UIKit Dynamics: Attachment inside UITableViewCell - ios

Question

My table view cells contain a circle in an UIView, indicating a value. I want to add the UIKit Dynamics attachment behaviour to that circle in order to for it to lag a bit when scrolling.
I don't want to attach the individual cells to each other but only the circle view to the UITableViewCell. The rest of the cell should scroll as usual.
Problem: The UITableViewCell has its origin always at (0, 0). How can I add the circle to a view that actually does move when scrolling?

Answer 1

I finally got it to work. The UITableView moves the coordinate system of every cell and of all views contained within that cell. Therefor I needed to manually move my view inside the UITableViewCell during scrolling while still referring to the initial anchor point.
The table view controller:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
BOOL scrollingUp = '\0';
if (self.lastContentOffset > scrollView.contentOffset.y) {
scrollingUp = YES;
}
else if (self.lastContentOffset < scrollView.contentOffset.y) {
scrollingUp = NO;
}
NSInteger offset = 64; // To compensate for the navigation bar.
if (scrollingUp) {
offset = offset - scrollView.contentOffset.y;
}
else {
offset = offset + scrollView.contentOffset.y;
}
// Limit the offset so the views will not disappear during fast scrolling.
if (offset > 10) {
offset = 10;
}
else if (offset < -10) {
offset = -10;
}
// lastContentOffset is an instance variable.
self.lastContentOffset = scrollView.contentOffset.y;
for (UITableViewCell *cell in self.tableView.visibleCells) {
// Use CoreAnimation to prohibit flicker.
[UIView beginAnimations:#"Display notification" context:nil];
[UIView setAnimationDuration:0.5f];
[UIView setAnimationBeginsFromCurrentState:YES];
cell.view.frame = CGRectMake(cell.view.frame.origin.x, offset, cell.view.frame.size.width, cell.view.frame.size.height);
[UIView commitAnimations];
[cell.dynamicAnimator updateItemUsingCurrentState:cell.view];
}
}
The table view cell:
-(void)layoutSubviews {
[super layoutSubviews];
// _view is the animated UIView.
UIDynamicItemBehavior *viewBehavior = [[UIDynamicItemBehavior alloc] initWithItems:#[_view]];
viewBehavior.elasticity = 0.9f;
UIAttachmentBehavior *attachmentBehaviorView = [[UIAttachmentBehavior alloc] initWithItem:_view attachedToAnchor:CGPointMake(_anchorView.frame.origin.x + _anchorView.frame.size.width / 2.0f, _anchorView.frame.origin.y + _anchorView.frame.size.height / 2.0f)];
attachmentBehaviorView.damping = 8.0f;
attachmentBehaviorView.frequency = 4.0f;
attachmentBehaviorView.length = 0.0f;
[_dynamicAnimator addBehavior:viewBehavior];
[_dynamicAnimator addBehavior:attachmentBehaviorView];
}

Answer 2

You can change the anchorPoint of UIAttachmentBehavior during -[scrollViewDidScroll:]. You may refer to the following code snippet:
- (void)viewDidLoad
{
UIDynamicAnimator *animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
UIAttachmentBehavior *behavior1 = [[UIAttachmentBehavior alloc] initWithItem:self.circleView
attachedToAnchor:[self tableViewAnchor]];
behavior1.length = 10.0;
behavior1.damping = 0.3;
behavior1.frequency = 2.5;
[animator addBehavior:behavior1];
}
- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
behavior1.anchorPoint = [self.tableView convertPoint:[self tableViewAnchor] toView:self.view];
}
- (CGPoint)tableViewAnchor
{
return CGPointMake(160.0, 154.0); // return your target coordination w.r.t. the table view
}
Preview: