Custom Transition Only Works First Time on Retina Simulator - ios

Question

I have a custom presentation using the UIViewControllerContextTransitioning protocol that splits the view of the initial controller in half (actually an image of that view), and slides the two haves off the screen to the left and right. The animation works properly for the presentation and dismissal the first time it is carried out, but is wrong on any subsequent presentations.
Specifically, the image views that hold the half images of the view don't respect the constraints they are given, and so are twice the size they should be (they're the size dictated by the retina image). This only happens on simulators with retina displays.
The animator object is deallocated after each presentation and dismissal, so it's not at all clear why it should behave differently on subsequent runs.
Here is the code, including the logs that show the size of one of the image views both before and after the constraints are applied,
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)context {
self.ctxContainerView = [context containerView];
UIView *toView = [context viewControllerForKey:UITransitionContextToViewControllerKey].view;
UIView *fromView = [context viewControllerForKey:UITransitionContextFromViewControllerKey].view;
// Create image from view
UIView *viewToImage = (self.isPresenting)? fromView : toView;
UIGraphicsBeginImageContextWithOptions(viewToImage.bounds.size, viewToImage.opaque, 0);
[viewToImage.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage * img = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
// Create left and right haves of the view image
CGImageRef tmp = CGImageCreateWithImageInRect([img CGImage], CGRectMake(0, 0, img.size.width * img.scale*0.5, img.size.height * img.scale));
UIImage *imgLeft = [UIImage imageWithCGImage:tmp];
CGImageRef tmp2 = CGImageCreateWithImageInRect([img CGImage], CGRectMake(img.size.width * img.scale*0.5, 0, img.size.width * img.scale*0.5, img.size.height * img.scale));
UIImage *imgRight = [UIImage imageWithCGImage:tmp2];
CGImageRelease(tmp);
CGImageRelease(tmp2);
self.snapshotViewLeft = [[UIImageView alloc] initWithImage:imgLeft];
self.snapshotViewRight = [[UIImageView alloc] initWithImage:imgRight];
NSLog(#"Before constraints %#", NSStringFromCGSize(self.snapshotViewLeft.frame.size)); //prints {375, 1334} all the time
if (self.isPresenting) {
[self.ctxContainerView insertSubview:toView belowSubview:fromView];
[self.ctxContainerView addSubview:self.snapshotViewLeft];
[self.ctxContainerView addConstraints: [self setupConstraintsForLeftView:self.snapshotViewLeft]];
[self.ctxContainerView addSubview:self.snapshotViewRight];
[self.ctxContainerView addConstraints: [self setupConstraintsForRightView:self.snapshotViewRight]];
[self.ctxContainerView layoutIfNeeded];
NSLog(#"After constraints %#", NSStringFromCGSize(self.snapshotViewLeft.frame.size)); // prints {187.5, 667} the first time (which is the correct size), but {375, 1334} the second time the controller is presented
// Remove the source view, and animate out the two image views
[fromView removeFromSuperview];
self.leftCon.constant = -self.ctxContainerView.frame.size.width/2;
self.rightCon.constant = self.ctxContainerView.frame.size.width/2;
[UIView animateWithDuration:ANIMATION_TIME animations:^{
[self.ctxContainerView layoutIfNeeded];
} completion:^(BOOL finished) {
[self.snapshotViewRight removeFromSuperview];
[self.snapshotViewLeft removeFromSuperview];
[context completeTransition:YES];
}];
}else{
[self.ctxContainerView addSubview:self.snapshotViewLeft];
[self.ctxContainerView addConstraints: [self setupConstraintsForLeftView:self.snapshotViewLeft]];
self.leftCon.constant = -self.ctxContainerView.frame.size.width/2;
[self.ctxContainerView addSubview:self.snapshotViewRight];
[self.ctxContainerView addConstraints: [self setupConstraintsForRightView:self.snapshotViewRight]];
self.rightCon.constant = self.ctxContainerView.frame.size.width/2;
[self.ctxContainerView layoutIfNeeded];
// Animate in the two image views
self.leftCon.constant = 0;
self.rightCon.constant = 0;
[UIView animateWithDuration:ANIMATION_TIME animations:^{
[self.ctxContainerView layoutIfNeeded];
} completion:^(BOOL finished) {
[self.snapshotViewRight removeFromSuperview];
[self.snapshotViewLeft removeFromSuperview];
[fromView removeFromSuperview];
[context completeTransition:YES];
}];
}
}
-(NSArray *)setupConstraintsForLeftView:(UIView *) inView {
inView.translatesAutoresizingMaskIntoConstraints = NO;
self.leftCon = [NSLayoutConstraint constraintWithItem:inView attribute:NSLayoutAttributeLeft relatedBy:0 toItem:self.ctxContainerView attribute:NSLayoutAttributeLeft multiplier:1 constant:0];
NSLayoutConstraint *con1 = [NSLayoutConstraint constraintWithItem:inView attribute:NSLayoutAttributeBottom relatedBy:0 toItem:self.ctxContainerView attribute:NSLayoutAttributeBottom multiplier:1 constant:0];
NSLayoutConstraint *con2 = [NSLayoutConstraint constraintWithItem:inView attribute:NSLayoutAttributeTop relatedBy:0 toItem:self.ctxContainerView attribute:NSLayoutAttributeTop multiplier:1 constant:0];
NSLayoutConstraint *con3 = [NSLayoutConstraint constraintWithItem:inView attribute:NSLayoutAttributeWidth relatedBy:0 toItem:self.ctxContainerView attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0];
NSArray *arr = #[self.leftCon,con1,con2,con3];
return arr;
}
-(NSArray *)setupConstraintsForRightView:(UIView *) inView {
inView.translatesAutoresizingMaskIntoConstraints = NO;
self.rightCon = [NSLayoutConstraint constraintWithItem:inView attribute:NSLayoutAttributeRight relatedBy:0 toItem:self.ctxContainerView attribute:NSLayoutAttributeRight multiplier:1 constant:0];
NSLayoutConstraint *con1 = [NSLayoutConstraint constraintWithItem:inView attribute:NSLayoutAttributeBottom relatedBy:0 toItem:self.ctxContainerView attribute:NSLayoutAttributeBottom multiplier:1 constant:0];
NSLayoutConstraint *con2 = [NSLayoutConstraint constraintWithItem:inView attribute:NSLayoutAttributeTop relatedBy:0 toItem:self.ctxContainerView attribute:NSLayoutAttributeTop multiplier:1 constant:0];
NSLayoutConstraint *con3 = [NSLayoutConstraint constraintWithItem:inView attribute:NSLayoutAttributeWidth relatedBy:0 toItem:self.ctxContainerView attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0];
NSArray *arr = #[self.rightCon,con1,con2,con3];
return arr;
}
The question is, why is the behavior different for runs after the first, even though the system should be back to the same start state after the first presentation and dismissal?