1.图片裁剪

//开启图形上下文

UIGraphicsBeginImageContextWithOptions(topic.pictureF.size, YES, 0.0);

//将下载完的Image对象绘制到图形上下文

CGFloat width = topic.pictureF.size.width;

CGFloat height = width * image.size.height/image.size.width;

[image drawInRect:CGRectMake(0, 0, width, height)];

//获得图片

self.imageView.image = UIGraphicsGetImageFromCurrentImageContext();

//结束图形上下文

UIGraphicsEndImageContext();

2.正方形图片设置圆角（比在图层layer要高效，不会出现卡顿，layer设置圆角会出现卡顿现象）

//开启图形上下文

UIGraphicsBeginImageContextWithOptions(self.size, NO, 0.0);

//获得上下文

CGContextRef ctx = UIGraphicsGetCurrentContext();

//添加一个圆

CGRect rect = CGRectMake(0, 0, self.size.width, self.size.height);

CGContextAddEllipseInRect(ctx, rect);

//裁剪

CGContextClip(ctx);

//将图片画上去

[self drawInRect:rect];

UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

//结束

UIGraphicsEndImageContext();