U3D提供了一套拓展编辑器的接口，可以用于直接在编辑器非播放模式运行程序。常用于运行一些工具程序，例如资源管理。在做技能编辑器等工具程序时，也可以使用运行模式接口会比较简单（这样也方便开放游戏创意工坊给玩家）。使用编辑器去做一些渲染相关的预览（如粒子系统，动画预览）会麻烦一点，有时候需要查询和反射使用U3D引擎未暴露的接口。
U3D编辑器相关官方文档查询链接:https://docs.unity3d.com/cn/current/Manual/GUIScriptingGuide.html

业务需求分析

这是常用的一些需求和接口，多数的用法比较简单的在这里简单介绍一下。

OnGUI

代码化在视口绘制UI，常用于绘制一些DebugUI，如官方示例：
OnGUI官方介绍

using UnityEngine;
using System.Collections;

public class GUITest : MonoBehaviour {
            
    void OnGUI ()
    {
        // 创建背景框
        GUI.Box(new Rect(10,10,100,90), "Loader Menu");
    
        // 创建第一个按钮。如果按下此按钮，则会执行 Application.Loadlevel (1)
        if(GUI.Button(new Rect(20,40,80,20), "Level 1"))
        {
            Application.LoadLevel(1);
        }
    
        // 创建第二个按钮。
        if(GUI.Button(new Rect(20,70,80,20),"Level 2")) 
        {
            Application.LoadLevel(2);
        }
    }
}

Editor和ExecuteInEditMode特性

ExecuteInEditMode特性：让脚本在编辑器模式运行。
Editor：封装一些UI绘制的接口，如（重新OnInspectorGUI以在Inspector窗口绘制UI界面）。

Editor和ExecuteInEditMode特性相关官方介绍

EditorWindow

继承EditorWindow重写以在编辑器创建绘制一个独立的编辑器窗口。

using UnityEditor;
using UnityEngine;

public class MyWindow : EditorWindow
{
    string myString = "Hello World";
    bool groupEnabled;
    bool myBool = true;
    float myFloat = 1.23f;
    
    // 将名为"My Window"的菜单项添加到 Window 菜单
    [MenuItem("Window/My Window")]
    public static void ShowWindow()
    {
        //显示现有窗口实例。如果没有，请创建一个。
        EditorWindow.GetWindow(typeof(MyWindow));
    }
    
    void OnGUI()
    {
        GUILayout.Label ("Base Settings", EditorStyles.boldLabel);
        myString = EditorGUILayout.TextField ("Text Field", myString);
        
        groupEnabled = EditorGUILayout.BeginToggleGroup ("Optional Settings", groupEnabled);
            myBool = EditorGUILayout.Toggle ("Toggle", myBool);
            myFloat = EditorGUILayout.Slider ("Slider", myFloat, -3, 3);
        EditorGUILayout.EndToggleGroup ();
    }
}

EditorWindow官方示例

Serializable属性绘制器

Serializable U3D官方翻译为属性绘制器，字面意思是可被序列化的。大概的效果是被Serializable特性标记可以被U3D序列化并在U3D进行绘制和提供更方便的操作，如下：

using System;
using UnityEngine;

enum IngredientUnit { Spoon, Cup, Bowl, Piece }

// 自定义 Serializable 类
[Serializable]
public class Ingredient
{
    public string name;
    public int amount = 1;
    public IngredientUnit unit;
}

public class Recipe : MonoBehaviour
{
    public Ingredient potionResult;
    public Ingredient[] potionIngredients;
}

属性绘制器官方介绍

外观风格/皮肤

GUIStyle和GUISkin提供了接口来定制化UI绘制的外观风格，区别在于GUIStyle常用于某个控件的风格绘制，而GUISkin则会应用于其下文中所有控件的绘制，具体使用细节见官方介绍
GUISkin官方介绍
 GUIStyle官方介绍

PreviewRenderUtility 预览窗口渲染工具

PreviewRenderUtility 个人认为它是一个预览窗口的渲染工具类，官方对其介绍几乎没有。但是在EditorWindow、Inspector窗口预览模型、动画、粒子特效会用到。例如这个特效的预览下文会具体介绍：

