iOS照相机获取到的图片自动旋转90度解决办法

情景再现：
iOS开发过程中，要实现用用手机摄像头拍一张照片，经过裁剪压缩，然后发出去。结果发出去的图片始终是逆时针旋转了90度的图片。

事出有因：
相机拍照后直接取出来的UIimage（用UIImagePickerControllerOriginalImage取出），它本身的imageOrientation属性是3，即UIImageOrientationRight。如果这个图片直接使用则没事，但是如果对它进行裁剪、缩放等操作后，它的这个imageOrientation属性会变成0。此时这张图片用在别的地方就会发生旋转。imageOrientation是只读的，不能直接修改其值。

解决办法：

1. 设置相机的一个属性allowsEditing为YES，设了这个值，你拍完照片后会在照片上出现一个框框，这就是对照片的裁剪编辑。在相机的代理方法中取照片的时候就别用UIImagePickerControllerOriginalImage来取了，要用UIImagePickerControllerEditedImage。用这个key取出来的照片，它的imageOrientation是0，所以之后的任何裁剪、缩放操作都不会造成旋转。这是第一种方法。

2.有时候你的需求是要照相结果的原图。那么，就要对图片进行手动旋转。这里有一个专门针对这个事的很好的category。

- (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;  
}  

这个分类是根据图片的imageOrientation属性，对它进行旋转。这个方法比较费内存。