Get center of any UIView in Swift

The center property is relative to a view's superview. So if your view's frame is {10, 10, 20, 20}, center is going to be {20, 20}.

I'm guessing you want to center the activityIndicator in view.

You can do

activityIndicator.center = CGPointMake(view.width/2, view.height/2)

Original Answer:

I had reproduce your problem easily (see my comment). I think it could be a problem of auto layout. So may be you can use constraints instead calculate position ?

func addActivityIndicatorToView(activityIndicator: UIActivityIndicatorView, view: UIView){

    self.view.addSubview(activityIndicator)

    //Don't forget this line
    activityIndicator.setTranslatesAutoresizingMaskIntoConstraints(false)
    view.addConstraint(NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal, toItem: view, attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant: 0))
    view.addConstraint(NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: view, attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant: 0))


    activityIndicator.startAnimating()


}

Updated Code for Swift 5.0:

• From edit by Rob
• NSLayoutAttribute.CenterX is now NSLayoutConstraint.Attribute.centerX
• NSLayoutRelation.Equal is now NSLayoutConstraint.Relation.equal
• NSLayoutAttribute.CenterY is now NSLayoutConstraint.Attribute.centerY

func addActivityIndicatorToView(activityIndicator: UIActivityIndicatorView, view: UIView){

    self.view.addSubview(activityIndicator)

    //Don't forget this line
    activityIndicatorView.translatesAutoresizingMaskIntoConstraints = false
    view.addConstraint(NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutConstraint.Attribute.centerX, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerX, multiplier: 1, constant: 0))
    view.addConstraint(NSLayoutConstraint(item: activityIndicator, attribute: NSLayoutConstraint.Attribute.centerY, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerY, multiplier: 1, constant: 0))

    activityIndicator.startAnimating()

}

Swift 3:

activityIndicator.center = CGPoint(x: view.width/2, y: view.height/2)

or you can try:

activityIndicator.center = view.center