利用CAShapeLayer可以制作出任意的几何图形，把它作为UIImageView的遮罩，达到把图片做成圆形效果。
imgView = [[UIImageView alloc]initWithFrame:CGRectMake(10, 35, 80, 80)];
imgView.image = [UIImage imageNamed:@"ma.jpg"];
UIBezierPath* path = [UIBezierPath bezierPathWithArcCenter:CGPointMake(40, 40) radius:40 startAngle:0 endAngle:2*M_PI clockwise:YES];
CAShapeLayer* shape = [CAShapeLayer layer];
shape.path = path.CGPath;
imgView.layer.mask = shape;
[self.view addSubview:imgView];