注意：AsyncOperation.isDone=false时，AsyncOperation. progress的最大值为0.9，=true时值才会变为1.0

1.代码如下：

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

public class LoadingAsync : MonoBehaviour
{
    private static string _nextSceneName = "";

    [SerializeField]
    private Slider loadingSlider;
    [SerializeField]
    private Text loadingText;

    /// <summary>
    /// 当前进度
    /// </summary>
    private int currentProgress;
    /// <summary>
    /// 目标进度
    /// </summary>
    private int targetProgress;

    /// <summary>
    /// 打开加载场景并且异步载入name场景
    /// </summary>
    /// <param name="name"></param>
    public static void EnterScene(string name)
    {
        _nextSceneName = name;
        SceneManager.LoadScene("LoadingScene");
    }

    private void Start()
    {
        DontDestroyOnLoad(this.gameObject);
        StartCoroutine(LoadScene());
    }

    private IEnumerator LoadScene()
    {
        Debug.Log("EnterScene:" + _nextSceneName);
        targetProgress = 0;
        currentProgress = 0;
        SetPercentUI(0);
        yield return new WaitForEndOfFrame();
        // 开始异步加载场景
        AsyncOperation asyncOperation = SceneManager.LoadSceneAsync(_nextSceneName);
        // 在未完全加载完之前不要让场景自动激活
        asyncOperation.allowSceneActivation = false;

        while (!asyncOperation.isDone)
        {
            // "asyncOperation.isDone = false"时"asyncOperation.progress"最大值是0。9
            // 而只有"asyncOperation.allowSceneActivation = true;"激活场景后isDone才=true
            if (asyncOperation.progress < 0.9f)
            {
                targetProgress = (int) (asyncOperation.progress * 100);
                yield return LoadProgress();
            }
            else
            {
                // 场景加载完成后激活场景
                asyncOperation.allowSceneActivation = true;
            }
            yield return null;
        }
        // 继续跑完进度条
        targetProgress = 100;
        yield return LoadProgress();
        // 延迟销毁是为了让进度条跑到100%
        Destroy(this.gameObject);
    }

    /// <summary>
    /// 由于需要两次调用，在这里进行简单封装
    /// </summary>
    /// <returns>等一帧</returns>
    private IEnumerator<WaitForEndOfFrame> LoadProgress()
    {
        while (currentProgress < targetProgress)
        {
            // 当前进度不断累加 （如果场景很小，可以调整这里的值 例如：+=10 +=20，来调节加载速度）
            currentProgress += 20;
            // 给UI进度条赋值
            SetPercentUI(currentProgress);
            //等一帧
            yield return new WaitForEndOfFrame();
        }
    }

    /// <summary>
    /// 更新进度显示
    /// </summary>
    /// <param name="progress"></param>
    private void SetPercentUI(int progress)
    {
        loadingSlider.value = (float) progress / 100;
        loadingText.text = $"{progress}%";
    }
}

2.UI结构图如下：

image.png