Anchoring objects to splines with the Spline Extension plugin

ReanchorSelected.cs

using sc.splines.extensions.runtime.components;
using UnityEditor;
using UnityEngine;
using UnityEngine.Splines;

public class ReanchorSelected
{
    [MenuItem("Tools/Spline Tricks/Re-anchor Selected Objects to nearest Splines")]
    private static void Perform()
    {
        GameObject[] gameObjects = Selection.gameObjects;

        Undo.SetCurrentGroupName("Re-anchor Selected Objects to nearest Splines");
        int currentGroup = Undo.GetCurrentGroup();
        Undo.RecordObjects(gameObjects, "add AttachTransformToSpline and reset transform");

        foreach (GameObject gameObject in gameObjects)
        {
            if (gameObject.GetComponent<AttachTransformToSpline>())
                continue;

            (SplineContainer nearestSpline, float nearestT, Vector3 nearestPositionWorld) =
                SplineTricks.FindNearestSpline(gameObject.transform);

            if (!nearestSpline)
                continue;

            Vector3 offset = new Vector3(0f, gameObject.transform.position.y - nearestPositionWorld.y, 0f);
            float customPosition = SplineUtility.ConvertIndexUnit(
                nearestSpline.Spline,
                nearestT,
                PathIndexUnit.Distance
            );

            AttachTransformToSpline newComponent = Undo.AddComponent<AttachTransformToSpline>(gameObject);
            newComponent.customPositionType = PathIndexUnit.Distance;
            newComponent.customPosition = customPosition;
            newComponent.offset = offset;
            newComponent.point = AttachTransformToSpline.ConnectionPoint.Custom;
            newComponent.splineContainer = nearestSpline;
            newComponent.splineIndex = 0;
            newComponent.alignRotation = false;
            newComponent.Apply();
        }

        Undo.CollapseUndoOperations(currentGroup);
    }
}

SplineTricks.cs

using Unity.Mathematics;
using UnityEngine;
using UnityEngine.Splines;

public class SplineTricks
{
    public static (SplineContainer, float, Vector3) FindNearestSpline(Transform transform)
    {
        Vector3 transformPositionWorld = transform.position;
        Vector3 nearestSplinePositionWorld = new Vector3(0, 0, 0);
        SplineContainer[] candidateSplines = Object.FindObjectsByType<SplineContainer>();
        SplineContainer nearestSpline = null;
        float nearestDistanceSquared = float.PositiveInfinity;
        float nearestT = 0f;

        foreach (SplineContainer candidateSpline in candidateSplines)
        {
            float3 transformPositionLocal = candidateSpline.transform.InverseTransformPoint(transform.position);
            Ray verticalRay = new Ray(transformPositionLocal, Vector3.down);

            SplineUtility.GetNearestPoint(
                candidateSpline.Spline,
                verticalRay,
                out float3 nearestCandidateSplinePointLocal,
                out float nearestCandidateT
            );

            Vector3 nearestCandidateSplinePointWorld = candidateSpline.transform.TransformPoint(
                nearestCandidateSplinePointLocal
            );

            // The distance is calculated in horizontal plane (i.e. without Y) to avoid false positives
            // when the object is far above the relevant spline and there are other splines on the side
            float distanceSquared = (
                new Vector3(nearestCandidateSplinePointWorld.x, 0, nearestCandidateSplinePointWorld.z)
                - new Vector3(transformPositionWorld.x, 0, transformPositionWorld.z)
            ).sqrMagnitude;

            if (distanceSquared < nearestDistanceSquared)
            {
                nearestSplinePositionWorld = nearestCandidateSplinePointWorld;
                nearestDistanceSquared = distanceSquared;
                nearestSpline = candidateSpline;
                nearestT = nearestCandidateT;
            }
        }

        Debug.DrawLine(nearestSplinePositionWorld, transformPositionWorld, Color.red, 10f);

        return (nearestSpline, nearestT, nearestSplinePositionWorld);
    }
}