UIScrollView - set negative contentOffset when bounce animation starts

Question

I am using a UITableView to achieve a scale/unblur effect on my user's profile picture (like in the Spotify App) when I pull down the UITableView to a negative contentOffset.y. This all works fine...

Now, when a user pulls down to a contentOffset.y smaller or equal to a certain value -let's call it maintainOffsetY = 70.0- and he "lets go" of the view, I would like to maintain this contentOffset until the user "pushes" the view up again, instead of the view to automatically bounce back to contentOffset = (0,0) again.

In order to achieve this, I must know when the touches began and ended, which I can do with the following delegate methods:

- (void) scrollViewWillBeginDragging:(UIScrollView *)scrollView {
    // is like touches begin
    if ([scrollView isEqual:_tableView]) {
        NSLog(@"touches began");
        _touchesEnded = NO;
    }
}
- (void) scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset {
// is like touches ended
NSLog(@"touches ended");
if ([scrollView isEqual:_tableView]) {
    _touchesEnded = YES;
}

}

Then in

- (void)scrollViewDidScroll:(UIScrollView *)scrollView

I check if the contentOffset.y is smaller than or equal to maintainOffsetY and if the user has already "let go" of the table, so it is about to bounce (or already bouncing) back to contentOffset = (0,0).

It does not seem to be possible to let it bounce back only to the maintainOffsetY. Does anybody know a workaround or have an idea on how to solve this?

Any help is much appreciated!

Answer 1

To set a negative offset to a scroll view you can use this.

You need to save the initial content inset of scroll view. It can be not 0 already, if you have translucent nav bar for example.

    - (void)viewDidAppear:(BOOL)animated {
        [super viewDidAppear:animated];

        contentInsetTopInitial = self.tableView.contentInset.top;
    }

Then write this.

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset {

    CGPoint contentOffset = scrollView.contentOffset; // need to save contentOffset before changing contentInset
    CGFloat contentOffsetY = contentOffset.y + contentInsetTopInitial; // calculate pure offset, without inset

    if (contentOffsetY < -maintainOffsetY) {
        scrollView.contentInset = UIEdgeInsetsMake(contentInsetTopInitial + maintainOffsetY, 0, maintainOffsetY, 0);
        [scrollView setContentOffset:contentOffset animated:NO];
    }
}

Hope this will help.