LookAtConstraintHelper.cs

//
// https://qiita.com/Akematty/items/04fadac40f7ea45fd5ab
// https://github.com/Unity-Technologies/UnityCsReference/blob/master/Runtime/Animation/ScriptBindings/LookAtConstraint.bindings.cs
// を参考に、LookAt Constraint版ペアレントを動的に追加するヘルパー
//

using UnityEngine;
using UnityEngine.Animations;

namespace UnityChan
{
  public class LookAtConstraintHelper : MonoBehaviour {

    //Warning CS0649が出る場合の対策.
    //https://qiita.com/nonkapibara/items/8cd26751e3c86451a563
    #pragma warning disable 0649
    //
    private ConstraintSource myConstraintSource;
    private LookAtConstraint myLookAtConstraint;

      [SerializeField]
      public Transform SourceTransform;

      // LookAtConstraint コンポーネントの親オブジェクトを動的に追加する
      private void SetObject2ConstraintSource(Transform parent) {
        
          // Constraintの参照元を設定(この処理が一つでも欠けると追尾せず即死する)
          this.myConstraintSource.sourceTransform = parent;
          this.myConstraintSource.weight = 1.0f; // 影響度を完全支配にする(Addの場合は0になるので)
          this.myLookAtConstraint.AddSource(this.myConstraintSource);
          this.myLookAtConstraint.useUpObject = false;
          this.myLookAtConstraint.roll = 0.0f;
          this.myLookAtConstraint.worldUpObject = null;
          this.myLookAtConstraint.locked = true;
          this.myLookAtConstraint.rotationAtRest = Vector3.zero;
          this.myLookAtConstraint.rotationOffset = Vector3.zero;
      }

      // Constraintの設定を解除する
      private void DeleteSource() {
      this.myLookAtConstraint.RemoveSource(0);// Sourceから削除
      this.myLookAtConstraint.enabled = false;// 無効にする
      }

      private void Awake() {
        this.myLookAtConstraint = GetComponent<LookAtConstraint>();
        if(this.SourceTransform == null){
          DeleteSource();
        }else{
          SetObject2ConstraintSource(this.SourceTransform);
        }
      }

  }
}