Keep list reloads in the same view branch as loaded data

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Changed
+- List screens switch on `DataState.listContent(isEmpty:)`, where a reload with data on screen is the same `.list` case as loaded data instead of a `switch` branch of its own. A separate branch makes SwiftUI rebuild the list on every reload, re-register its `navigationDestination` and re-run its `.task`; in MyTurnTagCreator that turned a reload after each tag change into a loop that hit the API's rate limit. Nothing here reloads that way yet, so this removes the trap rather than a live bug (Shops, number tags)
 - Error messages are localized (English and Japanese). A request that fails without the API's `error_message` shows "Something went wrong while communicating with the server. Please try again later." (`NATIVEAPPTEMPLATE-2001`); developer-only errors (`NATIVEAPPTEMPLATE-1001`, `2002`–`2008`, `4001`–`4004`) show "An unexpected error occurred." The API's `error_message` is shown without the " [Status: nnn]" suffix. Error codes are unchanged; status codes, field names and underlying errors go to the log (`CodedError.debugDescription`)
 
 ### Fixed

NativeAppTemplate/Data/DataState.swift

@@ -9,3 +9,31 @@ enum DataState {
     case hasData
     case failed
 }
+
+/// What a list screen shows for its `DataState`.
+enum ListContent {
+    case loading
+    case list
+    case failed
+}
+
+extension DataState {
+    /// A reload keeps the list already on screen instead of blinking a full-screen
+    /// LoadingView over it. It is the same `.list` case as `.hasData` on purpose: a
+    /// separate `switch` branch is a different view to SwiftUI, so the list would be
+    /// rebuilt on every reload, re-registering its `navigationDestination` and re-running
+    /// its `.task`. In MyTurnTagCreator that turned a reload after each tag change into a
+    /// loop that hit the API's rate limit.
+    func listContent(isEmpty: Bool) -> ListContent {
+        switch self {
+        case .initial:
+            .loading
+        case .loading:
+            isEmpty ? .loading : .list
+        case .hasData:
+            .list
+        case .failed:
+            .failed
+        }
+    }
+}

NativeAppTemplate/UI/Shop List/ShopListView.swift

@@ -62,18 +62,10 @@ extension ShopListView {
 private extension ShopListView {
     var contentView: some View {
         @ViewBuilder var contentView: some View {
-            switch viewModel.state {
-            case .initial:
-                LoadingView()
+            switch viewModel.state.listContent(isEmpty: viewModel.isEmpty) {
             case .loading:
-                // Keep showing the existing list while refreshing so re-appearance
-                // reloads don't blink a full-screen LoadingView over loaded data.
-                if viewModel.isEmpty {
-                    LoadingView()
-                } else {
-                    shopListView
-                }
-            case .hasData:
+                LoadingView()
+            case .list:
                 shopListView
             case .failed:
                 reloadView

NativeAppTemplate/UI/Shop Settings/ItemTag List/ItemTagListView.swift

@@ -30,18 +30,10 @@ private extension ItemTagListView {
             if viewModel.isBusy {
                 LoadingView()
             } else {
-                switch viewModel.state {
-                case .initial:
-                    LoadingView()
+                switch viewModel.state.listContent(isEmpty: viewModel.isEmpty) {
                 case .loading:
-                    // Keep showing the existing list while refreshing so re-appearance
-                    // reloads don't blink a full-screen LoadingView over loaded data.
-                    if viewModel.isEmpty {
-                        LoadingView()
-                    } else {
-                        itemTagListView
-                    }
-                case .hasData:
+                    LoadingView()
+                case .list:
                     itemTagListView
                 case .failed:
                     reloadView

NativeAppTemplateTests/Data/DataStateListContentTest.swift

@@ -0,0 +1,39 @@
+//
+//  DataStateListContentTest.swift
+//  NativeAppTemplate
+//
+
+@testable import NativeAppTemplate
+import Testing
+
+/// List screens switch on `listContent(isEmpty:)`. A reload of a list already on screen
+/// must stay `.list`, the same case as `.hasData`: as a branch of its own, SwiftUI rebuilds
+/// the list on every reload and re-runs its `.task`. In MyTurnTagCreator 2.1.0 that made the
+/// Shops tab reload about twice a second while a shop was open.
+@Suite
+struct DataStateListContentTest {
+    @Test
+    func reloadingWithDataKeepsTheList() {
+        // A reload with shops on screen (.loading), then .hasData: both the one `.list` case.
+        #expect(DataState.loading.listContent(isEmpty: false) == .list)
+        #expect(DataState.hasData.listContent(isEmpty: false) == .list)
+    }
+
+    @Test
+    func firstLoadShowsLoading() {
+        #expect(DataState.initial.listContent(isEmpty: true) == .loading)
+        #expect(DataState.loading.listContent(isEmpty: true) == .loading)
+    }
+
+    @Test
+    func loadedEmptyListIsStillTheList() {
+        // An empty result is shown by the list view itself (e.g. "no shops yet")
+        #expect(DataState.hasData.listContent(isEmpty: true) == .list)
+    }
+
+    @Test
+    func failureShowsTheErrorView() {
+        #expect(DataState.failed.listContent(isEmpty: true) == .failed)
+        #expect(DataState.failed.listContent(isEmpty: false) == .failed)
+    }
+}