// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System.Collections.Generic;

namespace Microsoft.AspNetCore.Mvc.Razor
{
    /// <summary>
    /// Specifies the contracts for a view location expander that is used by <see cref="RazorViewEngine"/> instances to
    /// determine search paths for a view.
    /// </summary>
    /// <remarks>
    /// Individual <see cref="IViewLocationExpander"/>s are invoked in two steps:
    /// (1) <see cref="PopulateValues(ViewLocationExpanderContext)"/> is invoked and each expander
    /// adds values that it would later consume as part of
    /// <see cref="ExpandViewLocations(ViewLocationExpanderContext, IEnumerable{string})"/>.
    /// The populated values are used to determine a cache key - if all values are identical to the last time
    /// <see cref="PopulateValues(ViewLocationExpanderContext)"/> was invoked, the cached result
    /// is used as the view location.
    /// (2) If no result was found in the cache or if a view was not found at the cached location,
    /// <see cref="ExpandViewLocations(ViewLocationExpanderContext, IEnumerable{string})"/> is invoked to determine
    /// all potential paths for a view.
    /// </remarks>
    public interface IViewLocationExpander
    {
        /// <summary>
        /// Invoked by a <see cref="RazorViewEngine"/> to determine the values that would be consumed by this instance
        /// of <see cref="IViewLocationExpander"/>. The calculated values are used to determine if the view location
        /// has changed since the last time it was located.
        /// </summary>
        /// <param name="context">The <see cref="ViewLocationExpanderContext"/> for the current view location
        /// expansion operation.</param>
        void PopulateValues(ViewLocationExpanderContext context);

        /// <summary>
        /// Invoked by a <see cref="RazorViewEngine"/> to determine potential locations for a view.
        /// </summary>
        /// <param name="context">The <see cref="ViewLocationExpanderContext"/> for the current view location
        /// expansion operation.</param>
        /// <param name="viewLocations">The sequence of view locations to expand.</param>
        /// <returns>A list of expanded view locations.</returns>
        IEnumerable<string> ExpandViewLocations(ViewLocationExpanderContext context,
                                                IEnumerable<string> viewLocations);
    }
}