Interpolate.swift

// Create interpolation object
let colorChange = Interpolate(from: UIColor.whiteColor(), to: UIColor.redColor(), apply: { [weak self] (color) in
      self?.view.backgroundColor = color
})

// For a gesture recognizer or delegate that reports every step of its progress (e.g. UIPanGestureRecognizer or a ScrollViewDidScroll) you can just apply the percentage directly to the Interpolate object
@IBAction func handlePan(recognizer: UIPanGestureRecognizer) {
    let translation = recognizer.translationInView(self.view)
    let translatedCenterY = view.center.y + translation.y
    let progress = translatedCenterY / self.view.bounds.size.height
    colorChange.progress = progress
}

// For other types of gesture recognizers that only report a beginning and an end (e.g. a UILongPressGestureRecognizer), you can animate directly to a target progress value with a given duration.
@IBAction func handleLongPress(recognizer: UILongPressGestureRecognizer) {
    switch recognizer.state {
        case .Began:
            colorChange.animate(1.0, duration: 0.3)
        case .Cancelled, .Ended, .Failed:
            colorChange.animate(0.0, duration: 0.3)
        default: break
    }
}

// To remove
colorChange.invalidate()