Implemented phase 5 for the POC prototype in the Website service.

Services/Website/Sources/Library/Public/Controllers/TradeController.swift

@@ -20,6 +20,9 @@ public struct TradeController<Context: RequestContext> {
     /// The repository listings and shops are read through.
     private let catalogue: CatalogueRepository
 
+    /// Told whenever the controller changes what is for sale, so pages cached elsewhere can catch up.
+    private let onCatalogueChange: @Sendable () async -> Void
+
     /// The store the signed-in account is read from.
     private let sessions: SessionStore
 
@@ -35,14 +38,17 @@ public struct TradeController<Context: RequestContext> {
     ///   - sessions: the store the signed-in account is read from.
     ///   - assetVersion: the version token appended to the pages' asset URLs, or `nil` (the default) to leave them unversioned.
     ///   - analytics: the analytics tracker the pages embed, or `nil` (the default) to omit it.
+    ///   - onCatalogueChange: told whenever the controller changes what is for sale; does nothing by default.
     public init(
         trade: TradeRepository,
         catalogue: CatalogueRepository,
         sessions: SessionStore,
         assetVersion: String? = nil,
-        analytics: Analytics? = nil
+        analytics: Analytics? = nil,
+        onCatalogueChange: @escaping @Sendable () async -> Void = {}
     ) {
         self.analytics = analytics
+        self.onCatalogueChange = onCatalogueChange
         self.assetVersion = assetVersion
         self.catalogue = catalogue
         self.sessions = sessions
@@ -95,7 +101,11 @@ private extension TradeController {
         }
 
         do {
-            return .redirect(to: .Route.sale(try await trade.buy(listing: slug, buyer: user)), type: .normal)
+            let number = try await trade.buy(listing: slug, buyer: user)
+
+            await onCatalogueChange()
+
+            return .redirect(to: .Route.sale(number), type: .normal)
         } catch let error as TradeError {
             return try await sheet(.buy, slug: slug, error: error, viewer: viewer, request: request, context: context)
         }
@@ -409,6 +419,10 @@ private extension TradeController {
         let form = try await URLEncodedFormDecoder().decode(AnswerForm.self, from: request, context: context)
         let number = try? await trade.respond(toOffer: id, accepting: form.answer == "accept", by: user)
 
+        if number.flatMap(\.self) != nil {
+            await onCatalogueChange()
+        }
+
         return .redirect(
             to: number.flatMap(\.self).map(String.Route.sale) ?? .Route.safeReturn(form.return),
             type: .normal