Get the center of screen with Navigation Bar considerations

Question

I am trying to center a UIActivityIndicator and reuse it throughout my app. I'm facing an issue where some of my views have to be under the navigation bar, and some aren't. This happens to give my activity indicator a lower than center presentation on views where the view is not under the navigation bar. For instance:

All the answers on StackOverflow seem to fall under picture #2.

I've simply made an extension of UIView to show and hide the activity indicator by adding it as a subview overlay. Checking multiple posts on StackOverflow, I didn't find anything that would duplicate this question.

My question is, in a production app, how would one successfully center this view for both of these situations utilizing only one function but still support IOS version back to 9.3

Answer 1

There are methods on UIView to convert points and frames between different view coordinate systems (supported on iOS 2.0+, https://developer.apple.com/documentation/uikit/uiview/1622424-convert).

extension UIView {

    func centerInContainingWindow() {
        guard let window = self.window else { return }

        let windowCenter = CGPoint(x: window.frame.midX, y: window.frame.midY)
        self.center = self.superview.convert(windowCenter, from: nil)
    }

}

A UIView specifies its coordinates relative to its superview, so centerInContainingWindow() uses UIView.convert(_:from:) to obtain the center of the window in coordinates relative to its superview.

• Picture #1, view under navigation bar: the view fills the entire window, so the center point before and after converting will be the same.
• Picture #2, view not under navigation bar: the view fills only part of the window, so the window center after converting will be shifted to remain in the same place on screen.