AssetImporterEditor.Apply

Description

Saves any changes from the Editor's control into the asset's import settings object.

Can be overridden as required. Gets called when changes to settings are being applied.

using UnityEngine;
using UnityEditor;
using UnityEditor.AssetImporters;

public class ExampleScript : AssetImporterEditor
{
    protected override void Apply()
    {
        base.Apply();
        Debug.Log("The Importer has been applied.");
    }
}