sharee.bike-App/SharedBusinessLogic/Services/ThemeNS/Theme.cs

using System;
using System.Collections.Generic;
using Xamarin.Forms;

namespace ShareeBike.Services.ThemeNS
{
	/// <summary>
	/// Manges themeing functionalty.
	/// Objects ResourceDictionary, Themes.ShareeBike, ... need Xamarin.Forms framework to be initialized which might break tests.
	/// </summary>
	public class Theme : ITheme
	{
		private ICollection<ResourceDictionary> _MergedDictionaries;

		public Theme(ICollection<ResourceDictionary> mergedDictionaries)
		{
			_MergedDictionaries = mergedDictionaries ?? throw new ArgumentNullException(nameof(mergedDictionaries));
		}

		/// <summary>
		/// Sets active theme. 
		/// </summary>
		/// <param name="themeName">Name of the new active theme.</param>
		public void SetActiveTheme(string themeName)
		{
			if (!Enum.TryParse(themeName, false, out ThemeSet theme))
				return;

			if (_MergedDictionaries == null)
				return;

			_MergedDictionaries.Clear();

			switch (theme)
			{
				case ThemeSet.LastenradBayern:
					_MergedDictionaries.Add(new Themes.LastenradBayern());
					break;

				default:
					_MergedDictionaries.Add(new Themes.ShareeBike());
					break;
			}
		}
	}
}