using System;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore.Storage;

namespace Microsoft.EntityFrameworkCore
{
	/// <summary>
	/// Extensions for the <see cref="DbContext"/>.
	/// </summary>
	public static class DbContextExensions
	{
		/// <summary>
		/// Starts a new transaction.
		/// </summary>
		/// <remarks>
		/// See <see href="https://aka.ms/efcore-docs-transactions">Transactions in EF Core</see> for more information.
		/// </remarks>
		/// <param name="dbContext">The current <see cref="DbContext"/>.</param>
		/// <returns>
		/// A <see cref="IDbContextTransaction" /> that represents the started transaction.
		/// </returns>
		public static IDbContextTransaction BeginTransaction(this DbContext dbContext)
		{
			if (dbContext.Database.GetProviderType() == DatabaseFacadeExtensions.DatabaseProvider.InMemory)
				return new DbContextTransactionStub();

			return dbContext.Database.BeginTransaction();
		}

		/// <summary>
		/// Asynchronously starts a new transaction.
		/// </summary>
		/// <remarks>
		/// <para>
		/// Entity Framework Core does not support multiple parallel operations being run on the same DbContext instance. This
		/// includes both parallel execution of async queries and any explicit concurrent use from multiple threads.
		/// Therefore, always await async calls immediately, or use separate DbContext instances for operations that execute
		/// in parallel. See <see href="https://aka.ms/efcore-docs-threading">Avoiding DbContext threading issues</see>
		/// for more information.
		/// </para>
		/// <para>
		/// See <see href="https://aka.ms/efcore-docs-transactions">Transactions in EF Core</see> for more information.
		/// </para>
		/// </remarks>
		/// <param name="dbContext">The current <see cref="DbContext"/>.</param>
		/// <param name="cancellationToken">A <see cref="CancellationToken" /> to observe while waiting for the task to complete.</param>
		/// <returns>
		/// A task that represents the asynchronous transaction initialization. The task result contains a <see cref="IDbContextTransaction" /> that represents the started transaction.
		/// </returns>
		/// <exception cref="OperationCanceledException">If the <see cref="CancellationToken" /> is canceled.</exception>
		public static Task<IDbContextTransaction> BeginTransactionAsync(this DbContext dbContext, CancellationToken cancellationToken)
		{
			if (dbContext.Database.GetProviderType() == DatabaseFacadeExtensions.DatabaseProvider.InMemory)
				return Task.FromResult<IDbContextTransaction>(new DbContextTransactionStub());

			return dbContext.Database.BeginTransactionAsync(cancellationToken);
		}

		/// <inheritdoc cref="IDbContextTransaction" />
		private class DbContextTransactionStub : IDbContextTransaction
		{
			/// <inheritdoc />
			public Guid TransactionId { get; private set; } = Guid.NewGuid();

			/// <inheritdoc />
			public void Commit()
			{ }

			/// <inheritdoc />
			public Task CommitAsync(CancellationToken cancellationToken = default)
				=> Task.CompletedTask;

			/// <inheritdoc />
			public void Dispose()
			{ }

			/// <inheritdoc />
			public ValueTask DisposeAsync()
				=> new(Task.CompletedTask);

			/// <inheritdoc />
			public void Rollback()
			{ }

			/// <inheritdoc />
			public Task RollbackAsync(CancellationToken cancellationToken = default)
				=> Task.CompletedTask;
		}
	}
}