Splitter

#include <windows.h>

#define MINSIZE        50 
  // The minimal width and height a childwindow should have
#define SBS            2  // Short for SplitterBarSize

LRESULT CALLBACK WndProc(HWND hwnd, UINT iMsg, WPARAM wParam, LPARAM lParam)
{
  static HWND hwndChild1, hwndChild2, hwndChild3;
  static CREATESTRUCT *cs;
  static HCURSOR hcSizeNS, hcSizeEW, hcArrow;
  static int xDiv, yDiv;
  static BOOL xSizing, ySizing;

  int xPos, yPos;
  RECT rect, focusrect;
  HDC hdc;

  switch (iMsg)
  {
    case WM_CREATE:
      cs = (CREATESTRUCT*)lParam;
      xDiv = 300;
      yDiv = 300;

      hwndChild1 = CreateWindowEx(WS_EX_CLIENTEDGE, "Pane 1", "Child 1",
        WS_CHILD | WS_VISIBLE | WS_CLIPCHILDREN, 0, 0, 0, 0, hwnd, NULL, cs
        ->hInstance, NULL);
      hwndChild2 = CreateWindowEx(WS_EX_CLIENTEDGE, "Pane 2", "Child 2",
        WS_CHILD | WS_VISIBLE | WS_CLIPCHILDREN, 0, 0, 0, 0, hwnd, NULL, cs
        ->hInstance, NULL);
      hwndChild3 = CreateWindowEx(WS_EX_CLIENTEDGE, "Pane 3", "Child 3",
        WS_CHILD | WS_VISIBLE | WS_CLIPCHILDREN, 0, 0, 0, 0, hwnd, NULL, cs
        ->hInstance, NULL);

      hcSizeNS = LoadCursor(NULL, IDC_SIZENS);
      hcSizeEW = LoadCursor(NULL, IDC_SIZEWE);
      hcArrow = LoadCursor(NULL, IDC_ARROW);
      break;

    case WM_SIZE:
      GetClientRect(hwnd, &rect);
      // Make sure splitterbars are visible
      if (xDiv > rect.right - MINSIZE)
      {
        xDiv = rect.right - MINSIZE;
      }
      else if (xDiv < MINSIZE)
      {
        xDiv = MINSIZE;
      }
      if (yDiv > rect.bottom - MINSIZE)
      {
        yDiv = rect.bottom - MINSIZE;
      }
      else if (yDiv < MINSIZE)
      {
        yDiv = MINSIZE;
      }
      MoveWindow(hwndChild1, rect.left, rect.top + 1, rect.left + xDiv - SBS,
        rect.top + yDiv - SBS, FALSE);
      MoveWindow(hwndChild2, rect.left + xDiv + SBS, rect.top + 1, rect.right -
        rect.left - xDiv - SBS, rect.top + yDiv - SBS, FALSE);
      MoveWindow(hwndChild3, rect.left, rect.top + yDiv + SBS, rect.right -
        rect.left, rect.bottom - rect. top - yDiv - SBS, FALSE);
      InvalidateRect(hwnd, &rect, TRUE);
      break;

    case WM_LBUTTONDOWN:
      xPos = (int)LOWORD(lParam);
      yPos = (int)HIWORD(lParam);

      xSizing = (xPos > xDiv - SBS && xPos < xDiv + SBS);
      ySizing = (yPos > yDiv - SBS && yPos < yDiv + SBS);

      if (xSizing || ySizing)
      {
        SetCapture(hwnd); // Track movement of the mousepointer 
        if (xSizing)
        {
          SetCursor(hcSizeEW);
        }
        else if (ySizing)
        {
          SetCursor(hcSizeNS);
        }
      }
      break;

    case WM_LBUTTONUP:
      if (xSizing || ySizing)
      {
        ReleaseCapture();
        hdc = GetDC(hwnd);
        GetClientRect(hwnd, &rect);
        if (xSizing)
        {
          SetRect(&focusrect, xDiv - SBS, rect.top, xDiv + SBS, yDiv);
          DrawFocusRect(hdc, &focusrect);
          xSizing = FALSE;
        }
        if (ySizing)
        {
          SetRect(&focusrect, rect.left + 1, yDiv - SBS, rect.right, yDiv + SBS)
            ;
          DrawFocusRect(hdc, &focusrect);
          ySizing = FALSE;
        }
        ReleaseDC(hwnd, hdc);
      }
      // Post a size message so the childwindows get their new size
      PostMessage(hwnd, WM_SIZE, 0, 0);
      break;

    case WM_MOUSEMOVE:
      xPos = (int)LOWORD(lParam);
      yPos = (int)HIWORD(lParam);

      if (wParam == MK_LBUTTON)
      // Is one of the splitterbars being moved?
      {
        if (xSizing || ySizing)
        {
          hdc = GetDC(hwnd);
          GetClientRect(hwnd, &rect);

          if (xSizing)
          {
            SetRect(&focusrect, xDiv - SBS, rect.top, xDiv + SBS, yDiv);
            DrawFocusRect(hdc, &focusrect);

            xDiv = xPos;

