应用场景：用户头像为空时根据后台返回字符串绘制图片
设置size
CGRect rect = CGRectMake(0, 0, size.width, size.height);
UIGraphicsBeginImageContextWithOptions(rect.size, NO, 0);
CGContextRef context = UIGraphicsGetCurrentContext();
设置形状（圆形）
if (isCircular) {//圆形
CGPathRef path = CGPathCreateWithEllipseInRect(rect, NULL);
CGContextAddPath(context, path);
CGContextClip(context);
CGPathRelease(path);
}
设置字体颜色：color
CGContextSetFillColorWithColor(context, color.CGColor);
CGContextFillRect(context, rect);
设置展示内容：text
CGSize textSize = [text sizeWithAttributes:textAttributes];
[text drawInRect:CGRectMake((size.width - textSize.width) / 2, (size.height - textSize.height) / 2, textSize.width, textSize.height) withAttributes:textAttributes];
上下文获取图像
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;
