iOS 照片上传服务器旋转90度解决办法

最近在处理移动端选择图片实时预览并上传时遇到一个问题：上传前图片预览正常，但上传到服务器上的图片展示到页面上时，有时会出现图片翻转的问题，一般是翻转 90 度。后经一翻研究找到了问题所在，特在此记录一下。
发现：如果选取的图片是在横屏状态下拍摄的，则上传后不会出现图片翻转的问题；反之，如果是在竖屏状态下拍摄的，上传后就会出现图片翻转的问题。

@interface UIImage (fixOrientation)
- (UIImage *)fixOrientation;
@end


  @implementation UIImage (fixOrientation)
- (UIImage *)fixOrientation:(UIImage *)aImage {  
// No-op if the orientation is already correct  
if (aImage.imageOrientation == UIImageOrientationUp)   
    return aImage;  
// 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 (aImage.imageOrientation) {  
    case UIImageOrientationDown:  
    case UIImageOrientationDownMirrored:  
        transform = CGAffineTransformTranslate(transform, aImage.size.width, aImage.size.height);  
        transform = CGAffineTransformRotate(transform, M_PI);  
        break;  
    case UIImageOrientationLeft:  
    case UIImageOrientationLeftMirrored:  
        transform = CGAffineTransformTranslate(transform, aImage.size.width, 0);  
        transform = CGAffineTransformRotate(transform, M_PI_2);  
        break;     
    case UIImageOrientationRight:  
    case UIImageOrientationRightMirrored:  
        transform = CGAffineTransformTranslate(transform, 0, aImage.size.height);  
        transform = CGAffineTransformRotate(transform, -M_PI_2);  
        break;  
    default:  
        break;  
}  
switch (aImage.imageOrientation) {  
    case UIImageOrientationUpMirrored:  
    case UIImageOrientationDownMirrored:  
        transform = CGAffineTransformTranslate(transform, aImage.size.width, 0);  
        transform = CGAffineTransformScale(transform, -1, 1);  
        break;          
    case UIImageOrientationLeftMirrored:  
    case UIImageOrientationRightMirrored:  
        transform = CGAffineTransformTranslate(transform, aImage.size.height, 0);  
        transform = CGAffineTransformScale(transform, -1, 1);  
        break;  
    default:  
        break;  
}  
// Now we draw the underlying CGImage into a new context, applying the transform  
// calculated above.  
CGContextRef ctx = CGBitmapContextCreate(NULL, aImage.size.width, aImage.size.height,  
                                         CGImageGetBitsPerComponent(aImage.CGImage), 0,  
                                         CGImageGetColorSpace(aImage.CGImage),  
                                         CGImageGetBitmapInfo(aImage.CGImage));  
CGContextConcatCTM(ctx, transform);  
switch (aImage.imageOrientation) {  
    case UIImageOrientationLeft:  
    case UIImageOrientationLeftMirrored:  
    case UIImageOrientationRight:  
    case UIImageOrientationRightMirrored:  
        // Grr...  
        CGContextDrawImage(ctx, CGRectMake(0,0,aImage.size.height,aImage.size.width), aImage.CGImage);  
        break;    
    default:  
        CGContextDrawImage(ctx, CGRectMake(0,0,aImage.size.width,aImage.size.height), aImage.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;  }