-(UIImage *)cutCenterSquareImage{

     CGSize imageSize = self.size;

     // 中间最大正方形尺寸

     CGRect centerRect;

     CGFloat centerRectWH;

     //根据图片的大小计算出图片中间矩形区域的位置与大小

     if (imageSize.width > imageSize.height) {

         centerRectWH = imageSize.height;

         float leftMargin = (imageSize.width - imageSize.height) * 0.5;

         centerRect = CGRectMake(leftMargin,,centerRectWH,centerRectWH);

     }else{

         centerRectWH = imageSize.width;

         float topMargin = (imageSize.height - imageSize.width)*0.5;

         centerRect = CGRectMake(,topMargin,centerRectWH,centerRectWH);

     }

     CGImageRef imageRef = self.CGImage;

     //在最大正方形尺寸范围内截取

     CGImageRef imageRefRect = CGImageCreateWithImageInRect(imageRef, centerRect);

     UIImage *tmp = [[UIImage alloc] initWithCGImage:imageRefRect];

     CGImageRelease(imageRefRect);// tmp是截取之后的image

     /*

     // 使用上下文，可以为上的tmp重新定位

     CGSize imageRectSize = CGSizeMake(centerRectWH, centerRectWH);

     CGRect imageRect = CGRectMake(0, 0, centerRectWH, centerRectWH);

     UIGraphicsBeginImageContext(imageRectSize);

     [tmp drawInRect:imageRect];

     // 从当前context中创建一个改变大小后的图片

     tmp = UIGraphicsGetImageFromCurrentImageContext();

     // 使当前的context出堆栈

     UIGraphicsEndImageContext();

     */

     return tmp;

 }