using System.Collections.Generic;

namespace Chernobyl.Values
{
    /// <summary>
    /// A basic implementation of an <see cref="IValue{T}"/> intended to make
    /// the creation of <see cref="IValue{T}"/> easier. This type is just like
    /// <see cref="BasicValue{T}"/> except the value is held, i.e.
    /// <see cref="BasicValue{T}.Value"/> and 
    /// <see cref="BasicValue{T}.PreviousValue"/> are implemented.
    /// </summary>
    /// <typeparam name="T">The type held by <see cref="HeldValue{T}"/>.</typeparam>
    public abstract class HeldValue<T> : BasicValue<T>
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="HeldValue{T}"/> class.
        /// </summary>
        protected HeldValue()
            : this(default(T))
        { }

        /// <summary>
        /// Initializes a new instance of the <see cref="HeldValue{T}" /> class.
        /// </summary>
        /// <param name="value">The value held by this instance.</param>
        protected HeldValue(T value)
            : this(value, EqualityComparer<T>.Default)
        {}

        /// <summary>
        /// Initializes a new instance of the <see cref="HeldValue{T}" /> class.
        /// </summary>
        /// <param name="value">The value held by this instance.</param>
        /// <param name="comparer">The instance used to compare new and previous
        /// values of <see cref="BasicValue{T}.Value" /> to determine if 
        /// <see cref="IValue{T}.Provide" /> should be raised.</param>
        protected HeldValue(T value, IEqualityComparer<T> comparer)
            : base(comparer)
        {
            _value = value;
        }

        /// <summary>
        /// The backing property or implementation to 
        /// <see cref="BasicValue{T}.Value"/>. This property should just set and 
        /// get the value. The <see cref="BasicValue{T}.Value"/> property handles 
        /// setting of <see cref="BasicValue{T}.IsReady"/> and 
        /// <see cref="PreviousValue"/> and the raising of 
        /// <see cref="BasicValue{T}.Provide"/>.
        /// </summary>
        protected override T BackingValue
        {
            get { return _value; }
            set { _value = value; }
        }

        /// <summary>
        /// The previous value of <see cref="BasicValue{T}.Value" />.
        /// </summary>
        protected override T PreviousValue { get; set; }

        /// <summary>
        /// The backing field to <see cref="BasicValue{T}.Value"/>.
        /// </summary>
        T _value;
    }
}