Unarchive UIImage object returns CGSizeZero image

I have this code working on iOS 7:

NSData *imageData = [NSKeyedArchiver archivedDataWithRootObject:self.imageView.image];
UIImage *imageCopy = [NSKeyedUnarchiver unarchiveObjectWithData:imageData];
NSLog(@"%@",NSStringFromCGSize(imageCopy.size));

but on iOS 8, imageCopy's size is always zero. Same thing happens when I archive UIImageView, the unarchived imageView's image has a zero size. I found out that in iOS 7, UIImage header is like:

UIImage : NSObject <NSSecureCoding, NSCoding>

but on iOS 8 it is :

UIImage : NSObject <NSSecureCoding>

It looks like the NSCoding protocol is missing on iOS 8. I have to encode the actual image data: UIImagePNGRepresentation(self.imageView.image) instead of the image to make sure I get a good image back.

Does anyone know why this happens? Is it for backward compatibility? I noticed in iOS earlier version UIImage doesn't conform to NSCoding.

UIImage : NSObject <NSSecureCoding> is not a problem because NSSecureCoding inherits NSCoding.

Anyway, I confirmed the problem can be reproduced with following code:

UIImage *img = [UIImage imageNamed: @"myImage"];
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:img];
UIImage *imgCopy = [NSKeyedUnarchiver unarchiveObjectWithData:data];
NSLog(@"%@, %@", imgCopy, NSStringFromCGSize(imgCopy.size)); // -> (null), {0, 0}

On the other hand, the following code works as expected:

UIImage *img = [UIImage imageNamed: @"myImage"];
UIImage *img2 = [UIImage imageWithCGImage:img.CGImage scale:img.scale orientation:img.imageOrientation];
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:img2];
UIImage *imgCopy = [NSKeyedUnarchiver unarchiveObjectWithData:data];
NSLog(@"%@, %@", imgCopy, NSStringFromCGSize(imgCopy.size)); // -> <UIImage: 0x7fa013e766f0>, {50, 53}

I don't know why, maybe bug?

I think, this is related to imageAsset or traitCollection property introduced in iOS8