title: Start and stop activity indicator swift3
body: <p>I am learning Swift and now working on my home task. 
  One of my buttons calls a function where I reload a collection view and it takes time. I am trying to implement an activity indicator to show the process.
  The problem that if I start the indicator, load the collection view, and then stop the indicator within the button action, the indicator doesn't appear.
  What would be the correct way to implement the indicator in this case? 
  Here is the piece of the code:</p>
  
  <pre><code>let activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: UIActivityIndicatorViewStyle.whiteLarge)
  @IBOutlet weak var onCIFilterView: UICollectionView!
  @IBOutlet var cifilterMenu: UIView!
  
  @IBAction func onCIFiltersButton(_ sender: UIButton) {
      if (sender.isSelected) {
          hideCIFilters()
          sender.isSelected = false
      }
      else {
          activityIndicator.startAnimating()
          showCIFilters()  //the menu with UIView collection
          activityIndicator.stopAnimating()
          sender.isSelected = true
      }
  }
  
  func showCIFilters () {
      onCIFilterView.reloadData() // reloading collection view
      view.addSubview (cifilterMenu)
      let bottomConstraint = cifilterMenu.bottomAnchor.constraint(equalTo: mainMenu.topAnchor)
      let leftConstraint = cifilterMenu.leftAnchor.constraint(equalTo: view.leftAnchor)
      let rightConstraint = cifilterMenu.rightAnchor.constraint(equalTo: view.rightAnchor)
      let heightConstraint = cifilterMenu.heightAnchor.constraint(equalToConstant: 80)
      NSLayoutConstraint.activate([bottomConstraint, leftConstraint, rightConstraint, heightConstraint])
      view.layoutIfNeeded()
  }
  
  func hideCIFilters () {
      currentImage = myImage.image
      cifilterMenu.removeFromSuperview()
  
  }
  
  override func viewDidLoad() {
      super.viewDidLoad()
      //.....
      activityIndicator.center = CGPoint(x: view.bounds.size.width/2, y: view.bounds.size.height/2)
      activityIndicator.color = UIColor.lightGray
      view.addSubview(activityIndicator)
  }
  </code></pre>