iOS开发，解决上传照片按原图旋转了90度的问题

做项目的时候发现, 如果把通过相机获取到的图片，直接进行操作, 比如裁剪, 缩放, 可能会把原图片向右旋转90度。
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。
如果我们忽略orientation信息，而直接对照片进行像素处理或者drawInRect等操作，得到的结果是翻转或者旋转90之后的样子。这是因为我们执行像素处理或者drawInRect等操作之后，imageOrientaion信息被删除了，imageOrientaion被重设为0，造成照片内容和imageOrientaion不匹配。
所以，在对照片进行处理之前，先将照片旋转到正确的方向，并且返回的imageOrientaion为0。

下面这个方法就是一个UIImage category中的方法，用它可以达到获取正确照片方向的目的。

- (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;  
}

但有时候我们获取图片时用的是AGImagePickerController，在其从ALAsset转为UIImage的时候可能原图就发生了旋转，因此，这里也给出了一个处理办法：

            for (ALAsset * asset in self.assetArray) {
                CGImageRef ref = [[asset defaultRepresentation]fullResolutionImage];
                UIImage *orgImage = [UIImage imageWithCGImage:[asset.defaultRepresentation fullScreenImage]
                                                        scale:[asset.defaultRepresentation scale] orientation:
                                     (UIImageOrientation)[asset.defaultRepresentation orientation]];
                // 这句解决了问题
                orgImage = [UIImage imageWithCGImage:ref scale:1 orientation:orgImage.imageOrientation];
//                XMLog(@"原始选完imageOrientation===%ld",orgImage.imageOrientation);
                // 让图片的旋转方向向上（即正确的方向）
                orgImage = [self fixOrientation: orgImage];
                [self.previewImageArray addObject:orgImage];
            }
            
            [delegate imagePickerDidSelectImagearray:self.previewImageArray];

但如果一开始用以下这种写法从ALAsset转为UIImage，就不会发生旋转的问题：

    ALAssetRepresentation *assetRep = [asset defaultRepresentation];
                CGImageRef imgRef = [assetRep fullResolutionImage];
                UIImage *orgImage = [UIImage imageWithCGImage:imgRef
                                                   scale:assetRep.scale
                                             orientation:(UIImageOrientation)assetRep.orientation];

参考资料：资料1 ， 资料2 。