Umbraco-CMS/src/Umbraco.Core/Components/ComposeAfterAttribute.cs

using System;

namespace Umbraco.Core.Components
{
    /// <summary>
    /// Indicates that a composer requires another composer.
    /// </summary>
    /// <remarks>
    /// <para>This attribute is *not* inherited. This means that a composer class inheriting from
    /// another composer class does *not* inherit its requirements. However, the runtime checks
    /// the *interfaces* of every composer for their requirements, so requirements declared on
    /// interfaces are inherited by every composer class implementing the interface.</para>
    /// <para>When targeting a class, indicates a dependency on the composer which must be enabled,
    /// unless the requirement has explicitly been declared as weak (and then, only if the composer
    /// is enabled).</para>
    /// <para>When targeting an interface, indicates a dependency on enabled composers implementing
    /// the interface. It could be no composer at all, unless the requirement has explicitly been
    /// declared as strong (and at least one composer must be enabled).</para>
    /// </remarks>
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface, AllowMultiple = true, Inherited = false)]
    public sealed class ComposeAfterAttribute : Attribute
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="ComposeAfterAttribute"/> class.
        /// </summary>
        /// <param name="requiredType">The type of the required composer.</param>
        public ComposeAfterAttribute(Type requiredType)
        {
            if (typeof(IComposer).IsAssignableFrom(requiredType) == false)
                throw new ArgumentException($"Type {requiredType.FullName} is invalid here because it does not implement {typeof(IComposer).FullName}.");
            RequiredType = requiredType;
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="ComposeAfterAttribute"/> class.
        /// </summary>
        /// <param name="requiredType">The type of the required composer.</param>
        /// <param name="weak">A value indicating whether the requirement is weak.</param>
        public ComposeAfterAttribute(Type requiredType, bool weak)
            : this(requiredType)
        {
            Weak = weak;
        }

        /// <summary>
        /// Gets the required type.
        /// </summary>
        public Type RequiredType { get; }

        /// <summary>
        /// Gets a value indicating whether the requirement is weak.
        /// </summary>
        /// <remarks>Returns <c>true</c> if the requirement is weak (requires the other composer if it
        /// is enabled), <c>false</c> if the requirement is strong (requires the other composer to be
        /// enabled), and <c>null</c> if unspecified, in which case it is strong for classes and weak for
        /// interfaces.</remarks>
        public bool? Weak { get; }
    }
}