拖动对象时，约束对象始终面向用户。

MRTK原版代码

原版代码有个缺点，拖动界面时，界面会根据头盔在Z轴上的旋转而绕Z轴旋转。

// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.

using Microsoft.MixedReality.Toolkit.Utilities;
using UnityEngine;

namespace Microsoft.MixedReality.Toolkit.UI
{
    /// <summary>
    /// Component for fixing the rotation of a manipulated object relative to the user
    /// </summary>
    public class FixedRotationToUserConstraint : TransformConstraint
    {
        #region Properties

        public override TransformFlags ConstraintType => TransformFlags.Rotate;

        private Quaternion startObjectRotationCameraSpace;

        #endregion Properties

        #region Public Methods

        /// <inheritdoc />
        public override void Initialize(MixedRealityTransform worldPose)
        {
            base.Initialize(worldPose);

            startObjectRotationCameraSpace = Quaternion.Inverse(CameraCache.Main.transform.rotation) * worldPose.Rotation;
        }

        /// <summary>
        /// Updates the objects rotation so that the rotation relative to the user
        /// is fixed
        /// </summary>
        public override void ApplyConstraint(ref MixedRealityTransform transform)
        {
            Vector3 euler = CameraCache.Main.transform.rotation.eulerAngles;
            // don't use roll (feels awkward) - just maintain yaw / pitch angle
            transform.Rotation = Quaternion.Euler(euler.x, euler.y, 0) * startObjectRotationCameraSpace;
        }

        #endregion Public Methods
    }
}

修改后的代码

去掉被操作对象在Z轴上的旋转。

// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.

using Microsoft.MixedReality.Toolkit.Utilities;
using UnityEngine;

namespace Microsoft.MixedReality.Toolkit.UI
{
    /// <summary>
    /// Component for fixing the rotation of a manipulated object relative to the user
    /// </summary>
    public class FixedRotationToUserConstraint2 : TransformConstraint
    {
        #region Properties

        public override TransformFlags ConstraintType => TransformFlags.Rotate;

        private Quaternion startObjectRotationCameraSpace;

        #endregion Properties

        #region Public Methods

        /// <inheritdoc />
        public override void Initialize(MixedRealityTransform worldPose)
        {
            base.Initialize(worldPose);

            startObjectRotationCameraSpace =  worldPose.Rotation;
        }

        /// <summary>
        /// Updates the objects rotation so that the rotation relative to the user
        /// is fixed
        /// </summary>
        public override void ApplyConstraint(ref MixedRealityTransform transform)
        {
            Vector3 euler = CameraCache.Main.transform.rotation.eulerAngles;
            // don't use roll (feels awkward) - just maintain yaw / pitch angle
            transform.Rotation = Quaternion.Euler(euler.x, euler.y, 0);
        }

        #endregion Public Methods
    }
}