A UIViewController Fade Animator

RVS_FadeAnimator.swift

import UIKit
/* ###################################################################################################################################### */
// MARK: Fade Transition Animator
/* ###################################################################################################################################### */
/**
 This allows us to do a fade between two screens.
 It was inspired by [this Medium post](https://medium.com/@ludvigeriksson/custom-interactive-uinavigationcontroller-transition-animations-in-swift-4-a4b5e0cefb1e)
 */
open class RVS_FadeAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    /* ################################################################## */
    /**
     This is set to true, if the operation is a "push," and false, if it is a "pop."
     */
    private var _presenting: Bool = false
    
    /* ################################################################## */
    /**
     The animation period, in seconds.
     */
    private var _animationPeriod: TimeInterval = 0.5

    /* ################################################################## */
    /**
     This is the "public face" of the duration of our transition, in seconds.
     
     - parameter using: The context (ignored).
     - returns: The time interval for the transition, in seconds.
     */
    public func transitionDuration(using: UIViewControllerContextTransitioning?) -> TimeInterval { _animationPeriod }
    
    /* ################################################################## */
    /**
     This actually performs the fade animation.
     
     - parameter using: The transition context, which we mine for "to" and "from" view controllers.
     */
    public func animateTransition(using inContext: UIViewControllerContextTransitioning) {
        guard let fromView = inContext.view(forKey: .from),
              let toView = inContext.view(forKey: .to)
        else { return }

        let container = inContext.containerView
        
        if _presenting {
            container.addSubview(toView)
            toView.alpha = 0.0
        } else {
            container.insertSubview(toView, belowSubview: fromView)
        }
        
        UIView.animate(withDuration: transitionDuration(using: inContext), animations: { [weak self] in
            if self?._presenting ?? false {
                toView.alpha = 1.0
            } else {
                fromView.alpha = 0.0
            }
        }, completion: { _ in
            let success = !inContext.transitionWasCancelled
            if !success {
                toView.removeFromSuperview()
            }
            inContext.completeTransition(success)
        })
    }
    
    /* ################################################################## */
    /**
     The initializer.
     
     - parameter presenting: True, if the operation is a "push."
     - parameter animationPeriodInSeconds: The animation period, in seconds. This is optional. If not provided, the period is half a second.
     */
    public init(presenting inPresenting: Bool, animationPeriodInSeconds inAnimationPeriod: TimeInterval! = nil) {
        _presenting = inPresenting
        guard let animationPeriod = inAnimationPeriod else { return }
        _animationPeriod = animationPeriod
    }
}