ZPosition Transition (#35)

Closes #32.

Author: davdroman

Demo/Demo/Swing.swift

@@ -14,6 +14,7 @@ struct Swing: NavigationTransition {
             let angle = 70.0
             let offset = 150.0
             OnInsertion {
+                ZPosition(1)
                 Rotate(.degrees(-angle))
                 Offset(x: offset)
                 Opacity()
@@ -24,10 +25,5 @@ struct Swing: NavigationTransition {
                 Offset(x: -offset)
             }
         }
-        OnPop {
-            OnRemoval {
-                SendToBack()
-            }
-        }
     }
 }

README.md

@@ -86,6 +86,7 @@ struct Swing: NavigationTransition {
             let angle = 70.0
             let offset = 150.0
             OnInsertion {
+                ZPosition(1)
                 Rotate(.degrees(-angle))
                 Offset(x: offset)
                 Opacity()
@@ -96,11 +97,6 @@ struct Swing: NavigationTransition {
                 Offset(x: -offset)
             }
         }
-        OnPop {
-            OnRemoval {
-                SendToBack()
-            }
-        }
     }
 }
 ```

Sources/Animator/AnimatorTransientView.swift

@@ -34,15 +34,18 @@ public class AnimatorTransientView {
 
     @_spi(package)public let uiView: UIView
 
-    /// Read-only proxy to underlying `UIView` instance.
+    /// Read-only proxy to underlying `UIView` properties.
     public subscript<T>(dynamicMember keyPath: KeyPath<UIView, T>) -> T {
         uiView[keyPath: keyPath]
     }
 
     @_spi(package)public init(_ uiView: UIView) {
         let properties = Properties(
             alpha: uiView.alpha,
-            transform: uiView.transform
+            transform: uiView.transform,
+            layer: .init(
+                zPosition: uiView.layer.zPosition
+            )
         )
         self.initial = properties
         self.animation = properties

Sources/Animator/AnimatorTransientViewLayerProperties.swift

@@ -0,0 +1,12 @@
+import UIKit
+
+/// Defines the allowed mutable properties in a transient view throughout each stage of the transition.
+public struct AnimatorTransientViewLayerProperties: Equatable {
+    /// A proxy for `CALayer.zPosition`.
+    @OptionalWithDefault
+    public var zPosition: CGFloat
+
+    func assignToUIView(_ uiView: UIView) {
+        $zPosition.assignTo(uiView, \.layer.zPosition)
+    }
+}

Sources/Animator/AnimatorTransientViewProperties.swift

@@ -2,22 +2,24 @@ import UIKit
 
 /// Defines the allowed mutable properties in a transient view throughout each stage of the transition.
 public struct AnimatorTransientViewProperties: Equatable {
+    public typealias Layer = AnimatorTransientViewLayerProperties
+
+    /// A proxy for `UIView.alpha`.
     @OptionalWithDefault
     public var alpha: CGFloat
+
+    /// A proxy for `UIView.transform`.
     @OptionalWithDefault
     public var transform: CGAffineTransform
 
+    /// A proxy for `UIView.layer`.
+    @OptionalWithDefault
+    public var layer: Layer
+
     func assignToUIView(_ uiView: UIView) {
         $alpha.assignTo(uiView, \.alpha)
         $transform.assignTo(uiView, \.transform)
-    }
-}
-
-private extension Optional {
-    func assignTo<Root: AnyObject>(_ root: Root, _ valueKeyPath: ReferenceWritableKeyPath<Root, Wrapped>) {
-        if let value = self {
-            root[keyPath: valueKeyPath] = value
-        }
+        $layer?.assignToUIView(uiView)
     }
 }
 

Sources/Animator/OptionalWithDefault.swift renamed to Sources/Animator/OptionalExtensions.swift

@@ -15,3 +15,11 @@ public struct OptionalWithDefault<Value> {
 }
 
 extension OptionalWithDefault: Equatable where Value: Equatable {}
+
+extension Optional {
+    func assignTo<Root: AnyObject>(_ root: Root, _ valueKeyPath: ReferenceWritableKeyPath<Root, Wrapped>) {
+        if let value = self {
+            root[keyPath: valueKeyPath] = value
+        }
+    }
+}

Sources/AtomicTransition/Opacity.swift

@@ -15,6 +15,7 @@ public struct Opacity: MirrorableAtomicTransition {
         }
     }
 
+    @inlinable
     public func mirrored() -> Self {
         self
     }

Sources/AtomicTransition/Scale.swift

@@ -28,6 +28,7 @@ public struct Scale: MirrorableAtomicTransition {
         }
     }
 
+    @inlinable
     public func mirrored() -> Self {
         self
     }

Sources/AtomicTransition/ZPosition.swift

@@ -1,5 +1,24 @@
 @_spi(package) import Animator
-import class UIKit.UIView
+import UIKit
+
+/// A transition that changes the view layer’s position on the z axis.
+public struct ZPosition: MirrorableAtomicTransition {
+    private var zPosition: CGFloat
+
+    public init(_ zPosition: CGFloat) {
+        self.zPosition = zPosition
+    }
+
+    public func transition(_ view: TransientView, for operation: TransitionOperation, in container: Container) {
+        view.animation.layer.zPosition = zPosition
+        view.completion.layer.zPosition = 0
+    }
+
+    @inlinable
+    public func mirrored() -> Self {
+        self
+    }
+}
 
 /// A transition that brings the view to the front, regardless of insertion or removal.
 public struct BringToFront: AtomicTransition {

Sources/NavigationTransition/Fade.swift

@@ -26,25 +26,17 @@ public struct Fade: NavigationTransition {
         case .in:
             MirrorPush {
                 OnInsertion {
+                    ZPosition(1)
                     Opacity()
                 }
             }
-            OnPop {
-                OnInsertion {
-                    BringToFront()
-                }
-            }
         case .out:
             MirrorPush {
                 OnRemoval {
+                    ZPosition(1)
                     Opacity()
                 }
             }
-            OnPush {
-                OnRemoval {
-                    BringToFront()
-                }
-            }
         case .cross:
             MirrorPush {
                 Opacity()