How can I save the high score for a 2D game

Question

I have a script that doesn't save the high score automatically and before I put the restart button my high score was saved without any code. I want the high score to be saved even if the player restarts the game .This is my code for the SCORE:

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

public class Score : MonoBehaviour
{
    public int score;
    public Text ScoreText;
    public int highscore;
    public Text highScoreText;


    void Start()
    {
        highscore = PlayerPrefs.GetInt("HighScore: " + highscore);
    }

    void Update()
    {
        ScoreText.text = "Score: " + score;
        highScoreText.text = highscore.ToString();
        if (score > highscore)
        {
            highscore = score;

            PlayerPrefs.SetInt("HighScore: ", highscore);
        }
    }
    void OnTriggerEnter(Collider other)
    {
        Debug.Log("collider is working");
        if (other.gameObject.tag == "Score: ")
        {
            score++;
        }
    }
}

And this is the code for the restart button:

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

public class RestartGame : MonoBehaviour
{
    public void RestartsGame()
    {
        SceneManager.LoadScene(SceneManager.GetActiveScene().name); // loads current scene
    }
}

Answer 1

You have problems with the keys used by PlayerPrefs :

void Start()
{
    highscore = PlayerPrefs.GetInt("HighScore");
}

void Update()
{
    ScoreText.text = "Score: " + score;
    highScoreText.text = highscore.ToString();
    if (score > highscore)
    {
        highscore = score;

        PlayerPrefs.SetInt("HighScore", highscore);

    }
}

Answer 2

As noted by Draco18s

you shouldn't be using player prefs to save game data (such as highscore).

Here is how you can save it in a json file if you want. It also adds more flexibility in case you want to add things like a top 10 score or something like that.

using UnityEngine;
using System.IO;

public class HighestScoreJson
{
    /// <summary>
    /// //Returns the path to the highestScoreFile
    /// </summary>
    /// <returns></returns>
    public static string GetPathToHighestScore() {
        return Path.Combine(Application.persistentDataPath, "HighestScore.json"); //You can look for info about the unity streamingAssets folder in the unity documentation
    }

    /// <summary>
    /// Get the highestScore
    /// </summary>
    /// <returns></returns>
    public static HighestScore GetHighestScore() {
        string path = GetPathToHighestScore();

        if (!File.Exists(path)) //Checks if the file exists
        {
            HighestScore highestScore = new HighestScore();
            string jsonObj = JsonUtility.ToJson(highestScore);
            //Debug.Log("SavedJsonFile: " + jsonObj);
            FileInfo defaultFile = new FileInfo(path);
            defaultFile.Directory.Create(); // If the directory already exists, this method does nothing.
            File.WriteAllText(path, jsonObj);
        }

        string file = File.ReadAllText(path);
        HighestScore highestScoreJson = JsonUtility.FromJson<HighestScore>(file);
        //Debug.Log(highestScoreJson.highestScore); //here you can check the value on the debug if you want
        return highestScoreJson;
    }

    /// <summary>
    /// Save a new highestScore
    /// </summary>
    /// <param name="highestScore"></param>
    public static void SaveHighestScore(int highestScoreValue) {
    string path = GetPathToHighestScore();
    HighestScore highestScore = new HighestScore();
    highestScore.highestScore = highestScoreValue;
    string jsonObj = JsonUtility.ToJson(highestScore);
    //Debug.Log("SavedJsonFile: " + jsonObj);
    FileInfo file = new FileInfo(path);
    file.Directory.Create(); // If the directory already exists, this method does nothing.
    File.WriteAllText(path, jsonObj);
    }
}

/// <summary>
/// This class holds the highestScore
/// </summary>
public class HighestScore
{
    public int highestScore;
    //You can also create a list if you want with the top 10 or something like that
}

Then you would use it like this

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

public class Score : MonoBehaviour
{
    public int score;
    public Text ScoreText;
    public int highscore;
    public Text highScoreText;


    void Start()
    {               
        highscore = HighestScoreJson.GetHighestScore().highestScore; 
    }

    void Update()
    {
        ScoreText.text = "Score: " + score;
        highScoreText.text = highscore.ToString();
        if (score > highscore)
        {
            highscore = score;
        HighestScoreJson.SaveHighestScore(highscore);
        }
    }
    void OnTriggerEnter(Collider other)
    {
        Debug.Log("collider is working");
        if (other.gameObject.tag == "Score: ")
        {
            score++;
        }
    }

}

You can probably optimize you calls if you take out the update and only call change your texts when the score changes.

void Update() //Don't need it
{
}
void OnTriggerEnter(Collider other)
{
    Debug.Log("collider is working");
    if (other.gameObject.tag == "Score: ")
    {
        score++;
        ScoreText.text = "Score: " + score;
        highScoreText.text = highscore.ToString();
        if (score > highscore)
        {
           highscore = score;
           HighestScoreJson.SaveHighestScore(highscore);
        }
    }
}