From ab77ad89b449b8c415441a6ac97e0922d6900dcc Mon Sep 17 00:00:00 2001
From: Shadowfacts <me@shadowfacts.net>
Date: Sat, 12 Sep 2020 13:02:19 -0400
Subject: [PATCH] Use cross fade transitions if reduce motion/prefer cross fade
 is enabled

---
 ...tContainerDismissAnimationController.swift | 26 ++++++++++++++++-
 ...ainerPresentationAnimationController.swift | 29 ++++++++++++++++++-
 .../UIAccessibility+BackwardsCompat.swift     | 18 ++++++++++++
 3 files changed, 71 insertions(+), 2 deletions(-)
 create mode 100644 Sources/SheetController/UIAccessibility+BackwardsCompat.swift

diff --git a/Sources/SheetController/SheetContainerDismissAnimationController.swift b/Sources/SheetController/SheetContainerDismissAnimationController.swift
index 3ad3426..7067974 100644
--- a/Sources/SheetController/SheetContainerDismissAnimationController.swift
+++ b/Sources/SheetController/SheetContainerDismissAnimationController.swift
@@ -11,7 +11,11 @@ import UIKit
 class SheetContainerDismissAnimationController: NSObject, UIViewControllerAnimatedTransitioning {
 
     func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
-        return 0.5
+        if UIAccessibility.prefersCrossFadeTransitionsBackwardsCompat && (transitionContext?.viewController(forKey: .from) as? SheetContainerViewController)?.dismissAnimationInitialVelocity == nil {
+            return 0.25
+        } else {
+            return 0.5
+        }
     }
     
     func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
@@ -19,6 +23,11 @@ class SheetContainerDismissAnimationController: NSObject, UIViewControllerAnimat
             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)
+        }
+    }
+    
 }
diff --git a/Sources/SheetController/SheetContainerPresentationAnimationController.swift b/Sources/SheetController/SheetContainerPresentationAnimationController.swift
index 981f76f..2b85c30 100644
--- a/Sources/SheetController/SheetContainerPresentationAnimationController.swift
+++ b/Sources/SheetController/SheetContainerPresentationAnimationController.swift
@@ -11,7 +11,11 @@ import UIKit
 class SheetContainerPresentationAnimationController: NSObject, UIViewControllerAnimatedTransitioning {
 
     func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
-        return 0.5
+        if UIAccessibility.prefersCrossFadeTransitionsBackwardsCompat {
+            return 0.25
+        } else {
+            return 0.5
+        }
     }
     
     func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
@@ -19,6 +23,11 @@ class SheetContainerPresentationAnimationController: NSObject, UIViewControllerA
             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)
+        }
+        
+    }
+    
 }
diff --git a/Sources/SheetController/UIAccessibility+BackwardsCompat.swift b/Sources/SheetController/UIAccessibility+BackwardsCompat.swift
new file mode 100644
index 0000000..08b2b0b
--- /dev/null
+++ b/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
+        }
+    }
+}