Swift version: 5.10
“Marching ants” is the informal name used for animation of a selection: you see a dashed line around whatever you selected, and the dashes slowly move around the selection to show that it’s active.
iOS can achieve most of this effect for you when you’re using a CAShapeLayer
. To try it out, first create a shape layer with a dashed stroke like this:
let layer = CAShapeLayer()
let bounds = CGRect(x: 50, y: 50, width: 250, height: 250)
layer.path = UIBezierPath(roundedRect: bounds, byRoundingCorners: .allCorners, cornerRadii: CGSize(width: 20, height: 20)).cgPath
layer.strokeColor = UIColor.black.cgColor
layer.fillColor = nil
layer.lineDashPattern = [8, 6]
view.layer.addSublayer(layer)
Now you need to create a CABasicAnimation
to animate the lineDashPhase
property. Annoyingly, the lineDashPattern
– the part that describes the way the dashed are drawn – is actually an array of NSNumber
so we need to boil it down to an integer with code like this:
layer.lineDashPattern?.reduce(0) { $0 - $1.intValue } ?? 0
With the line dash pattern used above – 8, 6 – that will result in toValue
being set to 14.
Here’s the animation you need to give the above shape layer a marching ants effect:
let animation = CABasicAnimation(keyPath: "lineDashPhase")
animation.fromValue = 0
animation.toValue = layer.lineDashPattern?.reduce(0) { $0 - $1.intValue } ?? 0
animation.duration = 1
animation.repeatCount = .infinity
layer.add(animation, forKey: "line")
I used .infinity
for the repeat count so that it lasts forever.
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Available from iOS 3.2
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.
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