Added example of MapCommand for extreme endpoints handling

[oskardudycz]

After the comment from @adrianiftode in #41.

I added an example of how to not need to define custom endpoints but have shortened extension method that will:

• deserialise command,
• get handler through DI,
• handle command,
• get the result (it there is),
• return it.

The main endpoint mapping function looks as follows:

internal static class EndpointsExtensions
{
    internal static IEndpointRouteBuilder MapCommand<TRequest>(
        this IEndpointRouteBuilder endpoints,
        HttpMethod httpMethod,
        string url,
        HttpStatusCode statusCode = HttpStatusCode.OK)
    {
        endpoints.MapMethods(url, new []{httpMethod.ToString()} , async context =>
        {
            var command = await context.FromBody<TRequest>();

            var commandResult = await context.SendCommand(command);

            if (commandResult == CommandResult.None)
            {
                context.Response.StatusCode = (int)statusCode;
                return;
            }

            await context.ReturnJSON(commandResult.Result, statusCode);
        });

        return endpoints;
    }
}

I had to add command result to the command handler.

public interface ICommandHandler<in T>
{
    ValueTask<CommandResult> Handle(T command, CancellationToken token);
}

public record CommandResult
{
    public object? Result { get; }

    private CommandResult(object? result = null)
        => Result = result;

    public static CommandResult None => new();

    public static CommandResult Of(object result) => new(result);
}

then you could register is as:

endpoints.MapCommand<RegisterProduct>(HttpMethod.Post, "/api/products", HttpStatusCode.Created)

Still, I'm not yet convinced that's the better option. By that, you're losing:

• Strongly typed Command (as you need to consider that you may get nulls),
• you're losing the ability to customise the request,
• generalising it might be hard in the long term.

I think that this could work if you're just doing the basic mapping for this particular case, and if you're not, you can fall back to the custom one.

I need to sleep through that :D I might add it into GoldenEye.