            SetRect(&focusrect, xDiv - SBS, rect.top, xDiv + SBS, yDiv);
            DrawFocusRect(hdc, &focusrect);
          }
          if (ySizing)
          {
            SetRect(&focusrect, rect.left + 1, yDiv - SBS, rect.right, yDiv +
              SBS);
            DrawFocusRect(hdc, &focusrect);

            yDiv = yPos;

            SetRect(&focusrect, rect.left + 1, yDiv - SBS, rect.right, yDiv +
              SBS);
            DrawFocusRect(hdc, &focusrect);
          }
          ReleaseDC(hwnd, hdc);
        }
        break; // Leave
      }

      if ((xPos > xDiv - SBS && xPos < xDiv + SBS) && GetCursor() != hcSizeEW)
      {
        SetCursor(hcSizeEW); // hovering above the vertical splitterbar
      }
      else if ((yPos > yDiv - SBS && yPos < yDiv + SBS) && GetCursor() !=
        hcSizeNS)
      {
        SetCursor(hcSizeNS); // hovering above the horizontal splitterbar
      }
      else if (GetCursor() != hcArrow)
      {
        SetCursor(hcArrow); // not hovering above a splitterbar
      }
      break;

    case WM_CLOSE:
      DestroyWindow(hwnd);
      break;

    case WM_DESTROY:
      PostQuitMessage(0);
      break;
  }
  return DefWindowProc(hwnd, iMsg, wParam, lParam);
}

LRESULT CALLBACK Child1WndProc(HWND hwnd, UINT iMsg, WPARAM wParam, LPARAM
  lParam)
{
  switch (iMsg)
  {
    case WM_CREATE:
      break;
  }
  return DefWindowProc(hwnd, iMsg, wParam, lParam);
}

LRESULT CALLBACK Child2WndProc(HWND hwnd, UINT iMsg, WPARAM wParam, LPARAM
  lParam)
{
  switch (iMsg)
  {
    case WM_CREATE:
      break;
  }
  return DefWindowProc(hwnd, iMsg, wParam, lParam);
}

LRESULT CALLBACK Child3WndProc(HWND hwnd, UINT iMsg, WPARAM wParam, LPARAM
  lParam)
{
  switch (iMsg)
  {
    case WM_CREATE:
      break;
  }
  return DefWindowProc(hwnd, iMsg, wParam, lParam);
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine,
  int iCmdShow)
{
  HWND hwnd;
  MSG msg;
  WNDCLASS wndclass;
  char szAppName[] = "SplitterApp";

  wndclass.style = 0;
  wndclass.lpfnWndProc = WndProc;
  wndclass.cbClsExtra = 0;
  wndclass.cbWndExtra = 0;
  wndclass.hInstance = hInstance;
  wndclass.hIcon = LoadIcon(hInstance, szAppName);
  wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
  wndclass.hbrBackground = (HBRUSH)(COLOR_BTNFACE + 1);
  wndclass.lpszMenuName = szAppName;
  wndclass.lpszClassName = szAppName;
  RegisterClass(&wndclass);

  wndclass.lpfnWndProc = Child1WndProc;
  wndclass.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
  wndclass.lpszClassName = "Pane 1";
  RegisterClass(&wndclass);

  wndclass.lpfnWndProc = Child2WndProc;
  wndclass.lpszClassName = "Pane 2";
  RegisterClass(&wndclass);

  wndclass.lpfnWndProc = Child3WndProc;
  wndclass.lpszClassName = "Pane 3";
  RegisterClass(&wndclass);

  hwnd = CreateWindow(szAppName, szAppName, WS_OVERLAPPEDWINDOW | SW_MAXIMIZE |
    WS_VISIBLE, 0, 0, 600, 500, NULL, NULL, hInstance, szCmdLine);

  while (GetMessage(&msg, NULL, 0, 0))
  {
    TranslateMessage(&msg);
    DispatchMessage(&msg);
  }
  return (int)msg.wParam;
}