iOS上传相册图片到服务器端显示位置翻转问题

iOS用相机拍摄出来的照片含有EXIF信息，UIImage的imageOrientation属性指的就是EXIF中的orientation信息。因为不是所有的工具都支持方向属性，这其中甚至包含了具有最多用户群体的Windows，所以要在保存之前将照片旋转至正常观看的方向即可，然后直接将最终具有正确方向的照片保存下来。
首先，为 UIImage 创建一个category，其中包含 fixOrientation 方法：

@interface UIImage (fixOrientation)

- (UIImage *)fixOrientation;

@end

@implementation UIImage (fixOrientation)

  - (UIImage *)fixOrientation {

      // No-op if the orientation is already correct
      if (self.imageOrientation == UIImageOrientationUp) return self;

      // We need to calculate the proper transformation to make the image upright.
      // We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored.
     CGAffineTransform transform = CGAffineTransformIdentity;

     switch (self.imageOrientation) {
         case UIImageOrientationDown:
         case UIImageOrientationDownMirrored:
             transform = CGAffineTransformTranslate(transform, self.size.width, self.size.height);
             transform = CGAffineTransformRotate(transform, M_PI);
             break;

         case UIImageOrientationLeft:
         case UIImageOrientationLeftMirrored:
             transform = CGAffineTransformTranslate(transform, self.size.width, 0);
             transform = CGAffineTransformRotate(transform, M_PI_2);
             break;

         case UIImageOrientationRight:
         case UIImageOrientationRightMirrored:
             transform = CGAffineTransformTranslate(transform, 0, self.size.height);
             transform = CGAffineTransformRotate(transform, -M_PI_2);
             break;
         case UIImageOrientationUp:
         case UIImageOrientationUpMirrored:
             break;
     }

     switch (self.imageOrientation) {
         case UIImageOrientationUpMirrored:
         case UIImageOrientationDownMirrored:
             transform = CGAffineTransformTranslate(transform, self.size.width, 0);
             transform = CGAffineTransformScale(transform, -1, 1);
             break;

         case UIImageOrientationLeftMirrored:
         case UIImageOrientationRightMirrored:
             transform = CGAffineTransformTranslate(transform, self.size.height, 0);
             transform = CGAffineTransformScale(transform, -1, 1);
             break;
         case UIImageOrientationUp:
         case UIImageOrientationDown:
         case UIImageOrientationLeft:
         case UIImageOrientationRight:
             break;
     }

     // Now we draw the underlying CGImage into a new context, applying the transform
     // calculated above.
     CGContextRef ctx = CGBitmapContextCreate(NULL, self.size.width, self.size.height,
                                              CGImageGetBitsPerComponent(self.CGImage), 0,
                                              CGImageGetColorSpace(self.CGImage),
                                              CGImageGetBitmapInfo(self.CGImage));
     CGContextConcatCTM(ctx, transform);
     switch (self.imageOrientation) {
         case UIImageOrientationLeft:
         case UIImageOrientationLeftMirrored:
         case UIImageOrientationRight:
         case UIImageOrientationRightMirrored:
             // Grr...
             CGContextDrawImage(ctx, CGRectMake(0,0,self.size.height,self.size.width), self.CGImage);
             break;

         default:
             CGContextDrawImage(ctx, CGRectMake(0,0,self.size.width,self.size.height), self.CGImage);
             break;
     }

     // And now we just create a new UIImage from the drawing context
     CGImageRef cgimg = CGBitmapContextCreateImage(ctx);
     UIImage *img = [UIImage imageWithCGImage:cgimg];
     CGContextRelease(ctx);
     CGImageRelease(cgimg);
     return img;
 }

@end