unity中制作滑动摇杆

准备素材

需要一个背景图片，和一个stick

实现原理

1.Stick跟随手指按下进行滑动，需要用到屏幕坐标转UI坐标。然后如果pos超过了圆的半径，需要将UI的Pos限制一下。

RectTransformUtility.ScreenPointToLocalPointInRectangle

2.Stick监听滑动和滑动结束事件，滑动结束回到原点。注意是Add New Event Type。而不是在一个Event Type里添加两个监听

3.最后可以将pos归一化处理得到一个Vector2值，返回给游戏处理移动

相关代码

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class JoyStick : MonoBehaviour {
    private Canvas cs;
    public Transform stick;
    //滑动的半径
    public float max_R = 80;

    private Vector2 touch_dir = Vector2.zero;
    public Vector2 dir {
        get {
            return this.touch_dir;
        }
    }

	// Use this for initialization
	void Awake () {
        this.cs = GameObject.Find("Canvas").GetComponent();
        this.stick.localPosition = Vector2.zero;
        this.touch_dir = Vector2.zero;
	}
	
	// Update is called once per frame
	void Update () {
        if (this.touch_dir.x == 0 && this.touch_dir.y == 0) {
            return;
        }
        Debug.Log(touch_dir);
        EventMgr.Instance.Emit("JoyStick", this.touch_dir);
	}

    public void OnStickDrag() {
        Vector2 pos = Vector2.zero;
        //屏幕坐标转UI坐标
        RectTransformUtility.ScreenPointToLocalPointInRectangle(this.transform as RectTransform, 
            Input.mousePosition, this.cs.worldCamera, out pos);
        //拖拽的半径
        float len = pos.magnitude;
        if (len <= 0) {
            this.touch_dir = Vector2.zero;
            return;
        }
        
        // 归一化, 
        this.touch_dir.x = pos.x / len; // cos(r)
        // this.touch_dir.y = pos.y / len; // (sinr) cos^2 + sin ^ 2 = 1;
        
        if (len >= this.max_R) { // this.max_R / len = x` / x = y` / y;
            pos.x = pos.x * this.max_R / len;
            pos.y = pos.y * this.max_R / len;
        }
        Debug.Log("OnStickDrag:"+pos);
        this.stick.localPosition = pos;
    }

    public void OnStickEndDrag() {
        this.stick.localPosition = Vector2.zero;
        this.touch_dir = Vector2.zero;
        EventMgr.Instance.Emit("JoyStick", this.touch_dir);
    }

}