Scaling current dot of UIPageControl and keeping it centered - ios

Question

I've subclassed UIPageControl in order to have its current dot bigger.
class CustomPageControl: UIPageControl {
override var currentPage: Int {
didSet {
updateDots()
}
}
func updateDots() {
let currentDot = subviews[currentPage]
let largeScaling = CGAffineTransform(scaleX: 3, y: 3)
subviews.forEach {
// apply the large scale of newly selected dot
// restore the normal scale of previously selected dot
$0.transform = $0 == currentDot ? largeScaling : .identity
}
}
}
But the result of the transform isn't centered (the red dot should be aligned with the others):
I've tried (on iOS 12):
changing the frame or center of currentDot has no effect.
changing the transform to include a translatedBy(x: CGFloat, y: CGFloat) has no effect.
changing the constraints like here is making the first dot jumping:
currentDot.translatesAutoresizingMaskIntoConstraints = false
currentDot.centerYAnchor.constraint(equalTo: self.centerYAnchor, constant: 0)
currentDot.centerXAnchor.constraint(equalTo: self.centerXAnchor, constant: 0)

Answer 1

I got it finally working by rewriting all the subviews constraints by myself.
// https://stackoverflow.com/a/55063316/1033581
class DefaultPageControl: UIPageControl {
override var currentPage: Int {
didSet {
updateDots()
}
}
override func sendAction(_ action: Selector, to target: Any?, for event: UIEvent?) {
super.sendAction(action, to: target, for: event)
updateDots()
}
private func updateDots() {
let currentDot = subviews[currentPage]
let largeScaling = CGAffineTransform(scaleX: 3.0, y: 3.0)
let smallScaling = CGAffineTransform(scaleX: 1.0, y: 1.0)
subviews.forEach {
// Apply the large scale of newly selected dot.
// Restore the small scale of previously selected dot.
$0.transform = $0 == currentDot ? largeScaling : smallScaling
}
}
override func updateConstraints() {
super.updateConstraints()
// We rewrite all the constraints
rewriteConstraints()
}
private func rewriteConstraints() {
let systemDotSize: CGFloat = 7.0
let systemDotDistance: CGFloat = 16.0
let halfCount = CGFloat(subviews.count) / 2
subviews.enumerated().forEach {
let dot = $0.element
dot.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.deactivate(dot.constraints)
NSLayoutConstraint.activate([
dot.widthAnchor.constraint(equalToConstant: systemDotSize),
dot.heightAnchor.constraint(equalToConstant: systemDotSize),
dot.centerYAnchor.constraint(equalTo: centerYAnchor, constant: 0),
dot.centerXAnchor.constraint(equalTo: centerXAnchor, constant: systemDotDistance * (CGFloat($0.offset) - halfCount))
])
}
}
}
System constants in the code (7.0 and 16.0) are respectively the size and the distance found for a default UIPageControl dot on iOS 12.

Answer 2

I tried the solution proposed by Cœur in Swift 5 and Xcode 11 and it works fine with a few notes:
The PageControl element in IB/Storyboard has to be positioned with constraints.
The dots are slightly off-center but it can be quickly fixed by changing the constant of the last constraint to systemDotDistance * ( CGFloat($0.offset) - (halfCount - 0.5)).
If the updateConstraints override is never called, you might need to call self.view.setNeedsUpdateConstraints() in the view controller.