Adds OrderSynchronizer protocol & definitions

Author: Ecarrion

WooCommerce/Classes/ViewRelated/Orders/Order Creation/Synchronizer/OrderSynchronizer.swift

@@ -1 +1,72 @@
 import Foundation
+import Yosemite
+import Combine
+
+/// Possible states of an `OrderSynchronizer` type.
+///
+enum OrderSyncState {
+    case syncing
+    case synced
+    case error(Error)
+}
+
+/// Product input for an `OrderSynchronizer` type.
+///
+struct OrderSyncProductInput {
+    let product: Product
+    let quantity: Int
+}
+
+/// Addresses input for an `OrderSynchronizer` type.
+///
+struct OrderSyncAddressesInput {
+    let billing: Address
+    let shipping: Address
+}
+
+/// A type that  receives "supported" order properties and keeps it synced against another source.
+///
+protocol OrderSynchronizer {
+
+    // MARK: Outputs
+
+    /// Defines the current sync state of the synchronizer.
+    ///
+    var state: Published<OrderSyncState> { get }
+
+    /// Defines the latest order to be synced or that is synced.
+    ///
+    var order: Published<Order> { get }
+
+    // MARK: Inputs
+
+    /// Changes the underlaying order status.
+    /// This property is not synched remotely until `commitAllChanges` method is invoked.
+    ///
+    var setStatus: PassthroughSubject<OrderStatusEnum, Never> { get }
+
+    /// Sets a product with it's quantity.
+    /// Set a `zero` quantity to remove a product.
+    ///
+    var setProduct: PassthroughSubject<OrderSyncProductInput, Never> { get }
+
+    /// Sets or removes the order shipping & billing addresses.
+    ///
+    var setAddresses: PassthroughSubject<OrderSyncAddressesInput?, Never> { get }
+
+    /// Sets or removes a shipping line.
+    ///
+    var setShipping: PassthroughSubject<ShippingLine?, Never> { get }
+
+    /// Sets or removes an order fee.
+    ///
+    var setFee: PassthroughSubject<OrderFeeLine?, Never> { get }
+
+    /// Retires the order sync. State needs to be in `.error` to initiate work.
+    ///
+    func retrySync()
+
+    /// Commits all order changes to the remote source. State needs to be in `.synced` to initiate work.
+    ///
+    func commitAllChanges(onCompletion: (Result<Order, Error>) -> Void)
+}