Stem.cs

using UnityEngine.EventSystems;
using UnityEngine;

public class Stem : MonoBehaviour, IPointerDownHandler, IBeginDragHandler, IDragHandler, IEndDragHandler, IPointerClickHandler
{
	Vector3 _pos;
	Vector3 _posOffset;
	RectTransform _thisRectTransform;
	RectTransform _parentRectTransform;
    Rigidbody2D _rb;

	void Awake() {
        if (_rb == null) {
            _rb = GetComponent<Rigidbody2D>();
        }
        Debug.Log(_rb);
        Canvas canvas = GetComponentInParent<Canvas>();
		if (canvas != null) {
			_parentRectTransform = canvas.GetComponent<RectTransform>();
			_thisRectTransform = GetComponent<RectTransform>();
            Debug.Log("_parentRectTransform " + _parentRectTransform + " exists");
		}
	}

	public void OnPointerDown(PointerEventData eventData) {
		RectTransformUtility.ScreenPointToWorldPointInRectangle(_thisRectTransform,
                                                                    eventData.position,
                                                                    eventData.pressEventCamera,
                                                                    out _posOffset);
	}

	public void OnPointerClick(PointerEventData eventData) {
		
	}

	// this method requires IBeginDragHandler
	public void OnBeginDrag(PointerEventData eventData) {

	}

	// this method requires IDragHandler
    public void OnDrag(PointerEventData eventData) {
		if (_parentRectTransform == null)
		{
			return;
		}

        if (RectTransformUtility.ScreenPointToWorldPointInRectangle(_thisRectTransform,
                                                                    eventData.position,
                                                                    eventData.pressEventCamera,
                                                                    out _pos)) {
            _rb.MovePosition(_pos);
        }
	}

	// this method requires IEndDragHandler
	public void OnEndDrag(PointerEventData eventData) {
		
	}
}