Intercepting UIScrollView Scrolls

Question

I have a UIScrollView and a UITableView inside the UIScrollView.

I would like to intercept scrolling of the UITableView and only allow scrolling if the super view (UIScrollView) have reached a specific contentOffset.

I have created subclass of both UIScrollView and UITableView, how do i catch scrolling event and intercepting the scrolling while the user is still scrolling?

Example of what i'm trying to accomplish: The UITableView is going to have a header, if i scroll down the header will collapse to 30% of original size and and stay visible at the top. After i have scrolled back to the top of the UITableView i want the header to expand. In other word i want to extend the scrolling of a UITableView with a header that can collapse/expand.

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There might be better way to accomplish this, i'm open for suggestions.

How about using scroll view delegate method scrollViewDidScroll:? — Valent Richie

Justin Bergen Justin Bergen · Accepted Answer · 2013-06-27T08:03:51

There may be a better way, but this solution is working for me.

UITableView (let's call it innerScrollView) inside of UIScrollView (let's call it scrollView) inside whatever the main view is (let's call it view).
Set an outlet in your view controller for the innerScrollView (self.innerScrollView).
Set bounce in XIB or code to: scrollView.bounces = YES and innerScrollView.bounces = NO.
Set scrolling to disabled initially for the innerScrollView.
In code (viewDidLoad:) set scrollView.contentSize.height to view.frame.size.height + the 70% of the header you want to disappear. Resize innerScrollView to fill the resized contentView to the bottom.

Set your view controller as the delegate for scrollView and implement scrollViewDidScroll: as follows:

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
   CGFloat maxYOffset = self.scrollView.contentSize.height - self.scrollView.frame.size.height;
   CGFloat minInnerYOffset = self.innerScrollView.contentInset.top;
   CGFloat currentYOffset = scrollView.contentOffset.y;
   CGFloat newYOffsetDelta = currentYOffset - maxYOffset;
   CGFloat currentInnerYOffset = self.innerScrollView.contentOffset.y;

   if (scrollView.contentOffset.y >= maxYOffset) {
       self.innerScrollView.scrollEnabled = YES;
       self.scrollView.contentOffset = CGPointMake(0, maxYOffset);

       if (currentYOffset != maxYOffset) {
           self.innerScrollView.contentOffset = CGPointMake(0, currentInnerYOffset + newYOffsetDelta);
       }
   }
   else if (currentInnerYOffset > minInnerYOffset) {
       self.scrollView.contentOffset = CGPointMake(0, maxYOffset);
       self.innerScrollView.contentOffset = CGPointMake(0, currentInnerYOffset + newYOffsetDelta);
   }
   else if (currentYOffset < scrollView.contentInset.top) {
       scrollView.contentOffset = CGPointMake(0, scrollView.contentInset.top);
   }
   else {
       self.lowerScrollView.scrollEnabled = NO;
   }

}

I just whipped this up and quickly tested it. It all seems to work, but there may be some improvements and tweaks needed. Regardless, this should get you started at least.

Intercepting UIScrollView Scrolls

2 Answers