Application.unloading

Description

Unity raises this event when Player is unloading.

Add an event handler to this event if you want to react to Application.Unload calls. The event is raised after a call to Application.Unload is made, right before the unloading starts.

Note: Application.unloading event is only supported on iOS, Android, and Universal Windows Platform.

using UnityEngine;
using System.Collections;

// Unload Unity when the user clicks the button. Exit is not applied to the application.

public class ExampleClass : MonoBehaviour
{
    void OnGUI()
    {
        if (GUI.Button(new Rect(10, 10, 200, 75), "Unload"))
            Application.Unload();
    }

    static void OnUnload()
    {
        Debug.Log("Unloading the Player");
    }

    [RuntimeInitializeOnLoadMethod]
    static void RunOnStart()
    {
        Application.unloading += OnUnload;
    }
}

Additional resources: Application.Unload.