Calculate new scroll view content inset when showing/hiding nav bar - ios

Question

I am replicating Facebook/Apple News hide/show nav bar functionality. I can hide/show the nav bar but I don't know how to calculate the scroll view's content inset as the nav bar frame.origin.y changes. I want the scroll view content inset to follow nav bar's position so that it all looks smooth.
Here's what I'm doing:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
CGFloat navBarTopLength = self.navigationController.topLayoutGuide.length; // 20
CGRect frame = self.navigationController.navigationBar.frame;
CGFloat size = frame.size.height - (navBarTopLength + 1);
CGFloat framePercentageHidden = ((navBarTopLength - frame.origin.y) / (frame.size.height - 1));
CGFloat scrollOffset = scrollView.contentOffset.y;
CGFloat scrollDiff = scrollOffset - self.previousScrollViewYOffset;
CGFloat scrollHeight = scrollView.frame.size.height;
CGFloat scrollContentSizeHeight = scrollView.contentSize.height + scrollView.contentInset.bottom;
if (scrollOffset <= -scrollView.contentInset.top) {
frame.origin.y = navBarTopLength; // showing nav bar
} else if ((scrollOffset + scrollHeight) >= scrollContentSizeHeight) {
frame.origin.y = -size;
} else {
frame.origin.y = MIN(navBarTopLength, MAX(-size, frame.origin.y - scrollDiff));
}
CGFloat top;
if (self.previousScrollViewYOffset > scrollOffset) {
NSLog(#"scrolling down"); //scrolling down
top = self.navigationController.navigationBar.frame.size.height + frame.origin.y;
} else {
NSLog(#"scrolling up"); //scrolling up
top = (navBarTopLength + 1) + frame.origin.y;
}
[self.navigationController.navigationBar setFrame:frame];
[self updateBarButtonItems:(1 - framePercentageHidden)];
scrollView.contentInset = UIEdgeInsetsMake(top, 0, 0, 0);
self.previousScrollViewYOffset = scrollOffset;
}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
[self stoppedScrolling];
}
- (void)updateBarButtonItems:(CGFloat)alpha {
[self.navigationItem.leftBarButtonItems enumerateObjectsUsingBlock:^(UIBarButtonItem *item, NSUInteger i, BOOL *stop) {
item.customView.alpha = alpha;
}];
[self.navigationItem.rightBarButtonItems enumerateObjectsUsingBlock:^(UIBarButtonItem *item, NSUInteger i, BOOL *stop) {
item.customView.alpha = alpha;
}];
self.navigationItem.titleView.alpha = alpha;
self.navigationController.navigationBar.tintColor =[self.navigationController.navigationBar.tintColor colorWithAlphaComponent:alpha];
}
- (void)animateNavBarTo:(CGFloat)y {
[UIView animateWithDuration:0.2
animations:^{
CGRect frame = self.navigationController.navigationBar.frame;
BOOL hide = frame.origin.y >= y;
CGFloat alpha = (hide ? 0 : 1);
frame.origin.y = y;
[self.navigationController.navigationBar setFrame:frame];
[self updateBarButtonItems:alpha];
self.navBarHidden = hide;
}];
}
- (void)stoppedScrolling {
CGRect frame = self.navigationController.navigationBar.frame;
if (frame.origin.y < 20) {
[self animateNavBarTo:-(frame.size.height - 21)];
}
}
I know that I should be changing the content inset in scrollViewDidScroll as I'm calculating a new frame for the nav bar. The way I'm currently calculating new top inset makes the whole UI shake and everything breaks shortly after with BAD ACCESS