常用控件速查表

名称	代码	示例
Label	GUI.Label (new Rect (25, 25, 100, 30), "Label");
Button	if (GUI.Button (new Rect (25, 25, 100, 30), "Button"))
TextField 单行输入框	str = GUI.TextField (new Rect (25, 25, 100, 30), str);
TextArea 多行输入框	str = GUI.TextArea (new Rect (25, 25, 100, 30), str);
Toggle 勾选框	b = GUI.Toggle (new Rect (25, 25, 100, 30), toggleBool, "Toggle");
HorizontalSlider 水平滑块	h = GUI.HorizontalSlider (new Rect (25, 25, 100, 30), hSliderValue, 0.0f, 10.0f);
Toolbar 页签	Idx = GUI.Toolbar (new Rect (25, 25, 250, 30), Idx, BtnNames);
SelectionGrid 平铺列表	Idx = GUI.SelectionGrid (new Rect (25, 25, 300, 60), Idx, Names, Cow);
ScrollView 滚动列表	pos = GUI.BeginScrollView (rect, scrollViewVector, rectContent); //XXX GUI.EndScrollView();
Window 小窗口	w = GUI.Window (0, wRect, drawDele, "win");

注意手动排版使用GUI.XXX，自动排版使用GUILayout.XXX

布局速查表

布局的一般用法是，例如组Group

GUI.BeginGroup
//控件A
//控件B ...
GUI.EndGroup
或
using (new GroupScope)
{
//控件A
//控件B ...
}

布局	用法	预览
组 Group	固定相对位置
区域 Area	用于自动布局一组控件，与组类似	-
水平布局 Horizontal	水平布局一组控件
垂直布局 Vertical	垂直布局一组控件

GUILayoutOption可以某些重写自动布局参数，例如：
GUILayout.Button ("My width has been overridden", GUILayout.Width (95));
GUILayout.Width重写了自动布局这个按钮的宽

特效预览窗口

U3D自带的特效预览方式是拖到Scene窗口上，有的时候做编辑器（例如技能编辑器）时需要预览的特效，切来切去太麻烦了。而且U3D拖到Scene才能预览这种方式也很麻烦，也可以自己拓展选中特效在Hir上预览。

使用反射调用库函数绑定特效
使用prevRU渲染GameObj

LibUtil 调用库函数

public class LibUtil
{
    public static Type ParticleSystemEditorUtils
    {
        get
        {
            var assembly = Assembly.GetAssembly(typeof(Editor));
            return assembly.GetType("UnityEditor.ParticleSystemEditorUtils");
        }
    }

    public static ParticleSystem lockedParticleSystem
    {
        get
        {
            var info = ParticleSystemEditorUtils.GetProperty("lockedParticleSystem", BindingFlags.Static | BindingFlags.NonPublic);
            return (ParticleSystem)info.GetValue(null, null);
        }
        set
        {
            var info = ParticleSystemEditorUtils.GetProperty("lockedParticleSystem", BindingFlags.Static | BindingFlags.NonPublic);
            info.SetValue(null, value, null);
        }
    }

    public static bool editorIsScrubbing
    {
        set
        {
            var info = ParticleSystemEditorUtils.GetProperty("playbackIsScrubbing", BindingFlags.Static | BindingFlags.NonPublic);
            info.SetValue(null, value, null);
        }
    }

    public static float editorPlaybackTime
    {
        get
        {
            var info = ParticleSystemEditorUtils.GetProperty("playbackTime", BindingFlags.Static | BindingFlags.NonPublic);
            return (float)info.GetValue(null, null);
        }
        set
        {
            var info = ParticleSystemEditorUtils.GetProperty("playbackTime", BindingFlags.Static | BindingFlags.NonPublic);
            info.SetValue(null, value, null);
        }
    }

    public static void StopEffect()
    {
        var assembly = Assembly.GetAssembly(typeof(Editor));
        var util = assembly.GetType("UnityEditor.ParticleSystemEffectUtils");
        var info = util.GetMethod("StopEffect", BindingFlags.Static | BindingFlags.NonPublic, null, new Type[] { }, new ParameterModifier[] { });
        info.Invoke(null, null);
    }
}

拓展ObjectPreview

public class MyView : ObjectPreview
{
    PreviewRenderUtility prevRU;
    GameObject ins;

    Mesh fm;
    Texture2D ft;
    Material fma;

    static int prevCulLay = 31;

    bool isRunning = false;

    Editor e;

