void Main()
{
	var configurationStore = new ConfigurationStore(new TypeMapFactory(), MapperRegistry.Mappers);
	var mapper = new MappingEngine(configurationStore);
	
	var server = new Server();
	
	configurationStore
		.CreateMap<From, To>()
		.ForMember(d => d.ServerThing, m => m.ResolveUsing(new FromServer1Resolver(server)).FromMember(s => s.ServerThingId))		
		.ForMember(d => d.AnotherServerThing, m => m.ResolveUsing(new FromServer2Resolver(server)).FromMember(s => s.AnotherServerThingId));

		var fromValue = new From { ServerThingId = "Id1", AnotherServerThingId = "Id2" }.Dump();
		mapper.AsyncMap<To>(fromValue).Result.Dump();
}

public static class AsyncExtensions
{
	public static Task<TTo> AsyncMap<TTo>(this IMappingEngine mapper, object source)
	{
		var asyncMapContext = new AsyncContext();
		// TODO the exception should be inside the task
		var result = mapper.Map<TTo>(source, o => o.Items.Add("AsyncContext", asyncMapContext));
		
		return asyncMapContext.MappingTask.ContinueWith(t => result);
	}
}


public class Server
{
	public Task<FromServer> GetThing(string id)
	{
		return Task.Run(() => new FromServer { Id = id, OtherData = Guid.NewGuid().ToString() });
	}
	public Task<FromServer2> GetAnotherThing(string id)
	{
		return Task.Run(() => new FromServer2 { Id = id, OtherData = Guid.NewGuid().ToString() });
	}
}

public class FromServer1Resolver : AsyncValueResolver<string, FromServer>
{
	Server server;
	
	public FromServer1Resolver(Server server)
	{
		this.server = server;
	}
	
	protected override Task<FromServer> GetValue(string fromValue)
	{
		return server.GetThing(fromValue);
	}
}
public class FromServer2Resolver : AsyncValueResolver<string, FromServer2>
{
	Server server;
	
	public FromServer2Resolver(Server server)
	{
		this.server = server;
	}
	
	protected override Task<FromServer2> GetValue(string fromValue)
	{
		return server.GetAnotherThing(fromValue);
	}
}
public abstract class AsyncValueResolver<TFrom, TTo> : IValueResolver
{
	public ResolutionResult Resolve(ResolutionResult source)
	{
		var contextItems = source.Context.Options.Items;
		if (!contextItems.ContainsKey("AsyncContext"))
			throw new InvalidOperationException("You must use mapper.AsyncMap when using async value resolvers");
			
		var asyncContext = (AsyncContext)contextItems["AsyncContext"];
		
		asyncContext.StartAsyncOperation();
		var parentSourceValue = source.Context.SourceValue;
		
		var sourceValue = source.Context.SourceType.GetProperty(source.Context.PropertyMap.SourceMember.Name).GetValue(source.Context.SourceValue);
		
		GetValue((TFrom)source.Value)
			.ContinueWith(r => 
			{
				source.Context.PropertyMap.DestinationProperty.SetValue(source.Context.DestinationValue, r.Result);
				asyncContext.OperationFinished();
			});
			
		return source.Ignore();
	}
	
	protected abstract Task<TTo> GetValue(TFrom fromValue);
}

public class AsyncContext
{
	object locker = new object();
	TaskCompletionSource<object> taskSource = new TaskCompletionSource<object>();
	int activeCalls = 0;

	public void StartAsyncOperation()
	{
		lock (locker) { activeCalls++; };
	}
	
	public void OperationFinished()
	{
		var isComplete = false;
		lock (locker) 
		{
			activeCalls--;
			
			if (activeCalls == 0)
			{
				isComplete = true;
			}
		};
		
		if (isComplete)
			taskSource.SetResult(null);
	}
	
	public Task MappingTask
	{
		get { return taskSource.Task; }
	}
}

public class To
{
	public FromServer ServerThing { get; set; }
	public FromServer2 AnotherServerThing { get; set; }
}

public class From
{
	public string ServerThingId {get;set;}
	public string AnotherServerThingId {get;set;}
}

public class FromServer
{
	public string Id { get; set; }
	public string OtherData { get; set; }
}

public class FromServer2
{
	public string Id { get; set; }
	public string OtherData { get; set; }
}