UVViewer

using UnityEngine;
using UnityEditor;

public class UVPreview : EditorWindow {

    private Mesh mesh;
    private Texture2D texture;
    private int selectedUV;

    private Rect rect;
    private Rect bounds;

    private float zoom = 1f;
    private Vector2 pan;

    private Event evt;
    private int triangle;

    private static Vector2 startSize = new Vector2(500, 560);

    private static Color edgeColor = new Color(0.3f, 0.8f, 1f, 1f);
    private static Color faceColor = new Color(0.3f, 0.8f, 1f, 0.25f);

    private static string[] uvLabels {
        get {
            return new string[] { "UV", "UV2", "UV3", "UV4" };
        }
    }

    private bool drawEdges {
        get {
            return EditorPrefs.GetBool("DrawEdges");
        }
        set {
            EditorPrefs.SetBool("DrawEdges", value);
        }
    }
    private bool drawFaces {
        get {
            return EditorPrefs.GetBool("DrawFaces");
        }
        set {
            EditorPrefs.SetBool("DrawFaces", value);
        }
    }

    [MenuItem("Window/UV Preview")]
    private static void Init() {
        var window = GetWindow<UVPreview>();
        window.titleContent = new GUIContent("UV Preview");
        window.wantsMouseMove = true;
        window.minSize = startSize;
        window.maxSize = startSize * 2;
        window.Show();
    }

    private void OnGUI() {
        EditorGUILayout.Separator();
        EditorGUILayout.BeginVertical("HelpBox");
        EditorGUILayout.LabelField("UV Preview", EditorStyles.boldLabel);
        EditorGUILayout.Separator();
        EditorGUILayout.BeginHorizontal();
        EditorGUILayout.BeginVertical(GUILayout.MaxWidth(200f));

        mesh = EditorGUILayout.ObjectField(mesh, typeof(Mesh), true) as Mesh;

        GUI.enabled = mesh;

        texture = EditorGUILayout.ObjectField(texture, typeof(Texture2D), true) as Texture2D;

        EditorGUILayout.EndVertical();
        EditorGUILayout.BeginVertical(GUILayout.MaxWidth(200f));
        selectedUV = GUILayout.Toolbar(selectedUV, uvLabels);

        if(GUILayout.Button("Set default position")) {
            zoom = 1f;
            pan = Vector2.zero;
        }

        EditorGUILayout.EndVertical();
        GUILayout.FlexibleSpace();
        EditorGUILayout.BeginVertical();

        drawEdges = EditorGUILayout.ToggleLeft("Edges", drawEdges, GUILayout.Width(50f));
        drawFaces = EditorGUILayout.ToggleLeft("Faces", drawFaces, GUILayout.Width(50f));

        EditorGUILayout.EndVertical();
        EditorGUILayout.EndHorizontal();
        EditorGUILayout.EndVertical();
        GUI.enabled = true;

        Vector2[] uv = new Vector2[0];
        if(mesh) {
            if(!mesh.isReadable) {
                EditorGUILayout.HelpBox("Mesh is not readable, you can change this on import settings", MessageType.Error);
                return;
            }
            switch(selectedUV) {
                case 1:
                uv = mesh.uv2;
                break;
                case 2:
                uv = mesh.uv3;
                break;
                case 3:
                uv = mesh.uv4;
                break;
                default:
                uv = mesh.uv;
                break;
            }
        }
        else {
            EditorGUILayout.HelpBox("No mesh selected", MessageType.Info);
            return;
        }

        if(uv.Length == 0) {
            EditorGUILayout.HelpBox(string.Format("Mesh doesn't have UV{0}", selectedUV + 1), MessageType.Warning);
            return;
        }

        EditorGUILayout.BeginVertical("HelpBox");
        bounds = EditorGUILayout.GetControlRect(GUILayout.ExpandHeight(true));
        PanAndZoom();

        if(texture)
            GUI.DrawTexture(bounds, texture);

        DrawUV(uv, mesh.triangles, rect);
        EditorGUILayout.EndVertical();
    }

    private void DrawUV(Vector2[] uv, int[] tris, Rect rect) {
        for(triangle = 0; triangle < tris.Length; triangle += 3) {
            var vertice0 = uv[tris[triangle + 0]];
            var vertice1 = uv[tris[triangle + 1]];
            var vertice2 = uv[tris[triangle + 2]];

            vertice0 = RelativeUVPosition(vertice0);
            vertice1 = RelativeUVPosition(vertice1);
            vertice2 = RelativeUVPosition(vertice2);

            Handles.color = edgeColor;
            if(drawEdges)
                Handles.DrawAAPolyLine(vertice0, vertice1, vertice2, vertice0);
            Handles.color = faceColor;
            if(drawFaces)
                Handles.DrawAAConvexPolygon(vertice0, vertice1, vertice2);
        }
    }

    private void PanAndZoom() {
        rect = bounds;
        evt = Event.current;

        switch(evt.type) {
            case EventType.ScrollWheel:
            zoom += -evt.delta.y * 0.05f;
            zoom = Mathf.Clamp(zoom, 0.3f, 2f);
            evt.Use();
            break;

            case EventType.MouseDrag:
            if(bounds.Contains(evt.mousePosition)) {
                EditorGUIUtility.AddCursorRect(bounds, MouseCursor.Pan);
                pan += evt.delta;
                pan.x = Mathf.Clamp(pan.x, -bounds.width / 2f, bounds.width / 2f);
                pan.y = Mathf.Clamp(pan.y, -bounds.height / 2f, bounds.height / 2f);
                evt.Use();
            }
            break;
        }

        rect.position += pan;
        var center = rect.center;
        rect.size *= zoom;
        rect.center = center;
    }

    private Vector3 RelativeUVPosition(Vector2 vertice) {
        vertice.x *= rect.width;
        vertice.y *= rect.height;
        vertice.y *= -1f;
        vertice.y += rect.height;

        vertice += rect.position;

        vertice.x = Mathf.Clamp(vertice.x, bounds.xMin, bounds.xMax);
        vertice.y = Mathf.Clamp(vertice.y, bounds.yMin, bounds.yMax);

        return vertice;
    }
}