Show the `ToString()` of a MonoBehavior in the preview GUI. https://twitter.com/shanecelis/status/1123263828983201794

PreviewGUIEditor.cs

/* Original code[1] Copyright (c) 2019 Shane Celis[2]
   Licensed under the MIT License[3]

   This comment generated by code-cite[4].

   [1]: https://gist.github.com/shanecelis/b012b573139ae83d40d93f6b402ab492
   [2]: https://github.com/shanecelis
   [3]: https://opensource.org/licenses/MIT
   [4]: https://github.com/shanecelis/code-cite
*/
// #define USE_RICHSTRING
using UnityEngine;
using UnityEditor;

/** PreviewGuiEditor shows the `ToString()` of a MonoBehavior in the preview GUI
    as demonstrated in this tweet[1]. Will work with any component. Just add an
    attribute to this editor like so:

```
    [CustomEditor(typeof(YourType))]
```

    Will also work with any component that implements IRichString. Advise
    placing the interface into a `Scripts` folder rather than an `Editor`
    folder.

    Modeled after Unity's EventSystemEditor[2].

    [1]: https://twitter.com/shanecelis/status/1123263828983201794
    [2]: https://bitbucket.org/Unity-Technologies/ui/raw/0651862509331da4e85f519de88c99d0529493a5/UnityEditor.UI/EventSystem/EventSystemEditor.cs
*/
// [CustomEditor(typeof(YourType))]
[CustomEditor(typeof(Objective), true)]
public class PreviewGUIEditor : Editor {
  /** Update every 15th frame. */
  private const int updateOnFrame = 15;

  private GUIStyle _previewLabelStyle;
  protected GUIStyle previewLabelStyle {
    get {
      if (_previewLabelStyle == null) {
        _previewLabelStyle = new GUIStyle("PreOverlayLabel") {
          richText = false,
          alignment = TextAnchor.UpperLeft,
          fontStyle = FontStyle.Normal
        };
        // Try to get a fixed-width font on macOS.
        var font = Font.CreateDynamicFontFromOSFont("Monaco", 12);
        // Failing that, try to get a fixed-width font on Windows.
        if (font == null)
          font = Font.CreateDynamicFontFromOSFont("Lucida Console", 12);
        // XXX What fixed-width font should I request if we're on Linux?
        if (font != null)
          _previewLabelStyle.font = font;
        // Debug.Log("Fonts: \n" + string.Join("\n", Font.GetOSInstalledFontNames()));
      }
      return _previewLabelStyle;
    }
  }

  public override bool HasPreviewGUI() {
    return Application.isPlaying;
  }

  public override bool RequiresConstantRepaint() {
    // Only repaint on the nth frame.
    return Application.isPlaying && Time.frameCount % updateOnFrame == 0;
  }

  public override void OnPreviewGUI(Rect rect, GUIStyle background) {
    // Cast to more specialized type if necessary.
#if USE_RICHSTRING
    string str;
    var richString = target as IRichString;
    if (richString != null) {
      str = richString.ToRichString();
      previewLabelStyle.richText = true;
    } else if (target != null) {
      str = target.ToString();
      previewLabelStyle.richText = false;
    } else {
      // No string available to preview.
      return;
    }
#else
    string str = target.ToString();
#endif
    GUI.Label(rect, str, previewLabelStyle);
  }
}

#if USE_RICHSTRING
/** This interface provides a string that is marked up with Unity's rich string
    format[3], e.g., <b> for bold, <i> for italics, etc.

    May need to copy and paste this interface into `IRichString.cs` in a
    `Scripts` folder (not `Editor`).

    [3]: https://docs.unity3d.com/Manual/StyledText.html
*/
public interface IRichString {
  string ToRichString();
}
#endif