CoreGraphics same code draw different - iphone

Question

I write a msg background in tableviewcell, but when scrolling, the view become different. seems like the "context" store some state.
The code draw a round rect with a arrow.The arrow size should be fixed,but in fact it will change sometimes then scrolling.
- (void)drawRect:(CGRect)rect
{
// Drawing code
CGContextRef context = UIGraphicsGetCurrentContext();
CGFloat width = self.frame.size.width;
CGFloat height = self.frame.size.height;
CGFloat arrowHeight = 10;
CGFloat arrowWidth = 5;
CGFloat arrowOffset = 5;
CGFloat radius = 5;
CGFloat shadowHeight = 2;
CGFloat shadowOffset = 4;
CGColorRef shadowColor = [UIColor colorWithRed:0.2f
green:0.2f
blue:0.2f
alpha:0.5f].CGColor;
CGColorRef color;
color = [UIColor whiteColor].CGColor;
CGContextSaveGState(context);
CGContextMoveToPoint(context, width/2, 0);
CGContextAddArcToPoint(context, width - arrowWidth, 0, width - arrowWidth, height - shadowOffset, radius);
CGContextAddLineToPoint(context, width - arrowWidth, arrowOffset);
CGContextAddLineToPoint(context, width, arrowOffset + arrowHeight/2);
CGContextAddLineToPoint(context, width - arrowWidth, arrowOffset + arrowHeight);
CGContextAddArcToPoint(context, width - arrowWidth, height - shadowOffset, shadowOffset, height - shadowOffset, radius);
CGContextAddArcToPoint(context, shadowOffset, height - shadowOffset, shadowOffset, 0, radius);
CGContextAddArcToPoint(context, shadowOffset, 0, width, 0, radius);
CGContextSetShadowWithColor(context, CGSizeMake(0, shadowHeight), 3.0, shadowColor);
CGContextSetFillColorWithColor(context, color);
CGContextFillPath(context);
CGContextRestoreGState(context);
}
the effect like this:

Answer 1

I find the problem. the view should redraw when cell is reused,or the view will use the view in last cell to show.
so, just call [theView setNeedsDisplay] to let the view call drawRect again.