Rewrite split order state machine article

guides/orders/order-state-machine.md

@@ -0,0 +1,72 @@
+# Order state machine
+
+Orders flow through a state machine, which is defined in the
+`Spree::Order::Checkout` module. It begins with a `cart` state and ends with
+at a `complete` state.
+
+There are six order states by default:
+
+1. `cart`
+2. `address`
+3. `delivery`
+4. `payment`
+5. `confirm`
+6. `complete`
+
+If you go through the checkout provided in the `solidus_frontend` gem, you can
+see that there is a clearly defined step for each of these states during checkout.
+
+In some circumstances, the `payment` and `confirm` states are optional:
+
+- The `payment` state is only triggered if `payment_required?` returns `true`.
+- The `confirm` state is only triggered if `confirmation_required?` returns
+  `true`.
+
+The `complete` state is triggered in one of two ways:
+
+1. No payment is required on the order. (The `total` equals `0`.)
+2. Payment is required on the order, and at least the order total has been
+   received as payment.
+
+## State criteria
+
+Each order state has criteria that must be met before the state can change. For
+example, before the state can change from `cart` to `address`, line items must be
+present on the order. Then, to change from `address` to `delivery`, the user
+must have a default address assigned.
+
+Once an order meets the criteria to go to the next state, you can call `next` on
+the order object to transition into the next state. For example, in the
+`solidus_frontend` gem, the [`Spree::CheckoutController`][checkout-controller]
+defines a `transition_forward` method that always calls `next` unless the order
+can be completed:
+
+```ruby
+# /frontend/app/controllers/spree/checkout_controller.rb
+def transition_forward
+  if @order.can_complete?
+    @order.complete
+  else
+    @order.next
+  end
+end
+``` 
+
+If `next` returns `false`, then the order does not meet the criteria and does
+not transition to the next state.
+
+[checkout-controller]: https://github.com/solidusio/solidus/blob/master/frontend/app/controllers/spree/checkout_controller.rb
+
+## Payments and shipments have their own state machines
+
+Note that a `Spree::Order` with the state `complete` does not mean that the
+payment has been processed or the order shipments have been shipped. See also the
+values of the `payment_state` and `shipment_state` attributes.
+
+<!-- TODO:
+  Once it's merged, link to documentation about checkout. This section only
+  attempts to summarize some of what happens during the checkout flow.
+
+  It may be useful to link to the payments documentation once it's merged, too.
+-->
+

guides/orders/overview.md

@@ -74,38 +74,6 @@ information, see the [Display totals methods][display-total-methods] article.
 [special-instructions]: ../shipments/user-interface-for-shipments.md#shipping-instructions
 [update-orders]: update-orders.md
 
-## The Order State Machine
-
-Orders flow through a state machine, beginning at a `cart` state and ending up
-at a `complete` state. The intermediary states can be configured using the
-[Checkout Flow API](checkout).
-
-The default states are as follows:
-
-* `cart`
-* `address`
-* `delivery`
-* `payment`
-* `confirm`
-* `complete`
-
-The `payment` state will only be triggered if `payment_required?` returns
-`true`.
-
-The `confirm` state will only be triggered if `confirmation_required?` returns
-`true`.
-
-The `complete` state can only be reached in one of two ways:
-
-1. No payment is required on the order.
-2. Payment is required on the order, and at least the order total has been
-received as payment.
-
-Assuming that an order meets the criteria for the next state, you will be able
-to transition it to the next state by calling `next` on that object. If this
-returns `false`, then the order does *not* meet the criteria. To work out why it
-cannot transition, check the result of an `errors` method call.
-
 ## Related models
 
 The `Spree::Order` model touches many other models. This section highlights