using System;
using System.Collections.Generic;
using System.Reflection;
using HotChocolate.Types;
using HotChocolate.Types.Descriptors;
using MediatR;
using Microsoft.Extensions.DependencyInjection;

namespace MyCompany.GraphQL
{
    /// <summary>
    /// This attribute allows us to tap into the field resolution middleware
    /// so that we can dynamically create an <see cref="IServiceScope"/> just for
    /// this field resolver method, and create an <see cref="IMediator"/> instance inside it.
    ///
    /// This is used to solve a problem with using EF Core's DbContextPool, which scopes
    /// DbContext instances per http request and will cause HotChocolate resolvers running
    /// in parallel to execute on the same DbContext, which isn't allowed.
    ///
    /// By resolving our <see cref="IMediator"/> instance inside an isolated scope,
    /// we ensure that if it depends itself on a DbContext that it will be unique in that scope
    /// and there won't be multiple threads using it simultaneously.
    /// </summary>
    [AttributeUsage(AttributeTargets.Method)]
    public class UseResolverScopedMediatorAttribute : ObjectFieldDescriptorAttribute
    {
        private static readonly string _injectedArgumentName = "mediator";

        private static readonly HashSet<string> _localSchemaNames = new HashSet<string>
        {
            SchemaNames.Content,
            SchemaNames.ShopTyre,
            SchemaNames.Task
        };

        public override void OnConfigure(
            IDescriptorContext descriptorContext,
            IObjectFieldDescriptor descriptor,
            MemberInfo member)
        {
            descriptor.Use(next => async context =>
            {
                // Temp workaround for: https://github.com/ChilliCream/hotchocolate/issues/2246
                using var scope = _localSchemaNames.Contains(context.Schema.Name)
                    ? context.Service<Microsoft.AspNetCore.Http.IHttpContextAccessor>().HttpContext.RequestServices.CreateScope()
                    : context.Service<IServiceProvider>().CreateScope();
                var mediator = scope.ServiceProvider.GetRequiredService<IMediator>();
                context.ModifyScopedContext(c => c.SetItem(_injectedArgumentName, mediator));

                await next(context);
            });
        }
    }
}