This is frustrating… I created a custom Progressbar like this
class myProgressBar : UIView {
private var zValue : CGFloat = 0.0
private let progressLayer = CALayer()
private let backgroundMask = CAShapeLayer()
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
setupLayers()
}
override init(frame:CGRect) {
super.init(frame:frame)
setupLayers()
}
private func setupLayers() { layer.addSublayer(progressLayer) }
override func layoutSubviews() {
super.layoutSubviews()
backgroundMask.path = UIBezierPath(roundedRect: self.bounds, cornerRadius: self.bounds.size.height * 0.25).cgPath
layer.mask = backgroundMask
update()
}
func update() {
let progressRect = CGRect(origin: .zero, size: CGSize(width: self.bounds.size.width * zValue, height: self.bounds.size.height))
progressLayer.frame = progressRect
progressLayer.backgroundColor = UIColor.green.cgColor
progressLayer.needsDisplayOnBoundsChange = true
print(progressRect)
}
public func setProgress(_ v:CGFloat) {
zValue = v
update()
}
}
and the setProgress function is called with increasing values (0.00 to 1.00) at various times during another function. The print command in the update() function is called at the correct times, and with the correct values. But the “drawing” of the layer doesn’t happen then… it happens ONCE when the routine is complete… Not much use that way as a “progress bar” 
for j in(0...f!.count-1) {
let pct = CGFloat(j+1)/CGFloat(f!.count)
updateLoadProgress(pct)
// the print command shows changing values here
.... do some stuff
}
// but the control doesn't really update until here,
// after its all done, which is too late
private func updateLoadProgress(_ value:CGFloat) {
loadProgress.setProgress(value)
loadProgress.layer.setNeedsDisplay()
loadProgress.layer.displayIfNeeded()
}
