Update the template screen to use the new(ish) Observation framework. (#4077)

* Update the template screen to use the new(ish) Observation framework.

* Add a variant of deferFulfillment that supports observables.

* Update snapshot fulfilment to work with either a publisher or a stream.

Tools/Scripts/Templates/SimpleScreenExample/ElementX/TemplateScreenModels.swift

@@ -16,6 +16,8 @@ enum TemplateScreenViewModelAction {
 struct TemplateScreenViewState: BindableState {
     var title: String
     var placeholder: String
+    var counter = 0
+    
     var bindings: TemplateScreenViewStateBindings
 }
 
@@ -27,5 +29,8 @@ enum TemplateScreenViewAction {
     case done
     case textChanged
     
+    case incrementCounter
+    case decrementCounter
+    
     // Consider adding CustomStringConvertible conformance if the actions contain PII
 }

Tools/Scripts/Templates/SimpleScreenExample/ElementX/TemplateScreenViewModel.swift

@@ -8,7 +8,7 @@
 import Combine
 import SwiftUI
 
-typealias TemplateScreenViewModelType = StateStoreViewModel<TemplateScreenViewState, TemplateScreenViewAction>
+typealias TemplateScreenViewModelType = StateStoreViewModelV2<TemplateScreenViewState, TemplateScreenViewAction>
 
 class TemplateScreenViewModel: TemplateScreenViewModelType, TemplateScreenViewModelProtocol {
     private let actionsSubject: PassthroughSubject<TemplateScreenViewModelAction, Never> = .init()
@@ -32,6 +32,16 @@ class TemplateScreenViewModel: TemplateScreenViewModelType, TemplateScreenViewMo
             actionsSubject.send(.done)
         case .textChanged:
             MXLog.info("View model: composer text changed to: \(state.bindings.composerText)")
+        case .incrementCounter:
+            Task {
+                try await Task.sleep(for: .seconds(.random(in: 1.0...2.0)))
+                state.counter += 1
+            }
+        case .decrementCounter:
+            Task {
+                try await Task.sleep(for: .seconds(.random(in: 1.0...2.0)))
+                state.counter -= 1
+            }
         }
     }
 }

Tools/Scripts/Templates/SimpleScreenExample/ElementX/View/TemplateScreen.swift

@@ -9,7 +9,7 @@ import Compound
 import SwiftUI
 
 struct TemplateScreen: View {
-    @ObservedObject var context: TemplateScreenViewModel.Context
+    @Bindable var context: TemplateScreenViewModel.Context
     
     var body: some View {
         Form {
@@ -18,9 +18,19 @@ struct TemplateScreen: View {
                         kind: .textField(text: $context.composerText))
                 
                 ListRow(label: .centeredAction(title: L10n.actionDone,
-                                               systemIcon: .doorLeftHandClosed),
+                                               icon: \.leave),
                         kind: .button { context.send(viewAction: .done) })
             }
+            
+            Section {
+                ListRow(label: .default(title: "Counter", icon: \.chart),
+                        details: .counter(context.viewState.counter),
+                        kind: .label)
+                ListRow(label: .default(title: "Increment", icon: \.plus),
+                        kind: .button { context.send(viewAction: .incrementCounter) })
+                ListRow(label: .default(title: "Decrement", icon: \.minus),
+                        kind: .button { context.send(viewAction: .decrementCounter) })
+            }
         }
         .compoundList()
         .navigationTitle(context.viewState.title)
@@ -33,10 +43,29 @@ struct TemplateScreen: View {
 // MARK: - Previews
 
 struct TemplateScreen_Previews: PreviewProvider, TestablePreview {
-    static let viewModel = TemplateScreenViewModel()
+    static let viewModel = makeViewModel()
+    static let incrementedViewModel = makeViewModel(counterValue: 1)
+    
     static var previews: some View {
         NavigationStack {
             TemplateScreen(context: viewModel.context)
         }
+        .previewDisplayName("Initial")
+        
+        NavigationStack {
+            TemplateScreen(context: incrementedViewModel.context)
+        }
+        .previewDisplayName("Incremented")
+        .snapshotPreferences(expect: incrementedViewModel.context.observe(\.viewState.counter).map { $0 == 1 }.eraseToStream())
+    }
+    
+    static func makeViewModel(counterValue: Int = 0) -> TemplateScreenViewModel {
+        let viewModel = TemplateScreenViewModel()
+        
+        for _ in 0..<counterValue {
+            viewModel.context.send(viewAction: .incrementCounter)
+        }
+        
+        return viewModel
     }
 }

Tools/Scripts/Templates/SimpleScreenExample/Tests/Unit/TemplateScreenViewModelTests.swift

@@ -22,13 +22,31 @@ class TemplateScreenViewModelTests: XCTestCase {
     }
 
     func testInitialState() {
-        XCTAssertFalse(context.viewState.placeholder.isEmpty)
         XCTAssertFalse(context.composerText.isEmpty)
+        XCTAssertEqual(context.viewState.counter, 0)
     }
 
-    func testCounter() async throws {
+    func testTextField() async throws {
         context.composerText = "123"
         context.send(viewAction: .textChanged)
         XCTAssertEqual(context.composerText, "123")
     }
+    
+    func testCounter() async throws {
+        var deferred = deferFulfillment(context.observe(\.viewState.counter)) { $0 == 1 }
+        context.send(viewAction: .incrementCounter)
+        try await deferred.fulfill()
+        XCTAssertEqual(context.viewState.counter, 1)
+        
+        deferred = deferFulfillment(context.observe(\.viewState.counter)) { $0 == 3 }
+        context.send(viewAction: .incrementCounter)
+        context.send(viewAction: .incrementCounter)
+        try await deferred.fulfill()
+        XCTAssertEqual(context.viewState.counter, 3)
+        
+        deferred = deferFulfillment(context.observe(\.viewState.counter)) { $0 == 2 }
+        context.send(viewAction: .decrementCounter)
+        try await deferred.fulfill()
+        XCTAssertEqual(context.viewState.counter, 2)
+    }
 }