API 实现风车的旋转,该怎么解决

API 实现风车的旋转
这个程序不太懂 问题：不知道它是通过什么实现了 旋转 这个功能 它没有设置Timer ，控制角度的变化来实现旋转  
希望高手解释下  
include<windows.h>
#include<cmath>
#define PI 3.1415926
double fAngle;
int nNum=0;
LRESULT CALLBACK WndProc(HWND,UINT,WPARAM,LPARAM);
int WINAPI WinMain(HINSTANCE hInstance,HINSTANCE hPrevInst,LPSTR lpszCmdLine,int
nCmdShow)
{
  HWND hwnd;
  MSG Msg;
  WNDCLASS wndclass;
  char lpszClassName[]="窗口";
  char lpszTitle[]="Simon's Windows";
  wndclass.style=0;
  wndclass.lpfnWndProc=WndProc;
  wndclass.cbClsExtra=0;
  wndclass.cbWndExtra=0;
  wndclass.hInstance=hInstance;
  wndclass.hIcon=LoadIcon(NULL,IDI_APPLICATION);
  wndclass.hCursor=LoadCursor(NULL,IDC_ARROW);
  wndclass.hbrBackground=(HBRUSH)GetStockObject(WHITE_BRUSH);
  wndclass.lpszMenuName=NULL;
  wndclass.lpszClassName=lpszClassName;

  if(!RegisterClass(&wndclass))
  {
  MessageBeep(0);
  return FALSE;
  }

  hwnd=CreateWindow(
  lpszClassName,
  lpszTitle,
  WS_OVERLAPPEDWINDOW,
  CW_USEDEFAULT,
  CW_USEDEFAULT,
  CW_USEDEFAULT,
  CW_USEDEFAULT,
  NULL,
  NULL,
  hInstance,
  NULL
  );
  ShowWindow(hwnd,nCmdShow);
  UpdateWindow(hwnd);
  while(GetMessage(&Msg,NULL,0,0))
  {
  TranslateMessage(&Msg);
  DispatchMessage(&Msg);
  }
  return Msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hwnd,UINT message,WPARAM wParam,LPARAM
lParam)
{
HDC hdc;
HBRUSH hBrush;
HPEN hp;
PAINTSTRUCT ps;
int nCenterX,nCenterY;
  switch(message)
  {
  case WM_PAINT:
  hdc=BeginPaint(hwnd,&ps);
  SetMapMode(hdc,MM_ANISOTROPIC);
  SetWindowExtEx(hdc,400,300,NULL);
  SetViewportExtEx(hdc,600,450,NULL);
  SetViewportOrgEx(hdc,200,200,NULL);
  hp=CreatePen(PS_SOLID,1,BLACK_PEN);
  LineTo(hdc,200,200);
  hp=CreatePen(PS_SOLID,1,RGB(255,0,255));
  SelectObject(hdc,hp);
  Ellipse(hdc,-100,-100,100,100);
  fAngle=2.0*PI/10*nNum;
  nCenterX=(int)(50*sin(fAngle));
  nCenterY=(int)(50*cos(fAngle));
  hBrush=CreateSolidBrush(RGB(255,0,0));
  SelectObject(hdc,hBrush);
  Pie(hdc,nCenterX-50,nCenterY-50,nCenterX+50,nCenterY+50,(int)100*sin(fAngle),(int)100*cos(fAngle),0,0);
   
   
  fAngle=2.0*PI/3+fAngle;
  nCenterX=(int)(50*sin(fAngle));
  nCenterY=(int)(50*cos(fAngle));
  hBrush=CreateSolidBrush(RGB(255,255,0));
  SelectObject(hdc,hBrush);
  Pie(hdc,nCenterX-50,nCenterY-50,nCenterX+50,nCenterY+50,100*sin(fAngle),100*cos(fAngle),0,0);
   
  fAngle=2.0*PI/3+fAngle;
  nCenterX=(int)(50*sin(fAngle));
  nCenterY=(int)(50*cos(fAngle));
  hBrush=CreateSolidBrush(RGB(0,255,255));
  SelectObject(hdc,hBrush);
  Pie(hdc,nCenterX-50,nCenterY-50,nCenterX+50,nCenterY+50,100*sin(fAngle),100*cos(fAngle),0,0);
  nNum++;
  Sleep(100);
  InvalidateRect(hwnd,NULL,1);
  DeleteObject(hBrush);
  EndPaint(hwnd,&ps);
  return 0;