Animations improvements

Shared/Modifiers/AnimationCompletionObserverModifier.swift

@@ -0,0 +1,51 @@
+import Foundation
+import SwiftUI
+
+/// An animatable modifier that is used for observing animations for a given animatable value.
+struct AnimationCompletionObserverModifier<Value>: AnimatableModifier where Value: VectorArithmetic {
+    /// While animating, SwiftUI changes the old input value to the new target value using this property. This value is set to the old value until the animation completes.
+    var animatableData: Value {
+        didSet {
+            notifyCompletionIfFinished()
+        }
+    }
+
+    /// The target value for which we're observing. This value is directly set once the animation starts. During animation, `animatableData` will hold the oldValue and is only updated to the target value once the animation completes.
+    private var targetValue: Value
+
+    /// The completion callback which is called once the animation completes.
+    private var completion: () -> Void
+
+    init(observedValue: Value, completion: @escaping () -> Void) {
+        self.completion = completion
+        animatableData = observedValue
+        targetValue = observedValue
+    }
+
+    /// Verifies whether the current animation is finished and calls the completion callback if true.
+    private func notifyCompletionIfFinished() {
+        guard animatableData == targetValue else { return }
+
+        /// Dispatching is needed to take the next runloop for the completion callback.
+        /// This prevents errors like "Modifying state during view update, this will cause undefined behavior."
+        DispatchQueue.main.async {
+            self.completion()
+        }
+    }
+
+    func body(content: Content) -> some View {
+        /// We're not really modifying the view so we can directly return the original input value.
+        return content
+    }
+}
+
+extension View {
+    /// Calls the completion handler whenever an animation on the given value completes.
+    /// - Parameters:
+    ///   - value: The value to observe for animations.
+    ///   - completion: The completion callback to call once the animation completes.
+    /// - Returns: A modified `View` instance with the observer attached.
+    func onAnimationCompleted<Value: VectorArithmetic>(for value: Value, completion: @escaping () -> Void) -> ModifiedContent<Self, AnimationCompletionObserverModifier<Value>> {
+        modifier(AnimationCompletionObserverModifier(observedValue: value, completion: completion))
+    }
+}