Improve gallery transitions when there is something displaying on top of the source view

See #520

Packages/GalleryVC/Sources/GalleryVC/GalleryDismissAnimationController.swift

@@ -47,6 +47,18 @@ class GalleryDismissAnimationController: NSObject, UIViewControllerAnimatedTrans
             container.addSubview(to.view)
         }
         
+        // This trick does not work as well here as it does for presentation, seemingly
+        // because the delayFactor:0.9 still results in the snapshot fading out too quickly :/
+        let sourceSnapshot = sourceView.snapshotView(afterScreenUpdates: false)
+        if let sourceSnapshot {
+            let snapshotContainer = sourceView.ancestorForInsertingSnapshot
+            snapshotContainer.addSubview(sourceSnapshot)
+            let sourceFrameInShapshotContainer = snapshotContainer.convert(sourceView.bounds, from: sourceView)
+            sourceSnapshot.frame = sourceFrameInShapshotContainer
+            sourceSnapshot.layer.opacity = 1
+            self.sourceView.layer.opacity = 0
+        }
+
         let sourceFrameInContainer = container.convert(sourceView.bounds, from: sourceView)
         let destFrameInContainer = container.convert(itemViewController.content.view.bounds, from: itemViewController.content.view)
         
@@ -59,6 +71,7 @@ class GalleryDismissAnimationController: NSObject, UIViewControllerAnimatedTrans
                 .translatedBy(x: destFrameInContainer.midX - sourceFrameInContainer.midX, y: destFrameInContainer.midY - sourceFrameInContainer.midY)
                 .scaledBy(x: scale, y: scale)
             sourceView.transform = sourceToDestTransform
+            sourceSnapshot?.transform = sourceToDestTransform
         } else {
             appliedSourceToDestTransform = false
         }
@@ -142,6 +155,7 @@ class GalleryDismissAnimationController: NSObject, UIViewControllerAnimatedTrans
             
             if appliedSourceToDestTransform {
                 self.sourceView.transform = origSourceTransform
+                sourceSnapshot?.transform = origSourceTransform
             }
             
             contentContainer.frame = sourceFrameInContainer
@@ -151,7 +165,16 @@ class GalleryDismissAnimationController: NSObject, UIViewControllerAnimatedTrans
             itemViewController.setControlsVisible(false, animated: false, dueToUserInteraction: false)
         }
         
+        if let sourceSnapshot {
+            animator.addAnimations({
+                self.sourceView.layer.opacity = 1
+                sourceSnapshot.layer.opacity = 0
+            }, delayFactor: 0.9)
+        }
+        
         animator.addCompletion { _ in
+            sourceSnapshot?.removeFromSuperview()
+            
             // Having dismissed, we don't need to undo any of the changes to the content VC.
             
             transitionContext.completeTransition(true)

Packages/GalleryVC/Sources/GalleryVC/GalleryPresentationAnimationController.swift

@@ -30,6 +30,25 @@ class GalleryPresentationAnimationController: NSObject, UIViewControllerAnimated
             return
         }
         
+        // Try to effectively "fade out" anything that's on top of the source view.
+        // The 0.05 duration makes this happen faster than the rest of the animation,
+        // and so less noticeable.
+        let sourceSnapshot = sourceView.snapshotView(afterScreenUpdates: false)
+        if let sourceSnapshot {
+            let snapshotContainer = sourceView.ancestorForInsertingSnapshot
+            snapshotContainer.addSubview(sourceSnapshot)
+            let sourceFrameInShapshotContainer = snapshotContainer.convert(sourceView.bounds, from: sourceView)
+            sourceSnapshot.frame = sourceFrameInShapshotContainer
+            sourceSnapshot.transform = sourceView.transform
+            sourceSnapshot.layer.opacity = 0
+            UIView.animate(withDuration: 0.05) {
+                sourceSnapshot.layer.opacity = 1
+                // Also fade out the actual source view underneath the snapshot to try
+                // and account for semi-transparent images.
+                self.sourceView.layer.opacity = 0
+            }
+        }
+        
         let container = transitionContext.containerView
         to.view.frame = container.bounds
         container.addSubview(to.view)
@@ -149,20 +168,23 @@ class GalleryPresentationAnimationController: NSObject, UIViewControllerAnimated
             itemViewController.setControlsVisible(true, animated: false, dueToUserInteraction: false)
 
             if let sourceToDestTransform {
+                sourceSnapshot?.transform = sourceToDestTransform
                 self.sourceView.transform = sourceToDestTransform
             }
         }
         
         animator.addCompletion { _ in
+            sourceSnapshot?.removeFromSuperview()
+            self.sourceView.layer.opacity = 1
+            if sourceToDestTransform != nil {
+                self.sourceView.transform = origSourceTransform
+            }
+
             contentContainer.removeFromSuperview()
             dimmingView.removeFromSuperview()
             
             to.view.backgroundColor = .black
             
-            if sourceToDestTransform != nil {
-                self.sourceView.transform = origSourceTransform
-            }
-            
             // Reset the properties we changed before re-adding the content to the scroll view.
             // (I would expect UIScrollView to effectively do this itself, but w/e.)
             content.view.transform = origContentTransform

Packages/GalleryVC/Sources/GalleryVC/UIView+Utilities.swift

@@ -0,0 +1,26 @@
+//
+//  UIView+Utilities.swift
+//  GalleryVC
+//
+//  Created by Shadowfacts on 11/24/24.
+//
+
+import UIKit
+
+extension UIView {
+    
+    var ancestorForInsertingSnapshot: UIView {
+        var view = self
+        while let superview = view.superview {
+            if superview.layer.masksToBounds {
+                return superview
+            } else if superview is UIScrollView {
+                return self
+            } else {
+                view = superview
+            }
+        }
+        return view
+    }
+    
+}