Umbraco-CMS/src/Umbraco.Core/Models/PublishedContent/IPublishedValueFallback.cs

namespace Umbraco.Cms.Core.Models.PublishedContent
{
    /// <summary>
    /// Provides a fallback strategy for getting <see cref="IPublishedElement"/> values.
    /// </summary>
    public interface IPublishedValueFallback
    {
        /// <summary>
        /// Tries to get a fallback value for a property.
        /// </summary>
        /// <param name="property">The property.</param>
        /// <param name="culture">The requested culture.</param>
        /// <param name="segment">The requested segment.</param>
        /// <param name="fallback">A fallback strategy.</param>
        /// <param name="defaultValue">An optional default value.</param>
        /// <param name="value">The fallback value.</param>
        /// <returns>A value indicating whether a fallback value could be provided.</returns>
        /// <remarks>
        /// <para>This method is called whenever property.Value(culture, segment, defaultValue) is called, and
        /// property.HasValue(culture, segment) is false.</para>
        /// <para>It can only fallback at property level (no recurse).</para>
        /// <para>At property level, property.GetValue() does *not* implement fallback, and one has to
        /// get property.Value() or property.Value{T}() to trigger fallback.</para>
        /// <para>Note that <paramref name="culture"/> and <paramref name="segment"/> may not be contextualized,
        /// so the variant context should be used to contextualize them (see our default implementation in
        /// the web project.</para>
        /// </remarks>
        bool TryGetValue(IPublishedProperty property, string? culture, string? segment, Fallback fallback, object? defaultValue, out object? value);

        /// <summary>
        /// Tries to get a fallback value for a property.
        /// </summary>
        /// <typeparam name="T">The type of the value.</typeparam>
        /// <param name="property">The property.</param>
        /// <param name="culture">The requested culture.</param>
        /// <param name="segment">The requested segment.</param>
        /// <param name="fallback">A fallback strategy.</param>
        /// <param name="defaultValue">An optional default value.</param>
        /// <param name="value">The fallback value.</param>
        /// <returns>A value indicating whether a fallback value could be provided.</returns>
        /// <remarks>
        /// <para>This method is called whenever property.Value{T}(culture, segment, defaultValue) is called, and
        /// property.HasValue(culture, segment) is false.</para>
        /// <para>It can only fallback at property level (no recurse).</para>
        /// <para>At property level, property.GetValue() does *not* implement fallback, and one has to
        /// get property.Value() or property.Value{T}() to trigger fallback.</para>
        /// </remarks>
        bool TryGetValue<T>(IPublishedProperty property, string? culture, string? segment, Fallback fallback, T? defaultValue, out T? value);

        /// <summary>
        /// Tries to get a fallback value for a published element property.
        /// </summary>
        /// <param name="content">The published element.</param>
        /// <param name="alias">The property alias.</param>
        /// <param name="culture">The requested culture.</param>
        /// <param name="segment">The requested segment.</param>
        /// <param name="fallback">A fallback strategy.</param>
        /// <param name="defaultValue">An optional default value.</param>
        /// <param name="value">The fallback value.</param>
        /// <returns>A value indicating whether a fallback value could be provided.</returns>
        /// <remarks>
        /// <para>This method is called whenever getting the property value for the specified alias, culture and
        /// segment, either returned no property at all, or a property with HasValue(culture, segment) being false.</para>
        /// <para>It can only fallback at element level (no recurse).</para>
        /// </remarks>
        bool TryGetValue(IPublishedElement content, string alias, string? culture, string? segment, Fallback fallback, object? defaultValue, out object? value);

        /// <summary>
        /// Tries to get a fallback value for a published element property.
        /// </summary>
        /// <typeparam name="T">The type of the value.</typeparam>
        /// <param name="content">The published element.</param>
        /// <param name="alias">The property alias.</param>
        /// <param name="culture">The requested culture.</param>
        /// <param name="segment">The requested segment.</param>
        /// <param name="fallback">A fallback strategy.</param>
        /// <param name="defaultValue">An optional default value.</param>
        /// <param name="value">The fallback value.</param>
        /// <returns>A value indicating whether a fallback value could be provided.</returns>
        /// <remarks>
        /// <para>This method is called whenever getting the property value for the specified alias, culture and
        /// segment, either returned no property at all, or a property with HasValue(culture, segment) being false.</para>
        /// <para>It can only fallback at element level (no recurse).</para>
        /// </remarks>
        bool TryGetValue<T>(IPublishedElement content, string alias, string? culture, string? segment, Fallback fallback, T? defaultValue, out T? value);

        /// <summary>
        /// Tries to get a fallback value for a published content property.
        /// </summary>
        /// <param name="content">The published element.</param>
        /// <param name="alias">The property alias.</param>
        /// <param name="culture">The requested culture.</param>
        /// <param name="segment">The requested segment.</param>
        /// <param name="fallback">A fallback strategy.</param>
        /// <param name="defaultValue">An optional default value.</param>
        /// <param name="value">The fallback value.</param>
        /// <param name="noValueProperty">The property that does not have a value.</param>
        /// <returns>A value indicating whether a fallback value could be provided.</returns>
        /// <remarks>
        /// <para>This method is called whenever getting the property value for the specified alias, culture and
        /// segment, either returned no property at all, or a property with HasValue(culture, segment) being false.</para>
        /// <para>In an <see cref="IPublishedContent"/>, because walking up the tree is possible, the content itself may not even
        /// have a property with the specified alias, but such a property may exist up in the tree. The <paramref name="noValueProperty"/>
        /// parameter is used to return a property with no value. That can then be used to invoke a converter and get the
        /// converter's interpretation of "no value".</para>
        /// </remarks>
        bool TryGetValue(IPublishedContent content, string alias, string? culture, string? segment, Fallback fallback, object? defaultValue, out object? value, out IPublishedProperty? noValueProperty);

        /// <summary>
        /// Tries to get a fallback value for a published content property.
        /// </summary>
        /// <typeparam name="T">The type of the value.</typeparam>
        /// <param name="content">The published element.</param>
        /// <param name="alias">The property alias.</param>
        /// <param name="culture">The requested culture.</param>
        /// <param name="segment">The requested segment.</param>
        /// <param name="fallback">A fallback strategy.</param>
        /// <param name="defaultValue">An optional default value.</param>
        /// <param name="value">The fallback value.</param>
        /// <param name="noValueProperty">The property that does not have a value.</param>
        /// <returns>A value indicating whether a fallback value could be provided.</returns>
        /// <remarks>
        /// <para>This method is called whenever getting the property value for the specified alias, culture and
        /// segment, either returned no property at all, or a property with HasValue(culture, segment) being false.</para>
        /// <para>In an <see cref="IPublishedContent"/>, because walking up the tree is possible, the content itself may not even
        /// have a property with the specified alias, but such a property may exist up in the tree. The <paramref name="noValueProperty"/>
        /// parameter is used to return a property with no value. That can then be used to invoke a converter and get the
        /// converter's interpretation of "no value".</para>
        /// </remarks>
        bool TryGetValue<T>(IPublishedContent content, string alias, string? culture, string? segment, Fallback fallback, T defaultValue, out T? value, out IPublishedProperty? noValueProperty);
    }
}