// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using Microsoft.Extensions.Options;

namespace Microsoft.AspNetCore.Mvc.Infrastructure
{
    // Guide to making breaking behavior changes in MVC:
    //
    // Hello, if you're reading this, you're probably trying to make a change in behavior in MVC in a minor
    // version. Every change in behavior is a breaking change to someone, even if a feature was buggy or
    // broken in some scenarios.
    //
    // To help make things easier for current users, we don't automatically opt users into breaking changes when
    // they upgrade applications to a new minor version of ASP.NET Core. It's a separate choice to opt in to new
    // behaviors in a minor release.
    //
    // To make things better for future users, we also want to provide an easy way for applications to get
    // access to the new behaviors. We make changes when they are improvements, and if we're changing something
    // we've already shipped, it must add value for all of our users (eventually). To this end, new applications
    // created using the template are always opted in to the 'current' version.
    //
    // This means that all changes in behavior should be opt-in.
    //
    // -----
    //
    // Moving on from general philosophy, here's how to implement a behavior change and corresponding
    // compatibility switch.
    //
    // Add a new property on options that uses a CompatibilitySwitch<T> as a backing field. Make sure the
    // new switch is exposed by implementing IEnumerable<ICompatibilitySwitch> on the options class. Pass the
    // property name to the CompatibilitySwitch constructor using nameof.
    //
    // Choose a boolean value or a new enum type as the 'value' of the property.
    //
    // If the new property has a boolean value, it should be named something like `SuppressFoo`
    // (if the new value deactivates some behavior) or like `AllowFoo` (if the new value enables some behavior).
    // Choose a name so that the old behavior equates to 'false'.
    //
    // If it's an enum, make sure you initialize the compatibility switch using the
    // CompatibilitySwitch(string, value) constructor to make it obvious the correct value is passed in. It's
    // a good idea to equate the original behavior with the default enum value as well.
    //
    // Then create (or modify) a subclass of ConfigureCompatibilityOptions appropriate for your options type.
    // Override the DefaultValues property and provide appropriate values based on the value of the Version
    // property. If you just added this class, register it as an IPostConfigureOptions<TOptions> in DI.
    //
    /// <summary>
    /// Infrastructure supporting the implementation of <see cref="CompatibilityVersion"/>. This is an
    /// implementation of <see cref="ICompatibilitySwitch"/> suitable for use with the <see cref="IOptions{T}"/>
    /// pattern. This is framework infrastructure and should not be used by application code.
    /// </summary>
    /// <typeparam name="TValue">The type of value associated with the compatibility switch.</typeparam>
    public class CompatibilitySwitch<TValue> : ICompatibilitySwitch where TValue : struct
    {
        private TValue _value;

        /// <summary>
        /// Creates a new compatibility switch with the provided name.
        /// </summary>
        /// <param name="name">
        /// The compatibility switch name. The name must match a property name on an options type.
        /// </param>
        public CompatibilitySwitch(string name)
            : this(name, default)
        {
        }

        /// <summary>
        /// Creates a new compatibility switch with the provided name and initial value.
        /// </summary>
        /// <param name="name">
        /// The compatibility switch name. The name must match a property name on an options type.
        /// </param>
        /// <param name="initialValue">
        /// The initial value to assign to the switch.
        /// </param>
        public CompatibilitySwitch(string name, TValue initialValue)
        {
            if (name == null)
            {
                throw new ArgumentNullException(nameof(name));
            }

            Name = name;
            _value = initialValue;
        }

        /// <summary>
        /// Gets a value indicating whether the <see cref="Value"/> property has been set.
        /// </summary>
        /// <remarks>
        /// This is used by the compatibility infrastructure to determine whether the application developer
        /// has set explicitly set the value associated with this switch.
        /// </remarks>
        public bool IsValueSet { get; private set; }

        /// <summary>
        /// Gets the name of the compatibility switch.
        /// </summary>
        public string Name { get; }

        /// <summary>
        /// Gets or set the value associated with the compatibility switch.
        /// </summary>
        /// <remarks>
        /// Setting the switch value using <see cref="Value"/> will set <see cref="IsValueSet"/> to <c>true</c>.
        /// As a consequence, the compatibility infrastructure will consider this switch explicitly configured by
        /// the application developer, and will not apply a default value based on the compatibility version.
        /// </remarks>
        public TValue Value
        {
            get => _value;
            set
            {
                IsValueSet = true;
                _value = value;
            }
        }

        // Called by the compatibility infrastructure to set a default value when IsValueSet is false.
        object ICompatibilitySwitch.Value
        {
            get => Value;
            set => Value = (TValue)value;
        }
    }
}
