How to add a sprite into spriteatlas with code?

Question

I have a sprite and a spriteatlas both with "Read/Write Enabled". I want to add this sprite into spriteatlas's "Objects for Packing" with code. And I found this U2D.SpriteAtlasExtensions.Add. But I don't know how to use it. Here is my code below, but it not works.

using UnityEngine;
using UnityEditor;
using UnityEngine.U2D;
using UnityEditor.U2D;

public class SpriteAtlasTool : EditorWindow 
{
    [MenuItem("Tools/SpriteAtlasTool")]
    static void DoIt () 
    {
        EditorWindow.GetWindow(typeof(SpriteAtlasTool));
        //EditorUtility.DisplayDialog("MyTool", "Do It in C# !", "OK", "");
    }

    private void OnGUI () 
    {
        if (true == GUILayout.Button("Test to Add")) 
        {
            Debug.Log("AddPic");
            SpriteAtlas temp = Resources.Load<SpriteAtlas>("Sprite Atlas/CharacterHeader/123");
            Sprite a = Resources.Load<Sprite>("TalkFile/ChoiceCenter/325709-130GH0214652");
            Debug.Log("before add  " + temp.spriteCount);
            SpriteAtlasExtensions.Add(temp, new Object[] { a});
            AssetDatabase.Refresh();
            Debug.Log("after add  " + temp.spriteCount);
        }
    }
}

And Console is

AddPic
before add 0
after add 0

Answer 1

After adding the new thing you would also need to save it back to the asset ... currently you only load the resources and make a change but until you save it it is only temporary afaik

private void OnGUI () 
{
    if (GUILayout.Button("Test to Add")) 
    {
        Debug.Log("AddPic");

        SpriteAtlas temp = Resources.Load<SpriteAtlas>("Sprite Atlas/CharacterHeader/123");
        Sprite a = Resources.Load<Sprite>("/TalkFile/ChoiceCenter/325709-130GH0214652");

        Debug.Log("before add  " + temp.spriteCount);

        SpriteAtlasExtensions.Add(temp, new Object[] { a});

        // save all changed assets (basically the same as CTRL+S)
        AssetDataBase.SaveAssets();

        AssetDatabase.Refresh();
        Debug.Log("after add  " + temp.spriteCount);
    }
}

See AssetDatabase.SaveAssets.

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Note: Typed on smartphone but I hope the idea gets clear