Added step by step constraints for TwistChain.

Also added step by step scenes mapping to these constraints.

Assets/Constraints/ConstraintsUtils.cs

+using System.Collections.Generic;
+using UnityEditor;
+using Unity.Collections;
+
+namespace UnityEngine.Animations.Rigging
+{
+    static class ConstraintsUtils
+    {
+        public static List<Transform> ExtractChain(Transform root, Transform tip)
+        {
+            var chain = new List<Transform>();
+
+            if (!tip.IsChildOf(root))
+                return chain;
+
+            Transform tmp = tip;
+            while (tmp != root)
+            {
+                chain.Add(tmp);
+                tmp = tmp.parent;
+            }
+            chain.Add(root);
+            chain.Reverse();
+
+            return chain;
+        }
+    }
+}

Assets/Constraints/ConstraintsUtils.cs.meta

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Assets/Constraints/TwistChainStep0.cs

+using System.Collections.Generic;
+using UnityEditor;
+using Unity.Collections;
+
+namespace UnityEngine.Animations.Rigging
+{
+    using Experimental.Animations;
+
+    [Unity.Burst.BurstCompile]
+    public struct TwistChainStep0Job : IWeightedAnimationJob
+    {
+        public FloatProperty jobWeight { get; set; }
+
+        public void ProcessRootMotion(AnimationStream stream) {}
+
+        public void ProcessAnimation(AnimationStream stream) {}
+    }
+
+    [System.Serializable]
+    public struct TwistChainStep0Data : IAnimationJobData
+    {
+        bool IAnimationJobData.IsValid() => true;
+
+        void IAnimationJobData.SetDefaultValues()
+        {
+        }
+    }
+
+    public class TwistChainStep0JobBinder : AnimationJobBinder<TwistChainStep0Job, TwistChainStep0Data>
+    {
+        public override TwistChainStep0Job Create(Animator animator, ref TwistChainStep0Data data, Component component)
+        {
+            // Build Job.
+            var job = new TwistChainStep0Job();
+            return job;
+        }
+
+        public override void Destroy(TwistChainStep0Job job)
+        {
+        }
+
+        public override void Update(TwistChainStep0Job job, ref TwistChainStep0Data data)
+        {
+        }
+    }
+
+    [DisallowMultipleComponent, AddComponentMenu("SIGGRAPH 2019/Twist Chain Step 0")]
+    public class TwistChainStep0 : RigConstraint<
+        TwistChainStep0Job,
+        TwistChainStep0Data,
+        TwistChainStep0JobBinder
+        >
+    {
+    }
+}

Assets/Constraints/TwistChainStep0.cs.meta

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Assets/Constraints/TwistChainStep1.cs

+using System.Collections.Generic;
+using UnityEditor;
+using Unity.Collections;
+
+namespace UnityEngine.Animations.Rigging
+{
+    using Experimental.Animations;
+
+    [Unity.Burst.BurstCompile]
+    public struct TwistChainStep1Job : IWeightedAnimationJob
+    {
+        public NativeArray<ReadWriteTransformHandle> chain;
+        public ReadWriteTransformHandle rootTarget;
+        public ReadWriteTransformHandle tipTarget;
+
+        public FloatProperty jobWeight { get; set; }
+
+        public void ProcessRootMotion(AnimationStream stream) {}
+
+        public void ProcessAnimation(AnimationStream stream)
+        {
+            // Retrieve root and tip rotation.
+            Quaternion rootRotation = rootTarget.GetRotation(stream);
+            Quaternion tipRotation = tipTarget.GetRotation(stream);
+
+            float mainWeight = jobWeight.Get(stream);
+
+            // Interpolate rotation on chain.
+            for (int i = 0; i < chain.Length; ++i)
+            {
+                float weight = Mathf.InverseLerp(0, chain.Length - 1, i);
+                chain[i].SetRotation(stream, Quaternion.Lerp(chain[i].GetRotation(stream), Quaternion.Lerp(rootRotation, tipRotation, weight), mainWeight));
+            }
+
+            // Update position of tip handle for easier visualization.
+            rootTarget.SetPosition(stream, chain[0].GetPosition(stream));
+            tipTarget.SetPosition(stream, chain[chain.Length - 1].GetPosition(stream));
+        }
+    }
+
+    [System.Serializable]
+    public struct TwistChainStep1Data : IAnimationJobData
+    {
+        public Transform root;
+        public Transform tip;
+
+        [SyncSceneToStream] public Transform rootTarget;
+        [SyncSceneToStream] public Transform tipTarget;
+
+        bool IAnimationJobData.IsValid() => !(root == null || tip == null || !tip.IsChildOf(root) || rootTarget == null || tipTarget == null);
+
+        void IAnimationJobData.SetDefaultValues()
+        {
+            root = null;
+            tip = null;
+
+            rootTarget = null;
+            tipTarget = null;
+        }
+    }
+
+    public class TwistChainStep1JobBinder : AnimationJobBinder<TwistChainStep1Job, TwistChainStep1Data>
+    {
+        public override TwistChainStep1Job Create(Animator animator, ref TwistChainStep1Data data, Component component)
+        {
+            // Retrieve chain in-between root and tip transforms.
+            List<Transform> chain = ConstraintsUtils.ExtractChain(data.root, data.tip);
+
+            // Build Job.
+            var job = new TwistChainStep1Job();
+            job.chain = new NativeArray<ReadWriteTransformHandle>(chain.Count, Allocator.Persistent, NativeArrayOptions.UninitializedMemory);
+            job.rootTarget = ReadWriteTransformHandle.Bind(animator, data.rootTarget);
+            job.tipTarget = ReadWriteTransformHandle.Bind(animator, data.tipTarget);
+
+            for (int i = 0; i < chain.Count; ++i)
+                job.chain[i] = ReadWriteTransformHandle.Bind(animator, chain[i]);
+
+            return job;
+        }
+
+        public override void Destroy(TwistChainStep1Job job)
+        {
+            job.chain.Dispose();
+        }
+
+        public override void Update(TwistChainStep1Job job, ref TwistChainStep1Data data)
+        {
+        }
+    }
+
+    [DisallowMultipleComponent, AddComponentMenu("SIGGRAPH 2019/Twist Chain Step 1")]
+    public class TwistChainStep1 : RigConstraint<
+        TwistChainStep1Job,
+        TwistChainStep1Data,
+        TwistChainStep1JobBinder
+        >
+    {
+    }
+}

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Assets/Scenes/TwistChainFinal.unity.meta

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Assets/Scenes/TwistChainSetup0.unity.meta

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