Handle for resizing rects in Unity.

RectBoundsHandle.cs

using UnityEditor;
using UnityEngine;

public class RectBoundsHandle
{
    public float HandleSize = 10f;
    public float CornerGrabRadius = 12f;
    public Color FillColor = new Color(0, 1, 1, 0.1f);
    public Color OutlineColor = Color.cyan;
    public Color HandleColor = Color.blue;


    // internal state  
    Corner _activeCorner = Corner.None;
    private Vector2 _initialPosition;
    private Vector2 _dragOffset;

    enum Corner
    {
        None = -1,
        TopLeft,
        TopRight,
        BottomRight,
        BottomLeft,
        Top,
        Right,
        Bottom,
        Left,
        Center
    }

    public Rect DrawHandle(Rect rect)
    {
        var rectCopy = rect;
        var e = Event.current;

        // Draw fill + outline
        Handles.DrawSolidRectangleWithOutline(rect, FillColor, OutlineColor);

        var corners = new[]
        {
            (Corner.TopLeft, MouseCursor.ResizeUpLeft, new Vector2(rect.xMin, rect.yMin)), // top-left
            (Corner.TopRight, MouseCursor.ResizeUpRight, new Vector2(rect.xMax, rect.yMin)), // top-right
            (Corner.BottomRight, MouseCursor.ResizeUpLeft, new Vector2(rect.xMax, rect.yMax)), // bottom-right
            (Corner.BottomLeft, MouseCursor.ResizeUpRight, new Vector2(rect.xMin, rect.yMax)), // bottom-left
            (Corner.Top, MouseCursor.ResizeVertical, new Vector2(rect.center.x, rect.yMin)), // top
            (Corner.Right, MouseCursor.ResizeHorizontal, new Vector2(rect.xMax, rect.center.y)), // right
            (Corner.Bottom, MouseCursor.ResizeVertical, new Vector2(rect.center.x, rect.yMax)), // bottom
            (Corner.Left, MouseCursor.ResizeHorizontal, new Vector2(rect.xMin, rect.center.y)), // left
            (Corner.Center, MouseCursor.MoveArrow,  new Vector2(rect.center.x, rect.center.y)) // center
        };


        foreach(var (corner, cursor, cpos) in corners)
        {
            var hr = new Rect(cpos.x - HandleSize / 2, cpos.y - HandleSize / 2, HandleSize, HandleSize);
            Handles.color = HandleColor;
            Handles.DrawSolidDisc(cpos, Vector3.forward, HandleSize / 2);
            Handles.color = Color.white;
            EditorGUIUtility.AddCursorRect(hr, cursor);
        }

        switch (e.type)
        {
            case EventType.MouseDown when e.button == 0:
                // see if we hit a corner
                foreach(var (corner, cursor, cpos) in corners)
                {
                    if (Vector2.Distance(e.mousePosition, cpos) < CornerGrabRadius / 2)
                    {
                        _activeCorner = corner;
                        _initialPosition = cpos;
                        _dragOffset = cpos - e.mousePosition;
                        e.Use();
                        return rect;
                    }
                }
                break;

            case EventType.MouseDrag when e.button == 0:
                var delta = e.mousePosition - _initialPosition;

                if (_activeCorner >= 0)
                {
                    // resize from that corner
                    var newCornerPos = (_initialPosition + _dragOffset) + delta;

                    switch (_activeCorner)
                    {
                        case Corner.TopLeft:
                            rect.xMin = newCornerPos.x;
                            rect.yMin = newCornerPos.y;
                            break;
                        case Corner.TopRight:
                            rect.xMax = newCornerPos.x;
                            rect.yMin = newCornerPos.y;
                            break;
                        case Corner.BottomRight:
                            rect.xMax = newCornerPos.x;
                            rect.yMax = newCornerPos.y;
                            break;
                        case Corner.BottomLeft:
                            rect.xMin = newCornerPos.x;
                            rect.yMax = newCornerPos.y;
                            break;
                        case Corner.Top:
                            rect.yMin = newCornerPos.y;
                            break;
                        case Corner.Right:
                            rect.xMax = newCornerPos.x;
                            break;
                        case Corner.Bottom:
                            rect.yMax = newCornerPos.y;
                            break;
                        case Corner.Left:
                            rect.xMin = newCornerPos.x;
                            break;
                        case Corner.Center:
                            rect.center = newCornerPos;
                            break;
                    }

                    e.Use();
                }
                
                break;

            case EventType.MouseUp when e.button == 0:
                _activeCorner = Corner.None;
                break;
        }

        return rect;
    }
}