'Twitching' when system stops

[marcregan]

If you stop scrolling the system, you'll see that it takes about 4 seconds for the system to come completely to a rest. You'll notice the blue boxes bouncing about one pixel up and down.

Somehow Apple has found a way around this as the Messages app doesn't have this problem. If you look carefully at the way the Messages cells work, there are subtle differences that I think could provide clues to their implementation:

• If you tap your finger down while the system is moving, the Messages app animates to a stop, while this implementation immediately comes to a stop.
• If you violently hit the end of the list, Messages does this really exaggerated bounce.

I suspect they are applying an animation to all the cells to bring them to a stop when the user releases their finger, but I haven't been able to reproduce the effect. They've added a spring based animation to UIKit in iOS7 that might be the key to this. If I have some time this week I'll see if I can help.

Thanks for building this!

[ashfurrow]

Hmm. I tried just modifying the properties of the attachment behaviour – no joy. Any one else have any ideas? Maybe an action on the attachment behaviours that sets their length to zero once they're close enough to zero?

[ashfurrow]

This is really intriguing. May have to burn one of my technical support tickets on this.

[ole]

A while ago I read on Twitter that the cells in Messages.app are actually not implemented with UIKit Dynamics. Don't know if true, though.

[marcregan]

There is a thread in the Apple developer forum about this problem (requires an Apple Developer Account):

https://devforums.apple.com/message/891070#891070

The last guy who responded claimed it could be fixed by adding multiple springs between each element and tuning the numbers, but I didn't have any luck there.

Marc Regan http://about.me/marcregan

On Tuesday, October 1, 2013 at 11:24 AM, Ash Furrow wrote:

This is really intriguing. May have to burn one of my technical support tickets on this.

— Reply to this email directly or view it on GitHub (https://github.com/TeehanLax/UICollectionView-Spring-Demo/issues/1#issuecomment-25458869).

[ashfurrow]

I think I've got it – set the length of the attachment behaviour to 1 so that it doesn't continue to "pull" as long after the finger has been released. @marcregan can you try this and confirm if it works for you too?

[ashfurrow]

Of course, this leaves the cells unevenly spaced, but for some applications it could be a viable solution.

[marcregan]

@AshFurrow Oh yeah! Nice! I switched the length = 1 for the attachment behavior and no more twitching.

Bummer about the cell spacing. In my particular app, it isn't that noticeable, so maybe it's OK.

I can't say I totally understand why this works - I'll have to watch the video again :)

[kovpas]

Twitching is really visible if you set pagingEnabled to YES. Cell spacing could also be crucial for someone (like me :)).

It seems that proper layout without twitching could be achieved with an additional spring, as it's said on apple dev forums (thanks @marcregan for the link):

    UIAttachmentBehavior *springBehaviour = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:center];
    springBehaviour.length = 0.0f; // <- set length back to 0, to make cell position properly
    springBehaviour.damping = 0.8f;
    springBehaviour.frequency = 1.0f;
    [self.dynamicAnimator addBehavior:springBehaviour];

    springBehaviour = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:CGPointMake(center.x, center.y - 1)];
    springBehaviour.length = 1.0f;
    springBehaviour.damping = 0.8f;
    springBehaviour.frequency = 1.0f;
    [self.dynamicAnimator addBehavior:springBehaviour];

The first spring is the original one (I set length back to 0), which puts cell back on it's position, the second one is a spring which doesn't let cell to go over the center.

Works pretty well for me.