    ParticleSystem ps
    {
        get
        {
            var p = ins.GetComponent<ParticleSystem>();
            return p;
        }
    }

    public void BindEditor(Editor editor)
    {
        e = editor;
    }

    public void Repaint()
    {
        if (e != null)
            e.Repaint();
    }

    void DrawPlayBtn()
    {
        if (GUILayout.Button("Play"))
            Play();
        if (GUILayout.Button("Stop"))
            Stop();
    }

    public override void OnPreviewSettings()
    {
        using (new EditorGUILayout.HorizontalScope())
        {
            DrawPlayBtn();
        }
    }

    public void Play()
    {
        var p = ps;
        Stop();
        if (p != null && isRunning == false)
        {
            isRunning = true;
            LibUtil.lockedParticleSystem = p;
            p.Play();
            LibUtil.editorIsScrubbing = false;
            EditorApplication.update += Update;
        }
    }

    public void Stop()
    {
        var p = ps;
        if (p != null && isRunning == true)
        {
            LibUtil.editorIsScrubbing = false;
            LibUtil.editorPlaybackTime = 0f;
            LibUtil.StopEffect();
            isRunning = false;
            p.Stop();
            EditorApplication.update -= Update;
        }
    }

	//PreviewRenderUtility渲染
    public void Render()
    {
        var flag = StartLight();

        var viewDir = new Vector2(120f, -20f);
        var cam = prevRU.camera;

        var zoomFactor = 1.0f;
        var avatarScale = 1.0f;

        cam.nearClipPlane = 0.5f * zoomFactor;
        cam.farClipPlane = 100f * avatarScale;
        Quaternion rot = Quaternion.Euler(-viewDir.y, -viewDir.x, 0f);
        var camPos = rot * (Vector3.forward * -5.5f * zoomFactor); // + bodyPos + pivotPosOff;
        cam.transform.position = camPos;
        cam.transform.rotation = rot;

        var refIns = ins;

        Matrix4x4 mat = Matrix4x4.TRS(refIns.transform.position, Quaternion.identity, Vector3.one * 5f * avatarScale);
        var refPos = refIns.transform.position;
        fma.mainTextureOffset = -new Vector2(refPos.x, refPos.z) * 5f * 0.08f * (1f / avatarScale);
        fma.SetVector("_Alphas", new Vector4(0.5f * 1f, 0.3f * 1f, 0f, 0f));
        Graphics.DrawMesh(fm, mat, fma, prevCulLay, cam, 0);

        cam.Render();
        EndLighting(flag);
    }

    public override void OnPreviewGUI(Rect r, GUIStyle background)
    {
        if (prevRU == null)
        {
            CreatePrevRU();
        }

        prevRU.BeginPreview(r, background);
        Render();
        prevRU.EndAndDrawPreview(r);
    }

    public void Update()
    {
        if (!isRunning)
            return;

        if (ps != null)
        {
            Repaint();
        }
    }

    public override void Initialize(UnityEngine.Object[] targets)
    {
        base.Initialize(targets);
    }

    public void CreatePrevRU()
    {
        if (prevRU != null)
            return;

        prevRU = new PreviewRenderUtility(true);
        prevRU.cameraFieldOfView = 30.0f;

        var cam = prevRU.camera;
        cam.cullingMask = 1 << prevCulLay;
        cam.allowHDR = false;
        cam.allowMSAA = false;

        CreateIns();
        CreateFloor();
    }

    void CreateIns()
    {
        DestoryIns();
        ins = GameObject.Instantiate(target) as GameObject;
        SetUpInsArr(ins);
        prevRU.AddSingleGO(ins);
    }

    void CreateFloor()
    {
        fm = Resources.GetBuiltinResource<Mesh>("New-Plane.fbx");
        ft = (Texture2D)EditorGUIUtility.Load("Avatar/Textures/AvatarFloor.png");
        var s = EditorGUIUtility.LoadRequired("Previews/PreviewPlaneWithShadow.shader") as Shader;
        fma = new Material(s);
        fma.mainTexture = ft;
        fma.mainTextureScale = Vector2.one * 20f;
        fma.SetVector("_Alphas", new Vector4(0.5f, 0.3f, 0f, 0f));
        fma.hideFlags = HideFlags.HideAndDontSave;
    }

    bool StartLight()
    {
        Light[] lights = prevRU.lights;

        lights[0].intensity = 1.4f;
        lights[0].transform.rotation = Quaternion.Euler(40f, 40f, 0f);
        lights[1].intensity = 1.4f;
        Color ambient = new Color(0.1f, 0.1f, 0.1f, 0f);
        InternalEditorUtility.SetCustomLighting(lights, ambient);
        bool fog = RenderSettings.fog;
        Unsupported.SetRenderSettingsUseFogNoDirty(false);
        return fog;
    }

    void EndLighting(bool old)
    {
        Unsupported.SetRenderSettingsUseFogNoDirty(old);
        InternalEditorUtility.RemoveCustomLighting();
    }

    public void SetUpInsArr(GameObject go)
    {
        go.hideFlags = HideFlags.HideAndDontSave;
        go.layer = prevCulLay;

        foreach (Transform t in go.transform)
            SetUpInsArr(t.gameObject);
    }

    public void DestoryIns()
    {
        if (ins == null)
            return;

