AnimationPreviewPlayer.cs

using System;
using System.Linq;
using System.Reflection;
using JetBrains.Annotations;
using UnityEditor;
using UnityEngine;
using Object = UnityEngine.Object;

namespace EditorScripts {

[InitializeOnLoad]
public static class AnimationPreviewPlayer {

    private const BindingFlags PRIVATE_FIELD_BINDING_FLAGS = BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField;
    private const BindingFlags PUBLIC_FIELD_BINDING_FLAGS = BindingFlags.Public | BindingFlags.Instance | BindingFlags.GetField;
    private const BindingFlags PUBLIC_PROPERTY_BINDING_FLAGS = BindingFlags.Public | BindingFlags.Instance | BindingFlags.GetProperty;
    
    private static Type animationClipEditorType;
    private static Type avatarPreviewType;
    private static Type timeControlType;

    private static Object selectedObject;
    private static bool shouldFindTimeControl;

    static AnimationPreviewPlayer () {
        animationClipEditorType = Type.GetType("UnityEditor.AnimationClipEditor,UnityEditor");
        avatarPreviewType = Type.GetType("UnityEditor.AvatarPreview,UnityEditor");
        timeControlType = Type.GetType("UnityEditor.TimeControl,UnityEditor");
    }

    private static void Update () {
        if (Selection.activeObject != selectedObject) {
            selectedObject = Selection.activeObject;

            if (selectedObject is AnimationClip) {
                shouldFindTimeControl = true;
            }
            else if (selectedObject is GameObject) {
                var assetPath = AssetDatabase.GetAssetPath(selectedObject);

                if (!string.IsNullOrWhiteSpace(assetPath)) {
                    foreach (var child in AssetDatabase.LoadAllAssetsAtPath(assetPath)) {
                        if (child is AnimationClip) {
                            shouldFindTimeControl = true;
                            break;
                        }
                    }
                }
            }

            return;
        }

        if (shouldFindTimeControl) {
            var animationClipEditor = Resources.FindObjectsOfTypeAll(animationClipEditorType).FirstOrDefault();
            if (animationClipEditor == null) return;

            var avatarPreview = animationClipEditorType.GetField("m_AvatarPreview", PRIVATE_FIELD_BINDING_FLAGS)?.GetValue(animationClipEditor);
            if (avatarPreview == null) return;

            var timeControl = avatarPreviewType.GetField("timeControl", PUBLIC_FIELD_BINDING_FLAGS)?.GetValue(avatarPreview);
            if (timeControl == null) return;
            
            shouldFindTimeControl = false;

            var playingProperty = timeControlType.GetProperty("playing", PUBLIC_PROPERTY_BINDING_FLAGS);
            if (playingProperty == null) return;
            
            playingProperty.SetValue(timeControl, true);
        }
    }

    [InitializeOnLoadMethod, UsedImplicitly]
    private static void SubscribeToUpdate () {
        EditorApplication.update += Update;
    }

}

}