To implement the Auto Update SDK, include the following callback methods:
MethodDescription
OnInitializationSuccessTriggered when SDK initialization is successful.
OnUpdateStatusTriggered when a new version of the app is available, and immediately after invoking the checkForUpdate() method.
OnDownloadStartedTriggered when permission to install has been granted by the player, and the download process begins. The SDK will attempt to download the new app version.
OnProgressTriggered when the download is initiated and when every new byte or packet is downloaded thereafter. You may use it to show download progression in real-time.
OnDownloadEndedTriggered when the download process is complete.
OnInstallationReadyTriggered when the SDK is ready to start the installation process.
OnErrorTriggered when an error occurs during the update process, including initialization failures. Follow the instructions in the Unity Auto Update SDK troubleshooting article and try to re-initialize again based on the error.

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Example code

Below is the example code for the implementation: 
using Appcharge.Updater;
using Appcharge.Updater.Interfaces;
using Appcharge.Updater.Models;
using UnityEngine;
using UnityEngine.UI;

public class Sample : MonoBehaviour, IUpdateCallback
{
    [SerializeField] private Text _logger;
    [SerializeField] private Button _btnCheckForUpdate;
    [SerializeField] private Button _btnInstallUpdate;

    private UpdateStatusModel _status;

    private void Start()
    {
        UpdateController.Instance.Init("sandbox", "checkoutPublicKey", "customerId", this);
    }

    #region Interface Callback Methods

    public void OnInitializationSuccess()
    {
        _logger.text = "Initialization successful.";
        _btnCheckForUpdate.gameObject.SetActive(true);
    }

    public void OnUpdateStatus(UpdateStatusModel status)
    {
        _status = status;
        _logger.text = $"A new version of your app is available: {status.Message}";
        _btnInstallUpdate.gameObject.SetActive(true);
    }

    public void OnDownloadStarted()
    {
        _logger.text = "Downloading the new app version.";
    }

    public void OnProgress(int progress)
    {
        _logger.text = $"Progress: {progress}%";
    }

    public void OnDownloadEnded()
    {
        _logger.text = "The new app version has been downloaded.";
    }

    public void OnInstallationReady()
    {
        _logger.text = "The new app version has been downloaded and is ready to install.";
    }

    public void OnError(int errorCode)
    {
        _logger.text = $"Error: {errorCode}";
    }

    #endregion

    public void CheckForUpdates()
    {
        UpdateController.Instance.CheckForUpdates();
    }

    public void InstallUpdate()
    {
        if (_status == null)
        {
            _logger.text = "No status";
            return;
        }

        if (_status.IsUpdateAvailable)
        {
            UpdateController.Instance.InstallUpdate();
        }
    }
}