How to show activity indicator while tableView loads?

Just try this:

var indicator = UIActivityIndicatorView()

func activityIndicator() {
    indicator = UIActivityIndicatorView(frame: CGRectMake(0, 0, 40, 40))
    indicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.Gray
    indicator.center = self.view.center
    self.view.addSubview(indicator)    
}

And where you want to start animating

indicator.startAnimating()
indicator.backgroundColor = UIColor.whiteColor()

For stop:

indicator.stopAnimating()
indicator.hidesWhenStopped = true

Note: Avoid the calling of start and stop at the same time. Just give some conditions.

SWIFT : 4.2
Just try this:

var indicator = UIActivityIndicatorView()

func activityIndicator() {
    indicator = UIActivityIndicatorView(frame: CGRect(x: 0, y: 0, width: 40, height: 40))
    indicator.style = UIActivityIndicatorView.Style.gray
    indicator.center = self.view.center
    self.view.addSubview(indicator)   
}

And where you want to start animating

activityIndicator()
indicator.startAnimating()
indicator.backgroundColor = .white

For stop:

indicator.stopAnimating()
indicator.hidesWhenStopped = true