Add VelocityTrackingAnimation (#458)

Author: Kyle-Ye

Sources/OpenSwiftUICore/Animation/Animation/VelocityTrackingAnimation.swift

@@ -2,16 +2,19 @@
 //  VelocityTrackingAnimation.swift
 //  OpenSwiftUICore
 //
-//  Audited for iOS 18.0
-//  Status: WIP
+//  Audited for 6.5.4
+//  Status: Complete
 //  ID: FD9125BC1E04E33D1D7BE4A31225AA98 (SwiftUICore)
 
+import Foundation
+
 // MARK: - TracksVelocityKey
 
 private struct TracksVelocityKey: TransactionKey {
     static var defaultValue: Bool { false }
 }
 
+@available(OpenSwiftUI_v5_0, *)
 extension Transaction {
     /// Whether this transaction will track the velocity of any animatable
     /// properties that change.
@@ -50,6 +53,68 @@ extension Transaction {
 }
 
 extension Animation {
-    // FIXME: VelocityTrackingAnimation
-    static let velocityTracking: Animation = .default
+    static let velocityTracking: Animation = Animation(VelocityTrackingAnimation())
+}
+
+private struct VelocityTrackingAnimation: CustomAnimation {
+    nonisolated func animate<V>(
+        value: V,
+        time: TimeInterval,
+        context: inout AnimationContext<V>
+    ) -> V? where V : VectorArithmetic {
+        var sampler = context.velocityState.sampler
+        if sampler.isEmpty { // FIXME: Verify this logic
+            sampler.addSample(value, time: .init(seconds: time))
+            context.velocityState = .init(sampler: sampler)
+        }
+        let newTime = (sampler.lastTime?.seconds ?? .zero) + 2.0
+        let velocity = velocity(
+            value: value,
+            time: time,
+            context: context
+        )
+        if let velocity, velocity == .zero {
+            return nil
+        }
+        guard newTime > time else {
+            return nil
+        }
+        return value
+    }
+
+
+    nonisolated func velocity<V>(
+        value: V,
+        time: TimeInterval,
+        context: AnimationContext<V>
+    ) -> V? where V : VectorArithmetic {
+        let timeDiff = time - (context.velocityState.sampler.lastTime?.seconds ?? .zero)
+        let scale = pow(0.998, timeDiff * 1000)
+        return context.velocityState.sampler.velocity.scaled(by: scale).valuePerSecond
+    }
+
+    nonisolated func shouldMerge<V>(
+        previous: Animation,
+        value: V,
+        time: TimeInterval,
+        context: inout AnimationContext<V>
+    ) -> Bool where V: VectorArithmetic {
+        context.velocityState.sampler.addSample(value, time: .init(seconds: time))
+        return true
+    }
+}
+
+extension AnimationContext {
+    fileprivate var velocityState: VelocityState<Value> {
+        get { state[VelocityState<Value>.self] }
+        set { state[VelocityState<Value>.self] = newValue }
+    }
+}
+
+private struct VelocityState<Value>: AnimationStateKey where Value: VectorArithmetic {
+    static var defaultValue: VelocityState {
+        VelocityState(sampler: .init())
+    }
+
+    var sampler: VelocitySampler<Value>
 }