UnityUIMarginsViewer.cs

using UnityEditor;
using UnityEngine;
using Object = UnityEngine.Object;


public class UIHelperGizmoDrawer
{
#if UNITY_USE_UI_MARGINS_VIEWER
    [DrawGizmo(GizmoType.Selected)]
    static void DrawGizmoForMyScript(RectTransform rect, GizmoType gizmoType)
    {
        UIMarginsViewer.DrawGizmo();
    }
#endif
}

/**
* Showing margins between two selected ui elements
* Showing sizes of selected ui elements
*/
class UIMarginsViewer
{
    static readonly int TextSize = 14;
    static readonly Color TextColor = Color.white;
    static readonly Color LinesColor = Color.red;

    static Object _firstSelected;
    static Object _secondSelected;

    public static void DrawGizmo()
    {
        OnDrawGizmos();
    }

    static void OnDrawGizmos()
    {
        _firstSelected = null;
        _secondSelected = null;

        for (int index = 0; index < Selection.objects.Length; index++) {
            var obj = Selection.objects[index];
            DrawRectTransformsAndSizes(obj);

            if (index == 0) {
                _firstSelected = obj;
            }

            if (index == 1) {
                _secondSelected = obj;
            }

            if (index > 1) {
                //not support selection more than 2
                continue;
            }
        }

        DrawMutualPositions(_firstSelected, _secondSelected);
    }

    static void DrawMutualPositions(Object firstSelected, Object secondSelected)
    {
        var t1 = (firstSelected as GameObject)?.transform;
        if (t1 == null) {
            return;
        }

        var t2 = (secondSelected as GameObject)?.transform;
        if (t2 == null) {
            return;
        }

        var rect1 = t1.GetComponent<RectTransform>();
        var rect2 = t2.GetComponent<RectTransform>();

        if (rect1 == null && rect2 == null) {
            return;
        }

        var offsets = CalculateOffsets(rect1, rect2);

        DrawOffsets(rect1, rect2, offsets);
    }

    static void DrawOffsets(RectTransform rect1, RectTransform rect2, Vector4 offsets)
    {
        DrawXOffsets(rect1, rect2, offsets);
        DrawYOffsets(rect1, rect2, offsets);
    }

    static void DrawYOffsets(RectTransform rect1, RectTransform rect2, Vector4 offsets)
    {
        //top
        var topCenter = new Vector2(rect1.rect.min.x + rect1.position.x + rect1.rect.width / 2,
            rect1.rect.max.y + rect1.position.y);


        //down
        var downCenter = new Vector2(rect1.rect.min.x + rect1.position.x + rect1.rect.width / 2,
            rect1.rect.min.y + rect1.position.y);

        bool oneSign = (offsets.y * offsets.w) > 0;
        if (oneSign) {
            if (offsets.y < 0) {
                var start = downCenter;
                var end = rect2.rect.yMax + rect2.position.y;
                var value = end - downCenter.y;
                DrawYOffsetLineWithText(start, value);
            } else {
                var start = topCenter;
                var end = rect2.rect.yMin + rect2.position.y;
                var value = end - topCenter.y;
                DrawYOffsetLineWithText(start, value);
            }
        } else {
            DrawYOffsetLineWithText(topCenter, offsets.y);
            DrawYOffsetLineWithText(downCenter, offsets.w);
        }
    }

    static void DrawXOffsets(RectTransform rect1, RectTransform rect2, Vector4 offsets)
    {
        //left
        var leftCenter = new Vector2(rect1.rect.min.x + rect1.position.x,
            rect1.rect.min.y + rect1.position.y + rect1.rect.height / 2);


