Remove Oscillation from UIAttachmentBehavior in UICollectionView - uicollectionview

Question

I am attempting to recreate the spring behavior that you see in the iOS Messages app in my UICollectionView. Like Messages it will have various cell sizes based on the text size. I have created a custom UICollectionViewFlowLayout which does add the behavior to the UICollectionView however the message bubbles continue to oscillate slightly after the user has stopped scrolling. I have tried any number of combinations in the length, damping and spring values but the oscillation never goes away.
After some reading of other stack questions I did find this comment
In order to prevent oscillation it's necessary to dynamically increase the damping factor on a quadratic scale as the attached views get closer and closer to their attachment points. <
But I am not really sure where to get started with implementing something like that on what I currently have. Any help or guidance would be appreciated.
Below is my code on the UICollectionViewFlowLayout that is creating the current effect.
- (void) prepareLayout {
[super prepareLayout];
CGRect originalRect = (CGRect){.origin = self.collectionView.bounds.origin, .size = self.collectionView.frame.size};
CGRect visibleRect = CGRectInset(originalRect, -50, -50);
NSArray *itemsInVisibleRectArray = [super layoutAttributesForElementsInRect:visibleRect];
NSSet *itemsIndexPathsInVisibleRectSet = [NSSet setWithArray:[itemsInVisibleRectArray valueForKey:#"indexPath"]];
NSPredicate *predicate = [NSPredicate predicateWithBlock:^BOOL(UIAttachmentBehavior *behaviour, NSDictionary *bindings) {
BOOL currentlyVisible = [itemsIndexPathsInVisibleRectSet member:[[[behaviour items] firstObject] indexPath]] != nil;
return !currentlyVisible;
}];
NSArray *noLongerVisibleBehaviours = [self.animator.behaviors filteredArrayUsingPredicate:predicate];
[noLongerVisibleBehaviours enumerateObjectsUsingBlock:^(id obj, NSUInteger index, BOOL *stop) {
[self.animator removeBehavior:obj];
[self.visibleIndexPathsSet removeObject:[[[obj items] firstObject] indexPath]];
}];
NSPredicate *newPredicate = [NSPredicate predicateWithBlock:^BOOL(UICollectionViewLayoutAttributes *item, NSDictionary *bindings) {
BOOL currentlyVisible = [self.visibleIndexPathsSet member:item.indexPath] != nil;
return !currentlyVisible;
}];
NSArray *newlyVisibleItems = [itemsInVisibleRectArray filteredArrayUsingPredicate:newPredicate];
CGPoint touchLocation = [self.collectionView.panGestureRecognizer locationInView:self.collectionView];
[newlyVisibleItems enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes *item, NSUInteger idx, BOOL *stop) {
CGPoint center = item.center;
UIAttachmentBehavior *springBehaviour = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:center];
springBehaviour.length = 0.1f;
springBehaviour.damping = 3.0f;
springBehaviour.frequency = 2.8f;
if (!CGPointEqualToPoint(CGPointZero, touchLocation)) {
CGFloat yDistanceFromTouch = fabs(touchLocation.y - springBehaviour.anchorPoint.y);
CGFloat xDistanceFromTouch = fabs(touchLocation.x - springBehaviour.anchorPoint.x);
CGFloat scrollResistance = (yDistanceFromTouch + xDistanceFromTouch) / 1500.0f;
if (self.latestDelta < 0) {
center.y += MAX(self.latestDelta, self.latestDelta*scrollResistance);
}
else {
center.y += MIN(self.latestDelta, self.latestDelta*scrollResistance);
}
item.center = center;
}
[self.animator addBehavior:springBehaviour];
[self.visibleIndexPathsSet addObject:item.indexPath];
}];
}

Answer 1

You can fix it with 2 steps.
1. add action for the behavior when initials to make sure the center of the cell doesn't change during the animation
springBehaviour.action = ^{
CGPoint itemCenter = item.center;
itemCenter.x = center.x;
item.center = itemCenter;
};
remove/re-add the behaviors when collectionview stops scrolling. To do this, you need implement a scrollview delegate method and in this method to remove/re-add behaviors.
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
ZZHCollectionFlowLayout *flowLayout = self.collectionView.collectionViewLayout;
if ([flowLayout isKindOfClass:[ZZHCollectionFlowLayout class]])
{
[flowLayout removeAnimationBehavior];
}
else
{
// Your NSAssertionHandler
}
}
- (void)removeAnimationBehavior
{
NSArray *behaviors = self.dynamicAnimator.behaviors;
[self.dynamicAnimator removeAllBehaviors];
for (UIDynamicBehavior *obj in behaviors)
{
[self.dynamicAnimator addBehavior:obj];
}
}
BTW, if there is way of changing damping to fix, would like to hear it!