Use cross fade transitions if reduce motion/prefer cross fade is enabled

Sources/SheetController/SheetContainerDismissAnimationController.swift

@@ -11,14 +11,23 @@ import UIKit
 class SheetContainerDismissAnimationController: NSObject, UIViewControllerAnimatedTransitioning {
 
     func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
+        if UIAccessibility.prefersCrossFadeTransitionsBackwardsCompat && (transitionContext?.viewController(forKey: .from) as? SheetContainerViewController)?.dismissAnimationInitialVelocity == nil {
+            return 0.25
+        } else {
             return 0.5
         }
+    }
     
     func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
         guard let fromVC = transitionContext.viewController(forKey: .from) as? SheetContainerViewController else {
             fatalError()
         }
         
+        if UIAccessibility.prefersCrossFadeTransitionsBackwardsCompat && fromVC.dismissAnimationInitialVelocity == nil {
+            animateCrossFadeTransition(using: transitionContext)
+            return
+        }
+        
         fromVC.view.alpha = 1.0
         fromVC.dimmingView.isHidden = true
         
@@ -57,4 +66,19 @@ class SheetContainerDismissAnimationController: NSObject, UIViewControllerAnimat
         animator.startAnimation()
     }
     
+    func animateCrossFadeTransition(using transitionContext: UIViewControllerContextTransitioning) {
+        guard let fromVC = transitionContext.viewController(forKey: .from) as? SheetContainerViewController else {
+            fatalError()
+        }
+        
+        transitionContext.containerView.addSubview(fromVC.view)
+        
+        let duration = transitionDuration(using: transitionContext)
+        UIView.animate(withDuration: duration) {
+            fromVC.view.alpha = 0
+        } completion: { (finished) in
+            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
+        }
+    }
+    
 }

Sources/SheetController/SheetContainerPresentationAnimationController.swift

@@ -11,14 +11,23 @@ import UIKit
 class SheetContainerPresentationAnimationController: NSObject, UIViewControllerAnimatedTransitioning {
 
     func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
+        if UIAccessibility.prefersCrossFadeTransitionsBackwardsCompat {
+            return 0.25
+        } else {
             return 0.5
         }
+    }
     
     func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
         guard let toVC = transitionContext.viewController(forKey: .to) as? SheetContainerViewController else {
             fatalError()
         }
         
+        if UIAccessibility.prefersCrossFadeTransitionsBackwardsCompat {
+            animateCrossFadeTransition(using: transitionContext)
+            return
+        }
+        
         toVC.dimmingView.isHidden = true
         
         let finalFrame = transitionContext.finalFrame(for: toVC)
@@ -56,4 +65,22 @@ class SheetContainerPresentationAnimationController: NSObject, UIViewControllerA
 
     }
     
+    func animateCrossFadeTransition(using transitionContext: UIViewControllerContextTransitioning) {
+        guard let toVC = transitionContext.viewController(forKey: .to) as? SheetContainerViewController else {
+            fatalError()
+        }
+        
+        transitionContext.containerView.addSubview(toVC.view)
+        
+        toVC.view.alpha = 0
+        
+        let duration = transitionDuration(using: transitionContext)
+        UIView.animate(withDuration: duration) {
+            toVC.view.alpha = 1
+        } completion: { (finished) in
+            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
+        }
+        
+    }
+    
 }

Sources/SheetController/UIAccessibility+BackwardsCompat.swift

@@ -0,0 +1,18 @@
+//
+//  File.swift
+//  
+//
+//  Created by Shadowfacts on 9/12/20.
+//
+
+import UIKit
+
+extension UIAccessibility {
+    static var prefersCrossFadeTransitionsBackwardsCompat: Bool {
+        if #available(iOS 14.0, *) {
+            return prefersCrossFadeTransitions
+        } else {
+            return isReduceMotionEnabled
+        }
+    }
+}