UIKit Dynamics: Attachment inside UITableViewCell

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

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];
}