        //right
        var rightCenter = new Vector2(rect1.rect.max.x + rect1.position.x,
            rect1.rect.min.y + rect1.position.y + rect1.rect.height / 2);


        bool oneSign = (offsets.x * offsets.z) > 0;
        if (oneSign) {
            if (offsets.x < 0) {
                var start = leftCenter;
                var end = rect2.rect.xMax + rect2.position.x;
                var value = end - leftCenter.x;
                DrawXOffsetLineWithText(start, value);
            } else {
                var start = rightCenter;
                var end = rect2.rect.xMin + rect2.position.x;
                var value = end - rightCenter.x;
                DrawXOffsetLineWithText(start, value);
            }
        } else {
            DrawXOffsetLineWithText(leftCenter, offsets.x);
            DrawXOffsetLineWithText(rightCenter, offsets.z);
        }
    }

    static void DrawXOffsetLineWithText(Vector2 start, float value)
    {
        var absValue = Mathf.Abs(value);
        if (Mathf.Approximately(absValue, 0)) {
            return;
        }

        var end = new Vector2(start.x + value, start.y);

        var center = new Vector2(start.x + value / 2, start.y);
        ;
        var textHeight = TextSize;
        var sizeTextContent = $"{absValue}";

        var textCenter = new Vector2(center.x, center.y + textHeight * 2f);

        DrawLine(start, end, 5, LinesColor);

        //draw text back
        var textLenght = sizeTextContent.Length;
        var textStart = new Vector3(textCenter.x - textLenght * textHeight * 1.5f, textCenter.y, 0);
        var textEnd = new Vector3(textCenter.x + textLenght * textHeight * 1.5f, textCenter.y, 0);
        DrawLine(textStart, textEnd, textHeight * 2, LinesColor);

        //draw text
        DrawText(UnityEngine.GUI.skin, sizeTextContent, textCenter, TextColor, textHeight);
    }

    static void DrawYOffsetLineWithText(Vector2 start, float value)
    {
        var absValue = Mathf.Abs(value);
        if (Mathf.Approximately(absValue, 0)) {
            return;
        }

        var end = new Vector2(start.x, start.y + value);

        var center = new Vector2(start.x, start.y + value / 2);
        var textHeight = TextSize;
        var sizeTextContent = $"{absValue}";

        var textCenter = new Vector2(center.x, center.y);

        DrawLine(start, end, 5, LinesColor);

        //draw text back
        var textLenght = sizeTextContent.Length;
        var textStart = new Vector3(textCenter.x - textLenght * textHeight * 1.5f, textCenter.y, 0);
        var textEnd = new Vector3(textCenter.x + textLenght * textHeight * 1.5f, textCenter.y, 0);
        DrawLine(textStart, textEnd, textHeight * 2, LinesColor);

        //draw text
        DrawText(UnityEngine.GUI.skin, sizeTextContent, textCenter, TextColor, textHeight);
    }

    static void DrawRectTransformsAndSizes(Object obj)
    {
        var transform = (obj as GameObject)?.transform;
        if (transform == null) {
            return;
        }

        var rect = transform.GetComponent<RectTransform>();
        if (rect == null) {
            return;
        }

        DrawRect(rect);
        var size = CalculateSize(rect);
        DrawSizeText(rect, size);
    }

    static void DrawRect(RectTransform rect)
    {
        Vector3[] points = new Vector3[]
        {
            new Vector3(rect.rect.min.x + rect.position.x,
                rect.rect.min.y + rect.position.y),
            new Vector3(rect.rect.min.x + rect.position.x,
                rect.rect.min.y + rect.position.y + rect.rect.height),
            new Vector3(rect.rect.min.x + rect.position.x,
                rect.rect.min.y + rect.position.y + rect.rect.height),
            new Vector3(rect.rect.max.x + rect.position.x,
                rect.rect.max.y + rect.position.y),
            new Vector3(rect.rect.max.x + rect.position.x,
                rect.rect.max.y + rect.position.y),
            new Vector3(rect.rect.max.x + rect.position.x,
                rect.rect.max.y + rect.position.y - rect.rect.height),
            new Vector3(rect.rect.max.x + rect.position.x,
                rect.rect.max.y + rect.position.y - rect.rect.height),
            new Vector3(rect.rect.min.x + rect.position.x,
                rect.rect.min.y + rect.position.y),
        };

