// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Collections.Generic;
using System.Diagnostics;
 
namespace System.Composition.Hosting.Core
{
    internal sealed class CycleBreakingExportDescriptor : ExportDescriptor
    {
        private readonly Lazy<ExportDescriptor> _exportDescriptor;
 
        public CycleBreakingExportDescriptor(Lazy<ExportDescriptor> exportDescriptor)
        {
            _exportDescriptor = exportDescriptor;
        }
 
        public override CompositeActivator Activator
        {
            get
            {
                if (!_exportDescriptor.IsValueCreated)
                    return Activate;
 
                return _exportDescriptor.Value.Activator;
            }
        }
 
        public override IDictionary<string, object> Metadata
        {
            get
            {
                if (!_exportDescriptor.IsValueCreated)
                    return new CycleBreakingMetadataDictionary(_exportDescriptor);
 
                return _exportDescriptor.Value.Metadata;
            }
        }
 
        private object Activate(LifetimeContext context, CompositionOperation operation)
        {
            if (!_exportDescriptor.IsValueCreated)
            {
                var ex = new NotImplementedException(SR.NotImplemented_MetadataCycles);
                Debug.WriteLine(SR.Diagnostic_ThrowingException, ex.ToString());
                throw ex;
            }
 
            Debug.WriteLine("[System.Composition] Activating via cycle-breaking proxy.");
            return _exportDescriptor.Value.Activator(context, operation);
        }
    }
}