TransitionCoordinator+ContentAnimation.swift

private func fadeInPlayerAnimator(with duration: TimeInterval) -> UIViewPropertyAnimator {
    let animator = UIViewPropertyAnimator(duration: duration, curve: .easeIn)
    addKeyframeAnimation(to: animator, withRelativeStartTime: 0.0, relativeDuration: 0.5) {
        self.updatePlayer(with: self.state)
    }
    animator.scrubsLinearly = false
    return animator
}

private func fadeOutMiniPlayerAnimator(with duration: TimeInterval) -> UIViewPropertyAnimator {
    let animator = UIViewPropertyAnimator(duration: duration, curve: .easeOut)
    addKeyframeAnimation(to: animator, withRelativeStartTime: 0.0, relativeDuration: 0.5) {
        self.updateMiniPlayer(with: self.state)
    }
    animator.scrubsLinearly = false
    return animator
}