implement previousScrollViewYOffset not work - objective-c

Question

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
CGRect frame = self.navigationController.navigationBar.frame;
CGFloat size = frame.size.height - 21;
CGFloat framePercentageHidden = ((20 - 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 = 20;
} else if ((scrollOffset + scrollHeight) >= scrollContentSizeHeight) {
frame.origin.y = -size;
} else {
frame.origin.y = MIN(20, MAX(-size, frame.origin.y - scrollDiff));
}
[self.navigationController.navigationBar setFrame:frame];
[self updateBarButtonItems:(1 - framePercentageHidden)];
self.previousScrollViewYOffset = scrollOffset;
}
ERROR:
Property 'previousScrollViewYOffset' not found on object of type 'HomeScreen'
#interface HomeScreen : UITableViewController <UIScrollViewDelegate,UIScrollViewAccessibilityDelegate>
I added ScrollViewDelegate why not see previousScrollViewYOffset any idea ?

Answer 1

Declaring that your class conforms to the UIScrollViewDelegate protocol does not give it access to additional properties. It means that your class will implement the methods specified in the delegate protocol, and that your class can therefore support a UIScrollView as its delegate. So, all that declaring your class is a UIScrollViewDelegate will do is enable the method you posted to get called.
Regarding the error you're getting, you should try creating the previousScrollViewYOffset property yourself and initializing it with a value such as 0.
I've never tried to create this specific effect, so I could be off, but it looks like previousScrollViewYOffset is a property on your class that you are calculating each time ScrollViewDidScroll is called. The result of that calculation is then used the next time this method is called. If that's the case, you'll need to create that property yourself, and initialize it in your class's initialization method.
Give this a shot [edited to add code] and let me know how it goes:
In your .h:
#property CGFloat thisFloat;
At the top of your .m:
#synthesize thisFloat;
In your initialization method:
thisFloat = 0.0;
Alternative to making it a property and synthesizing it - because no outside classes will access the property - you could declare it as a private variable at the top of your .m like this:
#implementation HomeScreen {
CGFloat previousScrollViewYOffset;
}