摇杆（magnitude）_deltaposition.magnitude

using UnityEngine;
using System.Collections;
using UnityEngine.EventSystems;

public class Joystick : MonoBehaviour, IDragHandler, IEndDragHandler
{
    private readonly int maxDistance = 150;

    public Vector2 joystickDirection { get { return this.transform.localPosition.normalized; } }

    public void OnDrag(PointerEventData eventData)
    {
        this.transform.position = Input.mousePosition;

        if (this.transform.localPosition.magnitude > maxDistance)
        {
            this.transform.localPosition = this.transform.localPosition.normalized * maxDistance;
        }
    }

    public void OnEndDrag(PointerEventData eventData)
    {
        this.transform.localPosition = Vector3.zero;
    }
}
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