Want UITableView to “snap to cell”

Question

I am displaying fairly large images in a UITableView. As the user scrolls, I\'d like to the table view to always snap the center-most photo in the middle. That

Answer 1

UITableView extends UIScrollView...

myTableView.pagingEnabled = YES 

Answer 2

for the swift peeps

override func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) 
{
   if decelerate == false
   {
       self.centerTable()
   }
 }

override func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    self.centerTable()
}

func centerTable()
{
   let midX:CGFloat = self.tableView.bounds.midX
   let midY:CGFloat = self.tableView.bounds.midY
   let midPoint:CGPoint = CGPoint(x: midX, y: midY)

   if let pathForCenterCell:IndexPath = self.tableView .indexPathForRow(at: midPoint)
    {
      self.tableView.scrollToRow(at: pathForCenterCell, at: .middle, animated: true)
    }
}//eom

Answer 3

There is a UIScrollView delegate method especially for this!

Edit: if you just want the code, look at the answers below which build off this.

The table view (which is a scroll view) will call - (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset when the user stops scrolling. You can set manipulate the targetContentOffset to ensure it ends up where you want, and it will decelerate ending at that position (just like a paging UIScrollView).

For example, if your cells were all 100 points high, you could do:

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset {
    targetContentOffset->y = 100 * (int)targetContentOffset->y/100;
}

Of course, you can also inspect the targetContentOffset passed in to see where it was going to land, and then find the cell that is in and alter it appropriately.

Answer 4

Building from what @jszumski posted, if you want the snap to occur mid drag, use this code:

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    [self centerTable];
}

- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView {
    [self centerTable];
}

- (void)centerTable {
    NSIndexPath *pathForCenterCell = [self.tableView indexPathForRowAtPoint:CGPointMake(CGRectGetMidX(self.tableView.bounds), CGRectGetMidY(self.tableView.bounds))];

    [self.tableView scrollToRowAtIndexPath:pathForCenterCell atScrollPosition:UITableViewScrollPositionMiddle animated:YES];
}

Answer 5

Extending @jesse-rusak's answer above, this is the code you would need to add to your UITableViewController subclass if you have cells with variable heights. This will avoid the double-scroll issue in the accepted answer.

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset {
    NSIndexPath *pathForTargetTopCell = [self.tableView indexPathForRowAtPoint:CGPointMake(CGRectGetMidX(self.tableView.bounds), targetContentOffset->y)];
    targetContentOffset->y = [self.tableView rectForRowAtIndexPath:pathForTargetTopCell].origin.y;
}

Answer 6

You can use the UIScrollViewDelegate methods on UITableView to do this:

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    // if decelerating, let scrollViewDidEndDecelerating: handle it
    if (decelerate == NO) {
        [self centerTable];
    }
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    [self centerTable];
}

- (void)centerTable {
    NSIndexPath *pathForCenterCell = [self.tableView indexPathForRowAtPoint:CGPointMake(CGRectGetMidX(self.tableView.bounds), CGRectGetMidY(self.tableView.bounds))];

    [self.tableView scrollToRowAtIndexPath:pathForCenterCell atScrollPosition:UITableViewScrollPositionMiddle animated:YES];
}