导入库头文件（重点）
#import <QuartzCore/QuartzCore.h>
//圆角设置
imageView.layer.cornerRadius = 6;
imageView.layer.masksToBounds = YES;
//边框宽度及颜色设置
[imageView.layer setBorderWidth:2];
[imageView.layer setBorderColor:[UIColor blueColor]]; //设置边框为蓝色
//自动适应,保持图片宽高比
imageView.contentMode = UIViewContentModeScaleAspectFit;