How can I center a UIActivityIndicatorView in a UICollectionViewCell?

Question

I am trying to put a UIActivityIndicatorView inside each collection view cell as it downloads its image. I have it appearing in each cell, but it refuses to center itself. It stays in the top left corner. How can I get it to center itself properly?

Here's how I'm doing it:

extension UIView {

    func showActivityIndicator(onView: UIView, withIndicator: UIActivityIndicatorView) {
        withIndicator.frame = CGRect(x: onView.frame.midX - 20, y: onView.frame.midY - 20, width: 40, height: 40)
        withIndicator.activityIndicatorViewStyle = .whiteLarge
        withIndicator.center = onView.center
        onView.addSubview(withIndicator)
        withIndicator.startAnimating()
    }
}

I call that function inside cellForItemAtIndexPath like:

showActivityIndicator(onView: cell.contentView, withIndicator: activityInd)

But nothing I do will move it from the top left corner. Any advice?

Answer 1

尝试这个

withIndicator.center = CGPointMake(self.frame.size.width/2, self.frame.size.height/2);

Answer 2

You need to add contraints to center it. For example use NSLayoutAnchor .

You need to set translatesAutoresizingMaskIntoConstraints to false , to set your constraints. And after adding it to the view set the constraints (hints in the code comments):

extension UIView {

    func showActivityIndicator(onView: UIView, withIndicator: UIActivityIndicatorView) {
        withIndicator.frame = CGRect(x: onView.frame.midX - 20, y: onView.frame.midY - 20, width: 40, height: 40)
        withIndicator.activityIndicatorViewStyle = .whiteLarge

        // set translatesAutoresizingMaskIntoConstraints to false to set your constraints
        withIndicator.translatesAutoresizingMaskIntoConstraints = false
        onView.addSubview(withIndicator)
        withIndicator.startAnimating()

        // add the constraints to center the indicator
        withIndicator.centerXAnchor.constraint(equalTo: onView.centerXAnchor).isActive = true
        withIndicator.centerYAnchor.constraint(equalTo: onView.centerYAnchor).isActive = true
    }

}

Answer 3

I suggest using constraints (aka auto layout):

indicator.translatesAutoresizingMaskIntoConstraints = false
"your cell".addSubview(indicator)

let widthConstraint = NSLayoutConstraint(item: indicator, attribute: .Width, relatedBy: .Equal,
                                                 toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 250)

let heightConstraint = NSLayoutConstraint(item: indicator, attribute: .Height, relatedBy: .Equal,
                                                  toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 100)

let xConstraint = NSLayoutConstraint(item: indicator, attribute: .CenterX, relatedBy: .Equal, toItem: self.tableView, attribute: .CenterX, multiplier: 1, constant: 0)

let yConstraint = NSLayoutConstraint(item: indicator, attribute: .CenterY, relatedBy: .Equal, toItem: self.tableView, attribute: .CenterY, multiplier: 1, constant: 0)

NSLayoutConstraint.activateConstraints([widthConstraint, heightConstraint, xConstraint, yConstraint])