13.3 The parcel

Once an order is placed, the customer can follow its parcel. A parcel goes through states of its own, so it is a machine of its own, a child: a state of the checkout starts it and waits for it to end, and takes an outcome from how it ends, as it does from a run.

Our courier is a fake, as the payment provider is: a parcel is packed, in transit a minute later, and delivered a minute after that, by timers. A real courier would move it with events.

Shopfront.Core/Machines/Parcel/
├── ParcelMachine.cs
├── ParcelState.cs
├── ParcelTrigger.cs
├── ParcelContext.cs
└── Models/
    └── ParcelResult.cs

Shopfront.Core/Machines/Parcel/ParcelState.cs:

namespace Shopfront.Machines.Parcel;
 
public enum ParcelState
{
    Packed,
    InTransit,
    Delivered,
}

Shopfront.Core/Machines/Parcel/ParcelTrigger.cs:

namespace Shopfront.Machines.Parcel;
 
// The fake courier moves a parcel by timers alone; a real one would send events.
public enum ParcelTrigger
{
    Scan,
}

Shopfront.Core/Machines/Parcel/ParcelContext.cs:

namespace Shopfront.Machines.Parcel;
 
public sealed record ParcelContext;

Shopfront.Core/Machines/Parcel/Models/ParcelResult.cs:

namespace Shopfront.Machines.Parcel.Models;
 
// What the checkout is told when the parcel ends: that it ended.
public sealed record ParcelResult;

Shopfront.Core/Machines/Parcel/ParcelMachine.cs:

using System.Text.Json.Nodes;
using Trax.StateMachine;
using Trax.StateMachine.Persistence;
 
namespace Shopfront.Machines.Parcel;
 
public sealed class ParcelMachine : Machine<ParcelState, ParcelTrigger>
{
    protected override void Configure(IMachineBuilder<ParcelState, ParcelTrigger> m)
    {
        m.Id("parcel").Version(1).StartsAt(ParcelState.Packed, () => new JsonObject());
 
        m.In(ParcelState.Packed)
            .Context<ParcelContext>()
            .After(TimeSpan.FromMinutes(1))
            .GoTo(ParcelState.InTransit);
        m.In(ParcelState.InTransit)
            .Context<ParcelContext>()
            .After(TimeSpan.FromMinutes(1))
            .GoTo(ParcelState.Delivered);
        m.In(ParcelState.Delivered).Context<ParcelContext>().Ends();
    }
}

Ends() marks a state the machine ends in, which gives its parent OnDone. The parcel is not served over GraphQL: only the checkout that spawned it reaches it. It belongs to the checkout's owner, and it cannot be thrown away while the checkout waits on it.

SDK Reference

Invoke a train from a state | Invoked runs | Spawn a child machine | Machine

Next: 13.4 The checkout invokes and spawns