2023-09-03: Persistent Data

Persistent Data

The mission today was to learn about persistent data between Scene and Session. The 2 simple scenes used to experiment were:

• Menu UI - To enter username
• Game UI - Displays username next to the score as well as all time high score.

Scene Data Persistence

Example: Username (mambo) entered in the UI scene is passed on to the game scene.

• Use singleton on Game Manager or any static class to store variable.
• The singleton GameObject should be loaded in the initial scene (Scene index 0) for most scenarios.
• Flag parent GameObject as “do not destroy on load” to persist cross different scenes.

    private void Awake()
    {
        Instance = this;
        DontDestroyOnLoad(gameObject);
    }

Session Data Persistence

Example: Previous high score from “mambo” is now displayed when E.T. starts a new game.

• E.T anatomy does not allow him(?) to play on human keyboard.

• Since the data is small, I opted for file based (JSON) saving mechanism.

  1. Create serializable inner class to hold information if using JSON format.

       [Serializable]
       private class SaveData
       {
              public string playerName;
              public int score;
       }
     

  2. Unity recommends storing these data to persistentDataPath. This file persists between game play.

       public void SaveHighScore(string highScorePlayer, int newHighScore)
       {
              SaveData data = new SaveData();
              data.playerName = highScorePlayer;
              data.score = newHighScore;
     
              string path = Application.persistentDataPath + "/saveData.json";
              string json = JsonUtility.ToJson(data);
              File.WriteAllText(path, json);
       }
     

  3. Similarly, do the reverse for loading.

       public void LoadSaveData()
       {
              string path = Application.persistentDataPath + "/saveData.json";
              if (File.Exists(path))
              {
                      string json = File.ReadAllText(path);
                      SaveData data = JsonUtility.FromJson<SaveData>(json);
                      Instance.HighScore = data.score;
                      Instance.HighScorePlayer = data.playerName;
              }
       }
     

Tip: Use null check on any class using the Instance so you can individually run the scene

//Inside UI script in the second scene
void Start() 
    {
        //For dev mode: in case we want to skip main menu
        if (MainManager.Instance != null)
        {
            HighScoreText.text = $"Best score: {MainManager.Instance.HighScore}";
        }
    }

Misc: Dev Mode (Conditional Compilation)

• The following snippet uses conditional compilation to do perform different operation based on where the application is running.

#if UNITY_EDITOR
    using UnityEditor;
#endif

    public void Exit()
    {
        MainManager.Instance.SaveColor();
#if UNITY_EDITOR
        EditorApplication.ExitPlaymode();
#else
        Application.Quit();
#endif
    }