Application.unloading

Description

Unity raises this event when the Player is unloading.

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

Note: The 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.