//
// Intelligencia.Utilities
// 
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// 
// http://www.apache.org/licenses/LICENSE-2.0
// 
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// 
using System;
using System.Reflection;
using System.ComponentModel;
using System.Collections.Generic;
using System.Web.UI.WebControls;

namespace Intelligencia.Utilities
{
	/// <summary>
	/// Helper class for creating list items from an enum data source.
	/// </summary>
	public class EnumDataSource
	{
		private EnumDataSource()
		{
		}

		/// <summary>
		/// Create an array of ListItem's from an Enum.
		/// </summary>
		/// <example>
		/// <remarks>
		/// This method returns an array of ListItem objects ready for passing to the
		/// AddRange method of a ListItemCollection.
		/// </remarks>
		/// <code>
		/// public enum PrivacyLevel
		/// {
		///		[Description("For your eyes only")]
		///		EyesOnly,
		/// 
		///		[Description("Top secret")]
		///		TopSecret,
		/// 
		///		Classified
		/// }
		/// 
		/// ...
		/// ListItem[] items = EnumDataSource.Create(typeof(PrivacyLevel));
		/// myDropDown.Items.AddRange(items);
		/// ...
		/// </code>
		/// <para>This will result in a dropdown list with the following:
		/// 0 "For your eyes only"
		/// 1 "Top secret"
		/// 2 "Classified"
		/// </para>
		/// </example>
		/// <param name="enumSource">The Type of the Enum to create list items from.</param>
		/// <returns>An array of ListItem objects, one for each Enum value.</returns>
		public static ListItem[] Create(Type enumSource)
		{
			// First check the cache in a thread-safe manner.
			if (!_cache.ContainsKey(enumSource))
			{
				lock (_cache)
				{
					if (!_cache.ContainsKey(enumSource))
					{
						// Not found in the cache, so create a new, empty list
						List<ListItem> values = new List<ListItem>();

						// Enumerate over all fields.  The Enum values appear as
						// fields in the Enum type.
						foreach (FieldInfo field in enumSource.GetFields())
						{
							// Pick out only the literal fields, which represent
							// the Enum values.
							if (field.IsLiteral)
							{
								// Look up any Description attributes
								DescriptionAttribute[] attribute = field.GetCustomAttributes(typeof(DescriptionAttribute), false) as DescriptionAttribute[];

								// Add either the description or the literal name
								values.Add(new ListItem(attribute.Length > 0 ? attribute[0].Description : field.Name, ((int)field.GetRawConstantValue()).ToString()));
							}
						}

						// Add the results to the cache for next time.
						_cache.Add(enumSource, values.ToArray());
					}

					// Not using the fall through, because we want this inside the lock().
					return _cache[enumSource];
				}
			}
			else
			{
				return _cache[enumSource];
			}
		}

		// A private static cache of the mappings from Enum type to ListItem[]
		private static Dictionary<Type, ListItem[]> _cache = new Dictionary<Type, ListItem[]>();
	}
}