iphone – CGContextAddLineToPoint：没有当前点

我想在点之间画线来打开锁,但是当我画画时,它会返回一个错误：

<错误>：CGContextAddLineToPoint：没有当前点

它在iOS 5.0和之前的工作正常但它在5.1中显示错误.

这是我的代码：

- (void)drawRect:(CGRect)rect
{
 NSLog(@"drawrect...%@",NSStringFromCGRect(rect));

 if (!self._trackPointValue)
 return;

CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context,10.0);
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
CGFloat components[] = {0.5,1.0,0.5,0.8};
CGColorRef color = CGColorCreate(colorspace,components);
CGContextSetStrokeColorWithColor(context,color);

CGPoint from;
UIView *lastDot;
for (UIView *dotView in self._dotViews) {  //_dotViews array of points
 from = dotView.center;      
 if (!lastDot)
 {
  CGContextMoveToPoint(context,from.x,from.y);

  }
 else
 {
    NSLog(@"from : %@",NSStringFromCGPoint(from));
   CGContextAddLineToPoint(context,from.y);

 }

 lastDot = dotView;
}

 CGPoint pt = [self._trackPointValue CGPointValue];  //_trackPointValue is current point

 CGContextAddLineToPoint(context,pt.x,pt.y);

 CGContextStrokePath(context);
 CGColorSpaceRelease(colorspace);
 CGColorRelease(color);

 self._trackPointValue = nil;//_trackPointValue is current point
 }