【Unity】アセットのmetaファイルの中身を見れるボタンを用意する

1_ProjectView.cs

using System.IO;
using UnityEditor;
using UnityEngine;

/// <summary>
/// ProjectView拡張
/// ボタンを追加する
/// </summary>
public class ProjectView
{
    private static Vector2 cacheSize;
    private static GUIContent cacheContent;
    private static GUIStyle cacheStyle;

    [InitializeOnLoadMethod]
    private static void ProjectWindow()
    {
        EditorApplication.projectWindowItemOnGUI += ProjectWindowGUI;
    }

    private static void ProjectWindowGUI(string guid, Rect rect)
    {
        if (string.IsNullOrEmpty(guid)) {
            return;
        }

        var asset = AssetDatabase.GUIDToAssetPath(guid);
        if (IsFolder(asset)) {
            return;
        }

        if (cacheStyle == null) {
            cacheStyle = (GUIStyle)"Box";
            cacheContent = EditorGUIUtility.IconContent("console.infoicon.inactive.sml");
            cacheSize = cacheStyle.CalcSize(cacheContent);
        }

        rect.xMin = rect.xMax - cacheSize.x;

        // ボタンを押したらmetaファイルの詳細を開く
        if (GUI.Button(rect, cacheContent, EditorStyles.iconButton)) {
            PopupWindow.Show(rect, new AssetMetaInfoPopupWindow(asset));
        }
    }

    /// <summary>
    /// フォルダかどうか判定
    /// </summary>
    private static bool IsFolder(string path)
    {
        if (string.IsNullOrEmpty(path)) {
            return false;
        }

        return File.GetAttributes(path).HasFlag(FileAttributes.Directory);
    }
}

2_AssetMetaInfoPopupWindow.cs

using UnityEngine;
using UnityEditor;
using System.IO;

/// <summary>
/// metaファイル詳細表示のポップアップウィンドウ
/// </summary>
public class AssetMetaInfoPopupWindow : PopupWindowContent
{
    string _assetPath;
    string _metaText;

    public AssetMetaInfoPopupWindow(string assetPath) : base()
    {
        _assetPath = assetPath;

        using (var sr = new StreamReader(assetPath + ".meta")) {
            _metaText = sr.ReadToEnd();
        }
    }

    public override Vector2 GetWindowSize()
    {
        return new Vector2(300, 200);
    }

    public override void OnGUI(Rect rect)
    {
        if (GUILayout.Button("CopyGUID")) {
            GUIUtility.systemCopyBuffer = AssetDatabase.AssetPathToGUID(_assetPath);
        }
        rect.xMin += 2;
        rect.yMin += EditorGUIUtility.singleLineHeight;

        // 一応コピペできるようにSelectableLabelを使う
        EditorGUI.SelectableLabel(rect, _metaText);
    }
}