Stroke animations on shape layers

LayerAnimations.swift

// Set duration for single animation
let duration: TimeInterval = 2
// Enumerate all the layers
layers.enumerated().forEach { offset, layer in
    // Create animation for each layer
    let animation = CABasicAnimation(keyPath: "strokeEnd")
    // Begin each animation after the one before
    animation.beginTime = duration * CFTimeInterval(offset) + delay
    animation.toValue = 1
    animation.duration = duration
    animation.fillMode = kCAFillModeForwards

    // Create animation group with the stroke basic animation
    let group = CAAnimationGroup()
    group.animations = [animation]
    group.isRemovedOnCompletion = false
    group.duration = CFTimeInterval(layers.count + 1) * duration
    group.repeatCount = .infinity
    group.autoreverses = true
    group.fillMode = kCAFillModeForwards

    // Add the animation group to the letter layer
    layer.add(group, forKey: "group")
}

https://blog.thefuntasty.com/swift-stroke-animations-e1b864917255