "myWindowClass";
HBITMAP Light = NULL;
// Step 4: the Window Procedure
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch (msg)
{
case WM_PAINT:
{
BITMAP bm;
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hwnd, &ps);
HDC hdcMem = CreateCompatibleDC(hdc);
HBITMAP hbmOld = (HBITMAP)SelectObject(hdcMem, Light);
GetObject(Light, sizeof(bm), &bm);
BitBlt(hdc, 20, 180, bm.bmWidth, bm.bmHeight, hdcMem, 0, 0, SRCCOPY);
SelectObject(hdcMem, hbmOld);
DeleteDC(hdcMem);
EndPaint(hwnd, &ps);
}
case WM_CREATE:
{
Light = LoadBitmap(GetModuleHandle(NULL), MAKEINTRESOURCE(IDB_BITMAP1));
}
break;
case WM_COMMAND:
switch (LOWORD(wParam))
{
}
break;
case WM_CLOSE:
DestroyWindow(hwnd);
break;
case WM_DESTROY:
{
DeleteObject(Light);
PostQuitMessage(0);
}
break;
default:
return DefWindowProc(hwnd, msg, wParam, lParam);
}
return 0;
}
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPSTR lpCmdLine, int nCmdShow)
{
WNDCLASSEX wc;
HWND hwnd;
MSG Msg;
int eventCode;
int i = 0;
wc.cbSize = sizeof(WNDCLASSEX);
wc.style = 0;
wc.lpfnWndProc = WndProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = hInstance;
wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wc.hCursor = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
wc.lpszMenuName = NULL;
wc.lpszClassName = (LPCSTR)g_szClassName;
wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);
if (!RegisterClassEx(&wc))
{
MessageBox(NULL, (LPCSTR)"Window Registration Failed!", (LPCSTR)"Error!",
MB_ICONEXCLAMATION | MB_OK);
return 0;
}
// Step 2: Creating the Window
hwnd = CreateWindowEx(
WS_EX_CLIENTEDGE,
(LPCSTR)g_szClassName,
(LPCSTR)"Test Picture",
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, CW_USEDEFAULT, 420, 280,
NULL, NULL, hInstance, NULL);
if (hwnd == NULL)
{
MessageBox(NULL, (LPCSTR)"Window Creation Failed!", (LPCSTR)"Error!",
MB_ICONEXCLAMATION | MB_OK);
return 0;
}
ShowWindow(hwnd, nCmdShow);
UpdateWindow(hwnd);
// Step 3: The Message Loop
while (GetMessage(&Msg, NULL, 0, 0) > 0)
{
TranslateMessage(&Msg);
DispatchMessage(&Msg);
if (i == 15)
{
Light = LoadBitmap(GetModuleHandle(NULL), MAKEINTRESOURCE(IDB_BITMAP3));
UpdateWindow(hwnd);
}
else if (i == 50)
{
Light = LoadBitmap(GetModuleHandle(NULL), MAKEINTRESOURCE(IDB_BITMAP2));
UpdateWindow(hwnd);
}
i++;
}
return Msg.wParam;
}
I thought that I could do
Light = LoadBitmap(GetModuleHandle(NULL), MAKEINTRESOURCE(IDB_BITMAP3));
UpdateWindow(hwnd);
to update the window picture, but I was wrong :( ...what could I use to change the image?

Answer 1

You need to call InvalidateRect to cause a repaint. UpdateWindow doesn't do anything if the window's invalid region is empty.