Umbraco-CMS/src/Umbraco.Core/Composing/CollectionBuilderBase.cs

using Microsoft.Extensions.DependencyInjection;
using Umbraco.Extensions;

namespace Umbraco.Cms.Core.Composing;

/// <summary>
///     Provides a base class for collection builders.
/// </summary>
/// <typeparam name="TBuilder">The type of the builder.</typeparam>
/// <typeparam name="TCollection">The type of the collection.</typeparam>
/// <typeparam name="TItem">The type of the items.</typeparam>
public abstract class CollectionBuilderBase<TBuilder, TCollection, TItem> : ICollectionBuilder<TCollection, TItem>
    where TBuilder : CollectionBuilderBase<TBuilder, TCollection, TItem>
    where TCollection : class, IBuilderCollection<TItem>
{
    private readonly Lock _locker = new();
    private readonly List<Type> _types = new();
    private Type[]? _registeredTypes;

    /// <summary>
    ///     Gets the collection lifetime.
    /// </summary>
    protected virtual ServiceLifetime CollectionLifetime => ServiceLifetime.Singleton;

    /// <inheritdoc />
    public virtual void RegisterWith(IServiceCollection services)
    {
        if (_registeredTypes != null)
        {
            throw new InvalidOperationException("This builder has already been registered.");
        }

        // register the collection
        services.Add(new ServiceDescriptor(typeof(TCollection), CreateCollection, CollectionLifetime));

        // register the types
        RegisterTypes(services);
    }

    /// <summary>
    ///     Creates a collection.
    /// </summary>
    /// <returns>A collection.</returns>
    /// <remarks>Creates a new collection each time it is invoked.</remarks>
    public virtual TCollection CreateCollection(IServiceProvider factory)
        => factory.CreateInstance<TCollection>(CreateItemsFactory(factory));

    /// <summary>
    ///     Gets the internal list of types as an IEnumerable (immutable).
    /// </summary>
    public IEnumerable<Type> GetTypes() => _types;

    /// <summary>
    ///     Gets a value indicating whether the collection contains a type.
    /// </summary>
    /// <typeparam name="T">The type to look for.</typeparam>
    /// <returns>A value indicating whether the collection contains the type.</returns>
    /// <remarks>
    ///     Some builder implementations may use this to expose a public Has{T}() method,
    ///     when it makes sense. Probably does not make sense for lazy builders, for example.
    /// </remarks>
    public virtual bool Has<T>()
        where T : TItem =>
        _types.Contains(typeof(T));

    /// <summary>
    ///     Gets a value indicating whether the collection contains a type.
    /// </summary>
    /// <param name="type">The type to look for.</param>
    /// <returns>A value indicating whether the collection contains the type.</returns>
    /// <remarks>
    ///     Some builder implementations may use this to expose a public Has{T}() method,
    ///     when it makes sense. Probably does not make sense for lazy builders, for example.
    /// </remarks>
    public virtual bool Has(Type type)
    {
        EnsureType(type, "find");
        return _types.Contains(type);
    }

    /// <summary>
    ///     Configures the internal list of types.
    /// </summary>
    /// <param name="action">The action to execute.</param>
    /// <remarks>Throws if the types have already been registered.</remarks>
    protected void Configure(Action<List<Type>> action)
    {
        lock (_locker)
        {
            if (_registeredTypes != null)
            {
                throw new InvalidOperationException(
                    "Cannot configure a collection builder after it has been registered.");
            }

            action(_types);
        }
    }

    /// <summary>
    ///     Gets the types.
    /// </summary>
    /// <param name="types">The internal list of types.</param>
    /// <returns>The list of types to register.</returns>
    /// <remarks>Used by implementations to add types to the internal list, sort the list, etc.</remarks>
    protected virtual IEnumerable<Type> GetRegisteringTypes(IEnumerable<Type> types) => types;

    /// <summary>
    ///     Creates the collection items.
    /// </summary>
    /// <returns>The collection items.</returns>
    protected virtual IEnumerable<TItem> CreateItems(IServiceProvider factory)
    {
        if (_registeredTypes == null)
        {
            throw new InvalidOperationException(
                "Cannot create items before the collection builder has been registered.");
        }

        return _registeredTypes // respect order
            .Select(x => CreateItem(factory, x))
            .ToArray(); // safe
    }

    /// <summary>
    ///     Creates a collection item.
    /// </summary>
    protected virtual TItem CreateItem(IServiceProvider factory, Type itemType)
        => (TItem)factory.GetRequiredService(itemType);

    protected Type EnsureType(Type type, string action)
    {
        if (typeof(TItem).IsAssignableFrom(type) == false)
        {
            throw new InvalidOperationException(
                $"Cannot {action} type {type.FullName} as it does not inherit from/implement {typeof(TItem).FullName}.");
        }

        return type;
    }

    private void RegisterTypes(IServiceCollection services)
    {
        lock (_locker)
        {
            if (_registeredTypes != null)
            {
                return;
            }

            Type[] types = GetRegisteringTypes(_types).ToArray();

            // ensure they are safe
            foreach (Type type in types)
            {
                EnsureType(type, "register");
            }

            // register them - ensuring that each item is registered with the same lifetime as the collection.
            // NOTE: Previously each one was not registered with the same lifetime which would mean that if there
            // was a dependency on an individual item, it would resolve a brand new transient instance which isn't what
            // we would expect to happen. The same item should be resolved from the container as the collection.
            foreach (Type type in types)
            {
                services.Add(new ServiceDescriptor(type, type, CollectionLifetime));
            }

            _registeredTypes = types;
        }
    }

    // used to resolve a Func<IEnumerable<TItem>> parameter
    private Func<IEnumerable<TItem>> CreateItemsFactory(IServiceProvider factory) => () => CreateItems(factory);
}