TextureAtlasSlicer.cs

using UnityEditor;
using UnityEngine;

public class TextureAtlasSlicer : EditorWindow {
    [MenuItem("CONTEXT/TextureImporter/Slice Sprite Using XML")]
    public static void SliceUsingXML(MenuCommand command)
    {
        TextureImporter textureImporter = command.context as TextureImporter;

        TextureAtlasSlicer window = ScriptableObject.CreateInstance<TextureAtlasSlicer>();

        window.importer = textureImporter;

        window.ShowUtility();
    }

    [MenuItem("CONTEXT/TextureImporter/Slice Sprite Using XML", true)]
    public static bool ValidateSliceUsingXML(MenuCommand command)
    {
        TextureImporter textureImporter = command.context as TextureImporter;

        //valid only if the texture type is 'sprite' or 'advanced'.
        return textureImporter && textureImporter.textureType == TextureImporterType.Sprite ||
               textureImporter.textureType == TextureImporterType.Advanced;
    }

    public TextureImporter importer;

    public TextureAtlasSlicer()
    {
        title = "Texture Atlas Slicer";
    }
}