GoogleDriveTest.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityGoogleDrive; // https://github.com/Elringus/UnityGoogleDrive

public class GoogleDriveTest : MonoBehaviour
{

    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.AfterSceneLoad)]
    static void AfterSceneLoad()
    {
        var camera = FindObjectOfType<Camera>();
        if (camera != null)
            camera.clearFlags = CameraClearFlags.SolidColor;

        var type = typeof(GoogleDriveTest);
        var obj = new GameObject(type.Name, type);
        DontDestroyOnLoad(obj);
    }

    Coroutine Coroutine;

    string Error = string.Empty;
    readonly List<string> FileList = new List<string>(10);

    GUIContext m_GUIContext;

    void OnGUI()
    {
        var cs = m_GUIContext ?? (m_GUIContext = new GUIContext());

        GUILayout.BeginHorizontal();
        bool original = GUI.enabled;
        GUI.enabled = Coroutine == null;

        if (GUILayout.Button("List", cs.Button))
            Coroutine = StartCoroutine(List());

        GUI.enabled = original;
        GUILayout.EndHorizontal();

        if (string.IsNullOrEmpty(Error))
            FileList.ForEach(f => GUILayout.Label(f, cs.Label));
        else
            GUILayout.Label("Error: " + Error, cs.Error);
    }

    IEnumerator List()
    {
        var op = GoogleDriveFiles.List();
        yield return op.Send();

        if (op.IsError)
            Error = op.Error;
        else
        {
            Error = string.Empty;

            var list = op.ResponseData;

            FileList.Clear();
            FileList.AddRange(list.Files.ConvertAll(f => f.Name));
        }

        Coroutine = null;
    }

    sealed class GUIContext
    {
        public GUIStyle Button;
        public GUIStyle Label;
        public GUIStyle Error;

        public GUIContext()
        {
            Button = new GUIStyle(GUI.skin.button) { fontSize = Screen.height / 18 };
            Label = new GUIStyle(GUI.skin.label) { fontSize = Screen.height / 16 };
            Error = new GUIStyle(GUI.skin.label) { fontSize = Screen.height / 16 };
            Error.normal.textColor = Color.red;
        }
    }
}