iOS 11 Large Title change event

Question

I want to get the event when iOS 11 large title changes. i.e. When it changes the place to & from navigation bar when user scrolls the view. I've checked the UINavigationBar class but nothing I can get it from.

The screen design I want to implement is like when Large title is visible I want transparent Navigation bar but when title scrolled up to navigation bar I want nav bar with solid colour.

Answer 1

I have added this observer in my UIViewController's subclass's ViewWillAppear. and setting the color according to its height as below:

In ViewWillAppear:

UINavigationBar *navBar = self.navigationController.navigationBar;
[navBar addObserver:self forKeyPath:@"frame" options:NSKeyValueObservingOptionNew context:nil];

In viewWillDisappear:

UINavigationBar *navBar = self.navigationController.navigationBar;
[navBar removeObserver:self forKeyPath:@"frame" context:NULL];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{
if ([keyPath isEqualToString:@"frame"]) {
    [self setNavigationBarColour];;
}

}

- (void)setNavigationBarColour
{
    UINavigationBar *navBar = self.navigationController.navigationBar;
    CGFloat height = navBar.bounds.size.height;
    if(height>44)
    {
        [navBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
        [navBar setTranslucent:YES];
        [navBar setShadowImage:[UIImage new]];
    }
    else
    {
        [navBar setBackgroundImage:[[UINavigationBar appearance] backgroundImageForBarMetrics:UIBarMetricsDefault] forBarMetrics:UIBarMetricsDefault];
        [navBar setTranslucent:NO];
        [navBar setShadowImage:[[UINavigationBar appearance] shadowImage]];
    }
}