kéo lê đối tượng bằng cách xóa và vẽ lại (chương 7)

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

#include "stdafx.h"
#include "resource.h"

#define MAX_LOADSTRING 100

// Global Variables:
HINSTANCE hInst;                                // current instance
TCHAR szTitle[MAX_LOADSTRING];                                // The title bar text
TCHAR szWindowClass[MAX_LOADSTRING];                                // The title bar text

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

int APIENTRY WinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPSTR     lpCmdLine,
                     int       nCmdShow)
{
     // TODO: Place code here.
    MSG msg;
    HACCEL hAccelTable;

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

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

    hAccelTable = LoadAccelerators(hInstance, (LPCTSTR)IDC_OBJECTDRAG);

    // Main message loop:
    while (GetMessage(&msg, NULL, 0, 0)) 
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg)) 
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return msg.wParam;
}



//
//  FUNCTION: MyRegisterClass()
//
//  PURPOSE: Registers the window class.
//
//  COMMENTS:
//
//    This function and its usage is 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)WndProc;
    wcex.cbClsExtra        = 0;
    wcex.cbWndExtra        = 0;
    wcex.hInstance        = hInstance;
    wcex.hIcon            = LoadIcon(hInstance, (LPCTSTR)IDI_OBJECTDRAG);
    wcex.hCursor        = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground    = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName    = (LPCSTR)IDC_OBJECTDRAG;
    wcex.lpszClassName    = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, (LPCTSTR)IDI_SMALL);

    return RegisterClassEx(&wcex);
}

//
//   FUNCTION: InitInstance(HANDLE, 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, unsigned, WORD, LONG)
//
//  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
//
//

// ==============
bool IsInsideEllipse(int x1, int y1, int x2, int y2, int x, int y) 
{ 
  double a = (double(x2)-x1)/2, b = (double(y2)-y1)/2; 
  if (!a || !b) return false; 
  double xx = x - (double(x1)+x2)/2, yy = y - (double(y1)+y2)/2; 
  double xr = xx/a, yr = yy/b; 
  return xr*xr+yr*yr <= 1; 
}
// ==============

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    int wmId, wmEvent;
    PAINTSTRUCT ps;
    HDC hdc;
    TCHAR szHello[MAX_LOADSTRING];
    LoadString(hInst, IDS_HELLO, szHello, MAX_LOADSTRING);

    // <!==
    static HCURSOR hCursorMove; 
    static x1,y1,x2,y2, px, py; 
    static HBRUSH hbr;
    // ===>

    switch (message) 
    {
        
        // <!==
        case WM_CREATE: 
            RECT rt; 
            GetClientRect(hWnd, &rt); 
            x1 = rt.right/4; x2 = 3*rt.right/4;  
            y1 = rt.bottom/4; y2 = 3*rt.bottom/4;  
 
            HBITMAP hBitmap; 
            hBitmap = LoadBitmap(hInst, (LPCTSTR)IDB_HEART); 
            hbr = CreatePatternBrush(hBitmap); 
            DeleteObject(hBitmap); 
 
            hCursorMove = LoadCursor(NULL, IDC_SIZEALL); 
            break;
            
        case WM_LBUTTONDOWN: 
            int x, y;       
            px = x = LOWORD(lParam); 
            py = y = HIWORD(lParam); 
            if (IsInsideEllipse(x1,y1,x2,y2,x,y)) 
                SetCapture(hWnd); 
            break; 

        case WM_LBUTTONUP: 
            if (GetCapture() == hWnd) 
                ReleaseCapture(); 
            break; 
        case WM_MOUSEMOVE: 
            x = LOWORD(lParam); 
            y = HIWORD(lParam); 
            if (IsInsideEllipse(x1,y1,x2,y2,x,y))
            { 
                SetCursor(hCursorMove); 
                if (GetCapture() == hWnd) 
                { 
                    int dx = x-px, dy = y-py; 
                    x1 += dx; y1 += dy; 
                    x2 += dx; y2 += dy; 
                    px = x; 
                    py = y; 
                    InvalidateRect(hWnd, NULL, TRUE); 
                } 
            } 
            break; 
        // ===>

        case WM_COMMAND:
            wmId    = LOWORD(wParam); 
            wmEvent = HIWORD(wParam); 
            // Parse the menu selections:
            switch (wmId)
            {
                case IDM_ABOUT:
                   DialogBox(hInst, (LPCTSTR)IDD_ABOUTBOX, hWnd, (DLGPROC)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...
        //    RECT rt;
            GetClientRect(hWnd, &rt);
            DrawText(hdc, szHello, strlen(szHello), &rt, DT_CENTER);

            // <!==
            SelectObject(hdc, hbr); 
            Ellipse(hdc,x1,y1,x2,y2); 
            // ===>

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

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

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