Swift: How to animate the rowHeight of a UITableView?

Question

I'm trying to animate the height of tableViewCell rows by calling startAnimation() inside the tableView function:

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {

    let cell = tableView.dequeueReusableCellWithIdentifier(cellIdentifier, forIndexPath: indexPath) as! TableViewCell

    tableView.rowHeight = 44.0

    startAnimation(tableView)

    return cell
}

//MARK: Animation function

func startAnimation(tableView: UITableView) {

    UIView.animateWithDuration(0.7, delay: 1.0, options: .CurveEaseOut, animations: {

        tableView.rowHeight = 88.0

    }, completion: { finished in

        print("Row heights changed!")
    })
}

The result: The row height does change but without any animation occurring. I don't understand why the animation doesn't work. Should I perhaps define some beginning and end state somewhere?

Answer 1

Don't change the height that way. Instead, when you know you want to change the height of a cell, call (in whatever function):

self.tableView.beginUpdates()
self.tableView.endUpdates()

These calls notify the tableView to check for height changes. Then implement the delegate override func tableView(tableView: UITableView, heightForHeaderInSection section: Int) -> CGFloat, and provide the proper height for each cell. The change in height will be animated automatically. You can return UITableViewAutomaticDimension for items you don't have an explicit height for.

I would not suggest doing such actions from within cellForRowAtIndexPath, however, but in one that responds to a tap didSelectRowAtIndexPath, for example. In one of my classes, I do:

override func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
    if indexPath == self.selectedIndexPath {
      self.selectedIndexPath = nil
    }else{
      self.selectedIndexPath = indexPath
    }
  }

internal var selectedIndexPath: NSIndexPath? {
    didSet{
      //(own internal logic removed)

      //these magical lines tell the tableview something's up, and it checks cell heights and animates changes
      self.tableView.beginUpdates()
      self.tableView.endUpdates()
    }
  }

override func tableView(tableView: UITableView, heightForRowAtIndexPath indexPath: NSIndexPath) -> CGFloat {
    if indexPath == self.selectedIndexPath {
      let size = //your custom size
      return size
    }else{
      return UITableViewAutomaticDimension
    }
  }