要實現一個slider控制text內容變化，需要一個元件onValueChanged回撥自定義方法
功能：滑塊滑動時，文字框內string隨之變化

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

public class TextChang : MonoBehaviour {
    public Slider slider;
    public Text text;
    public Toggle tog;
    //UnityAction<float> onSlider;
	// Use this for initialization
    void Awake()          
    {
        slider = GetComponent<Slider>();
        text = transform.Find("txtUseNum").GetComponent<Text>();
    }
	void Start () {
        slider.onValueChanged.AddListener(onSlider);    //當slider數值變化時，回撥onSlider方法
	}
	
	// Update is called once per frame
	void Update () {
        
	}

    void onSlider(float value)
    {
        text.text = value.ToString();
        text.text = text.text + "/20";
    }
}