UIScrollView subclass stops decelerating

Question

I have subclassed UIScrollView and I have two instances of MyScrollView each in their own view controller, with a segmented controller to switch between them. Both view controllers are added as child controllers in a main container view. They are switched by simply hiding/unhiding the view controller views, and calling the view appearance/disappearance methods on each controller at the correct time.

All of the above has been working fine for a couple of months. Nothing is unloaded in order to maintain state and responsiveness when switching.

My problem is quite a subtle one, illustrated as such:-

• launch app
• load view controller A into container view, which loads MyScrollView instance 1
• tap segmented controller
• hide view controller A's view
• load view controller B into container view, which loads MyScrollView instance 2
• tap segmented controller
• hide view controller B's view

• show view controller A, which shows MyScrollView instance 1

• Now, when I scroll any instance of MyScrollView, it stops sharply without decelerating.

When I hook up the delegate and add breakpoints, I see that scrollViewWillEndDragging:withVelocity:targetContentOffset: is called as expected, but the next call to scrollViewDidEndDragging:willDecelerate: has the decelerate argument as NO. Checking back, it was YES on the first two instances (i.e. when the scrollviews were freshly loaded).

This appears to be a "sticky" bug - once it has happened, it will not go away until I reload the scrollview. It happens to both scrollviews (they are the same class), and it seems to be to do with being shown twice. On the second showing, the scrolling will no longer decelerate.

I've tried setting all the appropriate properties on the scroll view each time its shown; I've tried different delegate objects; I've checked the contentSize is not changing dynamically, and that the scroll views are not being updated in any way during the scroll.

I haven't posted any code since this is a large project, and in any case there's no obvious focal point to the problem. If anyone can suggest a part they'd like to see, I can post that.

EDIT

I just tried removing the scrollview from view controller B, and the problem still exists. Therefore it looks like it's the act of hiding/showing MyScrollView that causes the problem, rather than any interaction between the two scroll views or their delegates.

Answer 1

The problem was notifications. I'm posting my answer here in case anyone else gets stung by this.

In my subclass, I have an active property to handle activation/deactivation of the grid (basically so the grid could ignore any global UI interaction notifications or delayed network responses etc if it was hidden). The setter looked like this:-

- (void)setActive:(BOOL)active
{
    if (_active != active)
    {
        _active = active;

        if (_active)
        {
            NSLog(@"enabling notifications for grid %@", self);
            [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(cellSelected:) name:kABIDataGridViewCellSelectedNotification object:nil];
            [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(headerCellSelected:) name:kABIDataGridViewHeaderCellSelectedNotification object:nil];
        }
        else
        {
            NSLog(@"disabling notifications for grid %@", self);
            // ** BUG HERE **
            [[NSNotificationCenter defaultCenter] removeObserver:self];
        }

    }
}

It seems that UIScrollView must use some internal NSNotifications for handling deceleration, because removing all notifications when hiding the grid caused the weird behaviour. Replacing the erroneous line with the more correct lines below fixed it.

    // Important to remove only the notifications we registered, otherwise we can cause weird scrolling behavious because UIScrollView must
    // use notifications internally.
    [[NSNotificationCenter defaultCenter] removeObserver:self name:kABIDataGridViewCellSelectedNotification object:nil];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:kABIDataGridViewHeaderCellSelectedNotification object:nil];

Of course, this is the more correct way of cancelling notifications. Many iOS devs find it's always much easier to cancel all of them when a view disappears, since you don't have to keep track of every notification you're using. But when subclassing, do not assume anything!