1. 设置圆角

-(UIImage *)avaterImageWithImage:(UIImage *)image size:(CGSize)size backColor:(UIColor *)backColor{
    CGRect rect = CGRectMake(0, 0, size.width, size.height);
    UIGraphicsBeginImageContextWithOptions(size, true, 0);
    //0.背景填充
    [backColor setFill];
    UIRectFill(rect);
    //1.实例化圆形路径
    UIBezierPath *path =  [UIBezierPath bezierPathWithOvalInRect:rect];
    [path addClip];
    //2.在指定区域拉伸屏幕
    [image drawInRect:rect];
    //3.绘制内切的圆形
    [[UIColor darkGrayColor] setStroke];
    path.lineWidth = 2;
    [path stroke];
    //4.获取image
    UIImage *result = UIGraphicsGetImageFromCurrentImageContext();
    //5.关闭上下文
    UIGraphicsEndImageContext();
    return result;
}

2. 栅格化

  缺点耗电会增加
//异步绘制
    self.layer.drawsAsynchronously = YES;
    //栅格化
    self.layer.shouldRasterize = YES;
    //指定分辨率
    self.layer.rasterizationScale = [UIScreen mainScreen].scale;

3.cell里不使用block
delegate运行成本低，block成本很高。
cell里的回调事件通过delegate处理，cell由于复用会反复执行赋值操作。