Animation and event questions in win32 (C++)

Question

I would like to create a small application which has two animated ellipses. One wanders around on its own using a custom function to determine its direction and behavior; the other is controlled by the arrow keys. When the two collide, there is an alert and the ellipses reset to their initial positions.

Using the method described in this video tutorial (found here: http://xoax.net/comp/cpp/win32/Lesson4.php), I have a successfully made a red ellipse so adding a second ellipse should not be too difficult. I would like for the ellipse I have made to move smoothly and continuously about the screen (for now just on its own and just towards the right). However I do not understand how or where I should insert the command to redraw the screen.

From Google searches, I've seen that InvalidateRect(handle of window, rectangular area to be redrawn, Boolean if window should be cleared first) should be used, but I do not understand where it should be called. In the main message loop? In the callback switch statement? I understand that NULL can be used for the whole window, but I don't know what to put for the window handle.

For the collision detection, where should I place the check? In the main loop? Or somewhere in the callback function's switch statement?

My code:

// MyGUI.cpp : Defines the entry point for the application.
//

#include "stdafx.h"
#include "MyGUI.h"

#define MAX_LOADSTRING 100

// Global Variables:
HINSTANCE hInst;                                // current instance
TCHAR szTitle[MAX_LOADSTRING];                  // The title bar text
TCHAR szWindowClass[MAX_LOADSTRING];            // the main window class name

float MyX = 10;
float MyY = 10;

// Forward declarations of functions included in this code module:
ATOM                MyRegisterClass(HINSTANCE hInstance);
BOOL                InitInstance(HINSTANCE, int);
LRESULT CALLBACK    WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK    About(HWND, UINT, WPARAM, LPARAM);

int APIENTRY _tWinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPTSTR    lpCmdLine,
                     int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    // TODO: Place code here.
    MSG msg;
    HACCEL hAccelTable;

    // Initialize global strings
    LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadString(hInstance, IDC_MYGUI, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);

    // Perform application initialization:
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }

    hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_MYGUI));

    // Main message loop:
    while (GetMessage(&msg, NULL, 0, 0))
    {
        MyX+=0.5;
//      InvalidateRect(hInst, NULL, true); ???
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}



//
//  FUNCTION: MyRegisterClass()
//
//  PURPOSE: Registers the window class.
//
//  COMMENTS:
//
//    This function and its usage are only necessary if you want this code
//    to be compatible with Win32 systems prior to the 'RegisterClassEx'
//    function that was added to Windows 95. It is important to call this function
//    so that the application will get 'well formed' small icons associated
//    with it.
//
ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEX wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);

    wcex.style          = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra     = 0;
    wcex.cbWndExtra     = 0;
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_MYGUI));
    wcex.hCursor        = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName   = MAKEINTRESOURCE(IDC_MYGUI);
    wcex.lpszClassName  = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

    return RegisterClassEx(&wcex);
}

//
//   FUNCTION: InitInstance(HINSTANCE, int)
//
//   PURPOSE: Saves instance handle and creates main window
//
//   COMMENTS:
//
//        In this function, we save the instance handle in a global variable and
//        create and display the main program window.
//
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   HWND hWnd;

   hInst = hInstance; // Store instance handle in our global variable

   hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);

   if (!hWnd)
   {
      return FALSE;
   }

   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}

//
//  FUNCTION: WndProc(HWND, UINT, WPARAM, LPARAM)
//
//  PURPOSE:  Processes messages for the main window.
//
//  WM_COMMAND  - process the application menu
//  WM_PAINT    - Paint the main window
//  WM_DESTROY  - post a quit message and return
//
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    int wmId, wmEvent;
    PAINTSTRUCT ps;
    HDC hdc;

    switch (message)
    {
    case WM_COMMAND:
        wmId    = LOWORD(wParam);
        wmEvent = HIWORD(wParam);
        // Parse the menu selections:
        switch (wmId)
        {
        case IDM_ABOUT:
            DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
            break;
        case IDM_EXIT:
            DestroyWindow(hWnd);
            break;
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
        }
        break;
    case WM_PAINT:
        hdc = BeginPaint(hWnd, &ps);
        // TODO: Add any drawing code here...

        HPEN hPenOld;

        // Draw a red line
        HPEN hEllipsePen;
        COLORREF qEllipseColor;
        qEllipseColor = RGB(255, 0,0);

        hEllipsePen = CreatePen(PS_SOLID, 3, qEllipseColor);
        hPenOld = (HPEN)SelectObject(hdc, hEllipsePen);

        Arc(hdc, MyX, MyY, MyX+10, MyY+10, 0, 0 ,0, 0);

        SelectObject(hdc, hPenOld);
        DeleteObject(hEllipsePen);

        EndPaint(hWnd, &ps);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
    UNREFERENCED_PARAMETER(lParam);
    switch (message)
    {
    case WM_INITDIALOG:
        return (INT_PTR)TRUE;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
        {
            EndDialog(hDlg, LOWORD(wParam));
            return (INT_PTR)TRUE;
        }
        break;
    }
    return (INT_PTR)FALSE;
}

Thanks.

Answer 1

The typical structure for games and other kind of graphical apps is this:

main()
{
    init();
    while (!exit) {
        process_input();
        update_state();
        draw();
    }
}

In process_input() you handle the input from the user. Since you get intput on the WndProc function, your handler in WndProc could just record the events in a queue, which is then processed by this function.

The update_state() function will take care of your animation. Any objects that move on their own will be updated here. You will also do collision detection and any other state related functions, like maybe updating the player score or stats.

Finally, the draw() function takes care of the drawing. If you do not need very low latency drawing in your app, then you could just call InvalidateRect() here, and let the system send the WM_PAINT message to your WndProc. If you need low latency, then you can just get a DC for your window and draw directly in this function.

I hope this helps.

Answer 2

Basically, both the collision detection and the invalidation should be done in the loop wherein the objects are placed or moved on the screen.

The window handle is the hWnd that you created in InitInstance. Pass it to wherever you need it, or make it a global variable or class member.

Answer 3

Your main function as you posted it uses GetMessage(). This function is only partly what you are looking for because it only returns if there is a message in the so called message queue. This means only if the user interacts with your program the GetMessage() functions returns and executes the code within the while() loop. Without any kind of input the program just "waits", and does not move the ellipse on its own. The other alternative is using PeekMessage() which just checks if any messages are available and returns. Using this gives you the opportunity to update the ellipses position very rapidly, but without any control how often the position is being updated/drawn.

In order to control how often user input and draw calls are processed you will need someting like a timer which execute the main loop at certain intervals. Have a look at SetWaitableTimer() and WaitForSingleObject() in the MSDN documentation. The basic structure of the applicatin loop has been described by Miguels post.