        GameObject.DestroyImmediate(ins);
        ins = null;
    }

    public override void Cleanup()
    {
        DestoryIns();
        if (prevRU != null)
        {
            prevRU.Cleanup();
            prevRU = null;
        }
        base.Cleanup();
    }
}

public class MyEditor : Editor
{
    MyView p;

    MyView preview
    {
        get
        {
            if (p == null)
            {
                p = new MyView();
                p.Initialize(targets);
                p.BindEditor(this);
            }
            return p;
        }
    }

    public override bool HasPreviewGUI()
    {
        return preview.HasPreviewGUI();
    }

    public override void OnPreviewSettings()
    {
        preview.OnPreviewSettings();
    }

    public override void OnPreviewGUI(Rect r, GUIStyle background)
    {
        preview.OnPreviewGUI(r, background);
    }

    public void Clearup()
    {
        p.Cleanup();
        p = null;
    }
}

EditorWindow拓展

public class MyWnd : EditorWindow
{
    [MenuItem("编辑器/MyWnd")]
    static public void PopUp()
    {
        var w = EditorWindow.GetWindow<MyWnd>("MyWnd");
        w.minSize = new Vector2(800, 600);
        w.Show();
    }

    MyEditor e;
    GameObject go;
    GameObject pf;

    private void OnGUI()
    {
        EditorGUI.BeginChangeCheck();
        pf = (GameObject)EditorGUILayout.ObjectField(pf, typeof(GameObject), false);
        if (EditorGUI.EndChangeCheck())
        {
            if (e != null)
            {
                e.Cleanup();
                e = null;
            }

            e = (MyEditor)Editor.CreateEditor(pf, typeof(MyEditor));
        }

        if (e)
        {
            e.OnPreviewSettings();
            e.OnPreviewGUI(GUILayoutUtility.GetRect(400, 400), EditorStyles.whiteLabel);
            Repaint();
        }
    }
}

动画预览相对会简单一点，U3D有一个AnimationClipEditor来预览动画，只是封在了库里没有暴露出来。AnimationClipEditor需要一个AnimClip文件和一个Avatar，比较麻烦。若是Avatar的GameObj上挂载Anim组件可以获取到AnimClip，可以优化下直接拖入Avatar预览动画。

使用AnimationClipEditor预览动画，使用反射调用库函数优化直接拖入Avatar预览动画
AnimationClipEditor类名需要从Debug工具中查看，比如VS断点。然后到U3D的官方C#库函数反射文件中查看
跳转链接：U3D官方C#反射文件

反射工具类

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;
using System.Reflection;

public class CSRefUtil
{
    static public void SetValuePublic(object obj, string name, params object[] param)
    {
        var filed = obj.GetType().GetField(name, BindingFlags.Instance | BindingFlags.Public);
        filed.SetValue(obj, param);
    }

    static public void SetValuePrivate(object obj, string name, params object[] param)
    {
        var filed = obj.GetType().GetField(name, BindingFlags.Instance | BindingFlags.NonPublic);
        filed.SetValue(obj, param);
    }

    static public T GetValuePublic<T>(object obj, string name, params object[] param)
    {
        return (T)obj.GetType().GetField(name, BindingFlags.Instance | BindingFlags.Public).GetValue(obj);
    }

    static public T GetValuePrivate<T>(object obj, string name, params object[] param)
    {
        return (T)obj.GetType().GetField(name, BindingFlags.Instance | BindingFlags.NonPublic).GetValue(obj);
    }

    static public void SetPropertyPublic(object obj, string name, params object[] param)
    {
        var property = obj.GetType().GetProperty(name, BindingFlags.Instance | BindingFlags.Public);
        property.SetMethod.Invoke(obj, param);
    }

    static public void SetPropertyPrivate(object obj, string name, params object[] param)
    {
        var property = obj.GetType().GetProperty(name, BindingFlags.Instance | BindingFlags.NonPublic);
        property.SetMethod.Invoke(obj, param);
    }

    static public T GetPropertyPublic<T>(object obj, string name)
    {
        var property = obj.GetType().GetProperty(name, BindingFlags.Instance | BindingFlags.Public);
        return (T)property.GetMethod.Invoke(obj, new object[] { });
    }

    static public T GetPropertyPrivate<T>(object obj, string name, params object[] param)
    {
        var property = obj.GetType().GetProperty(name, BindingFlags.Instance | BindingFlags.NonPublic);
        return (T)property.GetMethod.Invoke(obj, new object[] { });
    }

    static public T CallMethodPublic<T>(object obj, string name, params object[] param)
    {
        var method = obj.GetType().GetMethod(name, BindingFlags.Instance | BindingFlags.Public);
        return (T)method.Invoke(obj, param);
    }

    static public T CallMethodPrivate<T>(object obj, string name, params object[] param)
    {
        var method = obj.GetType().GetMethod(name, BindingFlags.Instance | BindingFlags.NonPublic);
        return (T)method.Invoke(obj, param);
    }
}

