LevelLoadManager.cs

using UnityEngine;
using System.Collections;
using UnityEngine.SceneManagement;

public class LevelLoadManager : MonoBehaviour {

    public GameObject player;
    public GameObject trigger;
    public Texture2D fadeOutTexture;    // the texture that will overlay the screen. This can be a black image or a loading graphic
    public float fadeSpeed = 0.2f;      // the fading speed

    public string levelToLoad;


    private Vector3 currentPos;
    private Vector3 currentRot;

    private int drawDepth = -1000;      // the texture's order in the draw hierarchy: a low number means it renders on top
    private float alpha = 1.0f;         // the texture's alpha value between 0 and 1
    private int fadeDir = -1;           // the direction to fade: in = -1 or out = 1
    private bool showGUI = false;



    void Start() {

        // store player's position at the beginning of game
        currentPos = new Vector3(player.transform.position.x, player.transform.position.y, player.transform.position.z);
        // store player's rotation value when game starts
        currentRot = new Vector3(player.transform.eulerAngles.x, player.transform.eulerAngles.y, player.transform.eulerAngles.y);

      
    }

    void OnGUI() {

        if (showGUI) {

            fadeOutTexture = (Texture2D)Resources.Load("blackFading");

            // fade out/in the alpha value using a direction, a speed and Time.deltaTime to convert the operation to seconds
            alpha += fadeDir * fadeSpeed * Time.deltaTime;

            // GUI.color uses Alpha values between 0 and 1
            // Clamps value between 0 and 1 and returns value.
            alpha = Mathf.Clamp01(alpha);


            // set color of our GUI (in this case our texture). All color values remain the same & the Alpha is set to the alpha variable
            GUI.color = new Color(GUI.color.r, GUI.color.g, GUI.color.b, alpha);
            GUI.depth = drawDepth;                                                              // make the black texture render on top (drawn last)
            GUI.DrawTexture(new Rect(0, 0, Screen.width, Screen.height), fadeOutTexture);       // draw the texture to fit the entire screen area

            // display name of current scene
            GUI.Label(new Rect(100, 100, 600, 600), "<size=60>" + SceneManager.GetActiveScene().name + "</size>");


        }

    }
    
    void LateUpdate() {

        // when player dies (just for demonstration, if player fall off the ground )
         if(player.transform.position.y < -15) {

        // StartCoroutine: loading scene with AsyncOperation
                StartCoroutine(DisplayLoading(levelToLoad));
          }

    }

    public void loadLevel() {

        StartCoroutine(DisplayLoading(levelToLoad));
    }

    IEnumerator DisplayLoading(string nextLevel) {

        // assigning Async task
        AsyncOperation async = SceneManager.LoadSceneAsync(nextLevel);

        // reset player's position 
        player.transform.position = currentPos;
        // reset player's rotation value
        player.transform.rotation = Quaternion.Euler(currentRot);

        // while async is running
        while (!async.isDone) {
            Debug.Log(async.progress);
            // enable GUI (fading screen)
            showGUI = true;
            // disable the game scene whil async
            async.allowSceneActivation = false;
            yield return null;
            
        }

    }

}