上代码：
func circleImageWithString(string: String, frame: CGRect, textFont: CGFloat, backgroundColor: UIColor, textColor: UIColor) -> UIImage {
let titleLabel = UILabel(frame: CGRectMake(0,0,frame.size.width * 3,frame.size.height * 3))
titleLabel.backgroundColor = UIColor.blueColor()
titleLabel.text = string
titleLabel.font = UIFont.systemFontOfSize(textFont * 3)
titleLabel.backgroundColor = backgroundColor
titleLabel.textAlignment = .Center
titleLabel.textColor = textColor
UIGraphicsBeginImageContext(titleLabel.frame.size)
let context: CGContextRef = UIGraphicsGetCurrentContext()!
CGContextSetLineWidth(context, 0)
CGContextSetStrokeColorWithColor(context, UIColor.redColor().CGColor)
let rect = CGRectMake(0, 0, titleLabel.frame.size.width, titleLabel.frame.size.height)
CGContextAddEllipseInRect(context, rect)
CGContextClip(context)
titleLabel.layer.drawInContext(context)
CGContextAddEllipseInRect(context, rect)
CGContextStrokePath(context)
let newImage = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
return newImage
}