编辑器窗口拓展

public class MyEditor : EditorWindow
{
    Editor previewAnimWnd;
    AnimationClip previewAnim;
    bool isPreviewAnimDirty = false;
    object avatarWnd;
    Vector2 AnimViewSize = new Vector2(800, 600);
    GameObject animGo;

    [MenuItem("编辑器/MyEditor")]
    public static void PopUp()
    {
        var win = GetWindow<MyEditor>();
        win.minSize = new Vector2(800, 800);
        win.Show();
    }

    void OnGUI()
    {
        DrawPreviewAnim();
    }
	// 清理资源
    void OnDisable()
    {
        animGo = null;
        previewAnim = null;

        if (previewAnimWnd != null)
            previewAnimWnd = null;

        if (avatarWnd != null)
            avatarWnd = null;
    }

    void SetPreviewAnim(AnimationClip anim)
    {
        previewAnim = anim;
        isPreviewAnimDirty = true;
    }
    

    public void DrawPreviewAnim()
    {
        EditorGUI.BeginChangeCheck();
        animGo = (GameObject)EditorGUILayout.ObjectField(animGo, typeof(GameObject), false);
        if (EditorGUI.EndChangeCheck())
        {
            if (animGo != null)
            {
                var animator = animGo.GetComponent<Animator>();
                var anim = animator.runtimeAnimatorController.animationClips[0];
                SetPreviewAnim(anim);
            }
        }

        if (isPreviewAnimDirty)
        {
            isPreviewAnimDirty = false;
            if (previewAnim != null)
            {
                previewAnimWnd = Editor.CreateEditor(previewAnim);
                previewAnimWnd.OnInspectorGUI();
                avatarWnd = CSRefUtil.GetValuePrivate<object>(previewAnimWnd, "m_AvatarPreview");
                CSRefUtil.CallMethodPrivate<object>(avatarWnd, "SetPreview", animGo);
            }
        }

        EditorGUILayout.BeginVertical(GUILayout.Width(AnimViewSize.x), GUILayout.Height(AnimViewSize.y));
		
		//绘制
        if (previewAnimWnd != null)
        {
            using (new EditorGUILayout.HorizontalScope())
            {
                GUILayout.FlexibleSpace();
                previewAnimWnd.OnPreviewSettings();
            }
            AnimationMode.StartAnimationMode();
            previewAnimWnd.OnInteractivePreviewGUI(GUILayoutUtility.GetRect(AnimViewSize.x, AnimViewSize.y), EditorStyles.whiteLabel);
            AnimationMode.StopAnimationMode();
        }
        EditorGUILayout.EndVertical();
    }
}

U3D对编辑器的很多功能都没有介绍清楚，碰到问题建议Google搜索或去讨论群去问下。
还有布局缩进等不是很常见的功能，建议用到的时候去Google或者浏览官方文档。
U3D有对编辑器UI树形控件支持
编辑器拓展开发量大可以考虑使用Unity编辑器扩展Odin插件，不过是收费的
新手在开发布局比较复杂的编辑器可以考虑参考下老司机的代码，给出两个供参考：

U3D编辑器开发&粒子特效/动画预览器示例 - 寡人正在Coding

业务需求分析

OnGUI

Editor和ExecuteInEditMode特性

EditorWindow

Serializable属性绘制器

外观风格/皮肤

PreviewRenderUtility 预览窗口渲染工具

常用控件速查表

布局速查表

特效预览窗口

LibUtil 调用库函数

拓展ObjectPreview

EditorWindow拓展

反射工具类

编辑器窗口拓展

Recommend

一起炫起来 -- 3D粒子动画篇

Three.js粒子特效，shader渲染初探

【译】带有Flutter的粒子动画

JavaScript动画实例：粒子文本

UGUI研究院之UI粒子特效自适应缩放（二十二）

强！腾讯开源的kotlin高性能特效动画组件！

HTML动画特效(第一部)

玩转Qml(14)-动画特效-梯度

玩转Qml(13)-动画特效-飞入

PS超炫酷粒子特效插件-Animation Particle Toolbox

About Joyk