Autolayout: UIImageView not rotating with device orientation - ios

Question

There's probably a dozen SO questions with similar titles, and as far as I can see, I've incorporated the advice from every one of them, with no luck. When I rotate the device from portrait to landscape, I want to change the background image. However, in every experiment I have tried, the UIImageView in landscape remains portrait sized.
I am using Autolayout, configured in IB like so:
My view hierarchy is configured like so:
When I rotate the device, I want to rotate everything in UIView viewBackground (the image and all of the buttons). I believe I can manage the button movements through constraints, but need to rotate UIImageView image view background myself. So, I added the following code:
-(void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator
{
int scaleFactor = self.traitCollection.displayScale;
NSString *source = #"drawn";
NSString *orientation;
if (size.width > size.height)
{
orientation = #"Landscape";
}
else
{
orientation = #"Portrait";
}
NSString *platform;
if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPad)
{
platform = #"iPad";
}
else
{
platform = #"iPhone";
}
NSString *filename = [NSString stringWithFormat:#"%# %# %# %.0fx%.0f.png", source, platform, orientation, size.width * scaleFactor, size.height * scaleFactor];
// CONFIRMED: we have assembled the correct new image name.
viewBackground.frame = CGRectMake(0, 0, size.width * scaleFactor, size.height * scaleFactor);
viewBackground.autoresizesSubviews = YES;
viewBackground.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
imageViewBackground.autoresizesSubviews = YES;
imageViewBackground.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
imageViewBackground.image = [UIImage imageNamed:filename];
}
The problem: The correct image appears on the screen, but as as mentioned above, it still has the portrait dimensions. I'm certain the image changes as the artwork is significantly different, with elements being stacked horizontally versus vertically. But the new picture is cut off horizontally in landscape mode.
I have tried putting all sorts of wild values into newRect, and none of them make any difference. The image never actually moves, even if I radically change the origin in newRect. so my current theory is that my changes to the view sizes are being ignored/over written.
The Question: What am I missing that's preventing the resizing of the image to landscape mode? Is there some other auto sizing aspect I'm not addressing?

Answer 1

So, the problem turned out to be yet another variable in the saga. There also must be a constraint on the UIImageView that is 0 on each side, with "Constrain to Margins unchecked." Apparently, when I did that step earlier, it didn't actually take.
I have to admit, though, I really don't understand what that constraint is doing for me. It clearly makes the rotation work, but I don't get why. Since that's technically the answer to my original question, I'll gladly award the answer if someone can explain how this constraint enables this scenario.
EDIT:
In case anyone stumbles upon this answer in the future... I see now that setting the aforementioned constraint with all 0 buffers between the UIImageView and the UIView pins the edges of UIImageView to the edges of the UIView. When the device rotates, the viewController resizes the UIView, and with this constraint, the UIImageView resizes also. I was able to remove the majority of my code. The only code I need is to select a portrait or landscape image, and I was able to remove everything that resized the UIView and UIImageView.
The image now changes size properly when rotating just by using the one constraint. I only need code to decide whether to show a landscape or portrait background. (And I vastly simplified that code by storing my images in an asset catalog so the code just selects the name of "portrait" or "landscape", but the size of the image is auto selected based on the device).