How to update one Bezier curve as another is moved using a custom editor

Question

I am creating Bézier curves using the code below which I got from here. I have also made a BezierPair game object which has two Bézier curves as child objects.

Answer 1

EDITED:

I dont think you'll need your BezierPair class. I suggest you to add a reference to the other BezierCurve object you want to "pair" as a public field on the BezierCurve class (paired). That other curve will get "paired" with this curve. Once paired, the constraints on the movement may apply. You can control the desired behaviors with the 3 public bool fields behavior1, behavior2 and behavior3.

Note#1: Instead of calling the method DrawBezierCurve from the Editor, I added the [ExecuteInEditMode] to the Component Class. This way, You don't mix responsibilities between the Component and the Editor: The BezierCurve component draws itself on the scene, while the BezierCurveEditor only manages edit logic, like applying the constraints and drawing the handlers.

BezierCurve:

using UnityEngine;

[RequireComponent(typeof(LineRenderer))]
[ExecuteInEditMode] // Makes Update() being called often even in Edit Mode
public class BezierCurve : MonoBehaviour
{

  public Vector3[] points;
  public int numPoints = 50;
  // Curve that is paired with this curve
  public BezierCurve paired;
  public bool behavior1; // check on editor if you desired behavior 1 ON/OFF
  public bool behavior2; // check on editor if you desired behavior 2 ON/OFF
  public bool behavior3; // check on editor if you desired behavior 3 ON/OFF
  LineRenderer lr;

  void Reset()
  {
    points = new Vector3[]
    {
      new Vector3(1f, 0f, 0f),
      new Vector3(2f, 0f, 0f),
      new Vector3(3f, 0f, 0f),
      new Vector3(4f, 0f, 0f)
    };
  }

  void Start()
  {
    lr = GetComponent();
  }

  void Update()
  {
    // This component is the only responsible for drawing itself.
    DrawBezierCurve();
  }

  // This method is called whenever a field is changed on Editor
  void OnValidate()
  {
    // This avoids pairing with itself
    if (paired == this) paired = null;
  }

  void DrawBezierCurve()
  {
    lr.positionCount = numPoints;
    for (int i = 0; i < numPoints; i++)
    {
      // This corrects the "strange" extra point you had with your script.
      float t = i / (float)(numPoints - 1);
      lr.SetPosition(i, GetPoint(t));
    }
  }

  public Vector3 GetPoint(float t)
  {
    return transform.TransformPoint(Bezier.GetPoint(points[0], points[1], points[2], points[3], t));
  }

  public Vector3 GetVelocity(float t)
  {
    return transform.TransformPoint(Bezier.GetFirstDerivative(points[0], points[1], points[2], points[3], t)) - transform.position;
  }

  public Vector3 GetDirection(float t)
  {
    return GetVelocity(t).normalized;
  }
}

Note#2: The desired behavior were coded inside the handler drawing methods, so you have access to Undo and other features.

Note#3: EditorUtility.SetDirty is considered obsolete since Unity 5.3 for marking objects as dirty for drawing, and should no-longer be used for modifying objects in scenes. Undo.RecordObject does the job.

BezierCurveEditor:

using UnityEngine;
using UnityEditor;

// This attribute allows you to select multiple curves and manipulate them all as a whole on Scene or Inspector
[CustomEditor(typeof(BezierCurve)), CanEditMultipleObjects]
public class BezierCurveEditor : Editor
{
  BezierCurve curve;
  Transform handleTransform;
  Quaternion handleRotation;
  const int lineSteps = 10;
  const float directionScale = 0.5f;

  BezierCurve prevPartner; // Useful later.

  void OnSceneGUI()
  {
    curve = target as BezierCurve;
    if (curve == null) return;
    handleTransform = curve.transform;
    handleRotation = Tools.pivotRotation == PivotRotation.Local ? handleTransform.rotation : Quaternion.identity;

    Vector3 p0 = ShowPoint(0);
    Vector3 p1 = ShowPoint(1);
    Vector3 p2 = ShowPoint(2);
    Vector3 p3 = ShowPoint(3);

    Handles.color = Color.gray;
    Handles.DrawLine(p0, p1);
    Handles.DrawLine(p2, p3);
    Handles.DrawBezier(p0, p3, p1, p2, Color.white, null, 2f);

    // Handles multiple selection
    var sel = Selection.GetFiltered(typeof(BezierCurve), SelectionMode.Editable);
    if (sel.Length == 1)
    {
      // This snippet checks if you just attached or dettached another curve,
      // so it updates the attached member in the other curve too automatically
      if (prevPartner != curve.paired)
      {
        if (prevPartner != null) { prevPartner.paired = null; }
        prevPartner = curve.paired;
      }
    }
    if (curve.paired != null & curve.paired != curve)
    {
      // Pair the curves.
      var partner = curve.paired;
      partner.paired = curve;
      partner.behavior1 = curve.behavior1;
      partner.behavior2 = curve.behavior2;
      partner.behavior3 = curve.behavior3;
    }
  }

  // Constraints for a curve attached to back
  // The trick here is making the object being inspected the "master" and the attached object is adjusted to it.
  // This way, you avoid the conflict of one object trying to move the other.
  // [ExecuteInEditMode] on component class makes it posible to have real-time drawing while editing.
  // If you were calling DrawBezierCurve from here, you would only see updates on the other curve when you select it
  Vector3 ShowPoint(int index)
  {
    var thisPts = curve.points;
    Vector3 point = handleTransform.TransformPoint(thisPts[index]);
    EditorGUI.BeginChangeCheck();
    point = Handles.DoPositionHandle(point, handleRotation);
    if (EditorGUI.EndChangeCheck())
    {
      if (curve.paired != null && curve.paired != curve)
      {
        Undo.RecordObjects(new Object[] { curve, curve.paired }, "Move Point " + index.ToString());
        var pairPts = curve.paired.points;
        var pairTransform = curve.paired.transform;
        switch (index)
        {
          case 0:
            {
              if (curve.behavior1)
              {
                pairPts[0] = pairTransform.InverseTransformPoint(point);
              }
              break;
            }
          case 1:
            {
              if (curve.behavior2)
              {
                var p1 = handleTransform.TransformPoint(thisPts[1]);
                pairPts[1] += pairTransform.InverseTransformVector(point - p1);
              }
              break;
            }
          case 2:
            {
              if (curve.behavior3)
              {
                var p0 = handleTransform.TransformPoint(thisPts[0]);
                var p3 = handleTransform.TransformPoint(thisPts[3]);
                var reflect = Vector3.Reflect(p3 - point, (p3 - p0).normalized);
                pairPts[2] = pairTransform.InverseTransformPoint(p3 + reflect);
              }
              break;
            }
          default:
            break;
        }
      }
      else
      {
        Undo.RecordObject(curve, "Move Point " + index.ToString());
      }
      thisPts[index] = handleTransform.InverseTransformPoint(point);
    }
    return point;
  }
}

For have it working, reference one BezierCurve to another's paired field through inspector, and set ON/OFF the behaviors you want.

Tips: tinker the properties of the LineRenderer to get cool gradients or width variation (like a brush stroke). If you have a cusp node and want it to look continuous, increase the value of End Cap Vertices on the Line Renderer. Use Sprites-Default as Material, for 2D.