FlexPanel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;

namespace SupportTool.Controls
{
	public class FlexPanel : Panel
	{
		public static bool GetFlex(DependencyObject obj) => (bool)obj.GetValue(FlexProperty);
		public static void SetFlex(DependencyObject obj, bool value) => obj.SetValue(FlexProperty, value);
		public static readonly DependencyProperty FlexProperty = DependencyProperty.RegisterAttached("Flex", typeof(bool), typeof(FlexPanel), new PropertyMetadata(false));

		public static int GetFlexWeight(DependencyObject obj) => (int)obj.GetValue(FlexWeightProperty);
		public static void SetFlexWeight(DependencyObject obj, int value) => obj.SetValue(FlexWeightProperty, value);
		public static readonly DependencyProperty FlexWeightProperty = DependencyProperty.RegisterAttached("FlexWeight", typeof(int), typeof(FlexPanel), new PropertyMetadata(1));

		public Orientation Orientation
		{
			get => (Orientation)GetValue(OrientationProperty);
			set => SetValue(OrientationProperty, value);
		}
		public static readonly DependencyProperty OrientationProperty = DependencyProperty.Register(nameof(Orientation), typeof(Orientation), typeof(FlexPanel), new PropertyMetadata(Orientation.Vertical));

		protected override Size MeasureOverride(Size availableSize)
		{
			var desiredSize = new Size();
			foreach (UIElement child in Children)
			{
				child.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));

				if (Orientation == Orientation.Vertical)
				{
					desiredSize.Height += child.DesiredSize.Height;
					desiredSize.Width = Math.Max(desiredSize.Width, child.DesiredSize.Width);
				}
				else
				{
					desiredSize.Width += child.DesiredSize.Width;
					desiredSize.Height = Math.Max(desiredSize.Height, child.DesiredSize.Height);
				}
			}

			if (double.IsPositiveInfinity(availableSize.Height) || double.IsPositiveInfinity(availableSize.Width)) return desiredSize;
			else return availableSize;
		}

		protected override Size ArrangeOverride(Size finalSize)
		{
			var currentLength = 0d;
			var totalLength = 0d;
			var flexChildrenWeightParts = 0;

			if (Orientation == Orientation.Vertical)
			{
				foreach (UIElement child in Children)
				{
					if (GetFlex(child)) flexChildrenWeightParts += GetFlexWeight(child);
					else totalLength += child.DesiredSize.Height;
				}

				var flexSize = Math.Max(0, (finalSize.Height - totalLength) / flexChildrenWeightParts);

				foreach (UIElement child in Children)
				{
					var arrangeRect = new Rect();
					if (GetFlex(child)) arrangeRect = new Rect(0, currentLength, finalSize.Width, flexSize * GetFlexWeight(child));
					else arrangeRect = new Rect(0, currentLength, finalSize.Width, child.DesiredSize.Height);

					child.Arrange(arrangeRect);
					currentLength += arrangeRect.Height;
				}
			}
			else
			{
				foreach (UIElement child in Children)
				{
					if (GetFlex(child)) flexChildrenWeightParts += GetFlexWeight(child);
					else totalLength += child.DesiredSize.Width;
				}

				var flexSize = Math.Max(0, (finalSize.Width - totalLength) / flexChildrenWeightParts);

				foreach (UIElement child in Children)
				{
					var arrangeRect = new Rect();
					if (GetFlex(child)) arrangeRect = new Rect(currentLength, 0, flexSize * GetFlexWeight(child), finalSize.Height);
					else arrangeRect = new Rect(currentLength, 0, child.DesiredSize.Width, finalSize.Height);

					child.Arrange(arrangeRect);
					currentLength += arrangeRect.Width;
				}
			}

			return finalSize;
		}
	}
}