func tweenTransform() {
    let transformA = CGAffineTransform.identity
    let transformB = CGAffineTransform(scaleX: 2.0, y: 2.0)
    let transformC = CGAffineTransform(rotationAngle: CGFloat(M_PI_2))
        
    controller.tween(from: transformA, at: 0.0)
        .to(transformB, at: 0.5)
        .then(to: transformC, at: 1.0)
        .with(action: tweenView.layer.twc_applyAffineTransform)
}

func tweenColor() {
    let colorA = UIColor.green
    let colorB = UIColor.blue
    let colorC = UIColor.red
    tweenView.backgroundColor = colorA
    
    controller.tween(from: colorA, at: 0.0)
        .to(colorB, at: 0.5)
        .then(to: colorC, at: 1.0)
        .with(action: tweenView.twc_applyBackgroundColor)
}

enum Step: Int {
    case One = 1, Two = 2, Three = 3, Four = 4
}

extension Step: Tweenable {
    static func valueBetween(_ val1: Step, _ val2: Step, percent: Double) -> Step {
        let val = Int(round(Double(val2.rawValue - val1.rawValue) * percent + Double(val1.rawValue)))
        return Step(rawValue: max(min(val, 4), 1)) ?? .One
    }
}

func tweenStep() {
    tweenController.tween(from: Step.One, at: 0.0)
        .to(.Four, at: 0.25)
        .thenHold(until: 0.75)
        .then(to: .Two, at: 1.0)
        .with { step in
            // use step
    }
}

// can be called in response to user interaction
// such as a gesture recognizer or scroll view
// or something else, like a CADisplayLink

// progress range is arbitrary
// you might choose to use a percentage, like 0.0 - 1.0
// or the content size of a scroll view

controller.update(progress: newProgress)

controller.tween(from: transformA, at: 0.0)
	.to(transformB, at: 1.0, withEasing: Easing.easeInOutQuart)
	.with(action: tweenView.layer.twc_applyAffineTransform)

controller.tween(from: 0.0, at: 0.0)
    .to(M_PI * 8.0, at: 1.0)
    .with(object: tweenView.layer, keyPath: "transform.rotation.z")

func observeBoundaries() {
    controller.observeForward(progress: 0.5) { progress in
        // halfway finished!
    }
    controller.observeBoth(progress: 0.75) { progress in
        // this is called when moving backwards or forwards
    }
}