using Chernobyl.Event;
using Chernobyl.Readiness;
using Chernobyl.Utility;

namespace Chernobyl.Values
{
    /// <summary>
    /// An <see cref="IValue{T}"/> type that holds the non-null value held 
    /// by a <see cref="IValue{T}"/>.
    /// </summary>
    /// <typeparam name="T">The type that is to be held.</typeparam>
    public class NotNull<T> : HeldValue<T>
        where T : class
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="NotNull{T}"/> class.
        /// </summary>
        /// <param name="value">The instance whose contained value is to be 
        /// considered ready when it is no longer null.</param>
        public NotNull(IValue<T> value)
        {
            value.ThrowIfNull("value");
            value.Provide += OnValueProvide;
        }

        /// <summary>
        /// An event handler that is invoked when the value is changed. This 
        /// method ensures the value of this instance is maintained.
        /// </summary>
        /// <param name="sender">The sender of the event.</param>
        /// <param name="e">The arguments containing the changed value.</param>
        void OnValueProvide(object sender, ValueChangedEventArgs<T> e)
        {
            if (e.NewValue != null)
                Value = e.NewValue;
            else
                IsReady = false;
        }
    }

    /// <summary>
    /// Utility methods and extensions for <see cref="NotNull{T}"/>.
    /// </summary>
    public static class NotNullExtensions
    {
        /// <summary>
        /// Returns an <see cref="NotNull{T}"/> instance created with the 
        /// <see cref="IValue{T}"/> provided.
        /// </summary>
        /// <typeparam name="T">The type contained within <paramref name="value"/>.</typeparam>
        /// <param name="value">The <see cref="IValue{T}"/> with which to create 
        /// the <see cref="NotNull{T}"/>.</param>
        /// <returns>The <see cref="NotNull{T}"/> instance.</returns>
        public static IValue<T> NotNull<T>(this IValue<T> value)
            where T : class
        {
            return new NotNull<T>(value);
        }
    }
}