        GizmosUtils.DrawLines(points, 10, LinesColor);
    }

    static void DrawSizeText(RectTransform rect, Vector2 size)
    {
        var rectCenter = new Vector2(rect.rect.center.x + rect.position.x, rect.rect.center.y + rect.position.y);
        var textHeight = TextSize;
        var sizeTextContent = $"{size.x}x{size.y}";

        var offset = 1.5f * textHeight;
        var textPosition = new Vector2(rectCenter.x, rectCenter.y - rect.rect.height / 2 - offset);

        //draw text back
        var textLenght = sizeTextContent.Length;
        var textStart = new Vector3(textPosition.x - textLenght * textHeight * 1.5f, textPosition.y, 0);
        var textEnd = new Vector3(textPosition.x + textLenght * textHeight * 1.5f, textPosition.y, 0);
        DrawLine(textStart, textEnd, textHeight * 2, LinesColor);

        //draw text
        DrawText(UnityEngine.GUI.skin, sizeTextContent, textPosition, TextColor, textHeight);
    }

    #region Logic
    static Vector4 CalculateOffsets(RectTransform rect1, RectTransform rect2)
    {
        var leftOffset = (rect2.rect.min.x + rect2.position.x) - (rect1.rect.min.x + rect1.position.x);
        var rightOffset = (rect2.rect.max.x + rect2.position.x) - (rect1.rect.max.x + rect1.position.x);

        var downOffset = (rect2.rect.min.y + rect2.position.y) - (rect1.rect.min.y + rect1.position.y);
        var topOffset = (rect2.rect.max.y + rect2.position.y) - (rect1.rect.max.y + rect1.position.y);

        return new Vector4(leftOffset, topOffset, rightOffset, downOffset);
    }

    static Vector2 CalculateSize(RectTransform rect)
    {
        var size = Vector2.zero;
        size.x = rect.rect.width;
        size.y = rect.rect.height;

        return size;
    }
    #endregion
        
    public static void DrawLine(Vector3 p1, Vector3 p2, float width = 1, Color? color = null)
    {
        var prevColor = Handles.color;

        var useColor = Color.white;
        if (color != null) {
            Handles.color = color.Value;
            useColor = color.Value;
        }

        Handles.DrawBezier(p1, p2, p1, p2, useColor, null, width);

        Handles.color = prevColor;
    }

    public static void DrawLines(Vector3[] lineSegments, float width = 1, Color? color = null)
    {
        if (lineSegments.Length % 2 != 0) {
            Debug.LogError("unexpected format");
            return;
        }

        for (int line = 0; line < lineSegments.Length; line += 2) {
            var p1 = lineSegments[line];
            var p2 = lineSegments[line + 1];
            DrawLine(p1, p2, width, color);
        }
    }
    
    public static void DrawText(GUISkin guiSkin, string text, Vector3 position, Color? color = null, int fontSize = 0,
        float yOffset = 0)
    {
        var prevSkin = GUI.skin;
        if (guiSkin == null)
            Debug.LogWarning("editor warning: guiSkin parameter is null");
        else
            GUI.skin = guiSkin;

        GUIContent textContent = new GUIContent(text);

        GUIStyle style = (guiSkin != null) ? new GUIStyle(guiSkin.GetStyle("Label")) : new GUIStyle();
        if (color != null)
            style.normal.textColor = (Color) color;
        if (fontSize > 0)
            style.fontSize = fontSize;

        Vector2 textSize = style.CalcSize(textContent);
        Vector3 screenPoint = Camera.current.WorldToScreenPoint(position);

        // checks necessary to the text is not visible when the camera is pointed in the opposite direction relative to the object
        if (screenPoint.z > 0) 
        {
            var worldPosition = Camera.current.ScreenToWorldPoint(new Vector3(screenPoint.x - textSize.x * 0.5f,
                screenPoint.y + textSize.y * 0.5f + yOffset, screenPoint.z));
            UnityEditor.Handles.Label(worldPosition, textContent, style);
        }

        GUI.skin = prevSkin;
    }
}