[QoL] Allow RenderTester to recover and continue from missing AnyScreen-producing child workflows

Package.swift

@@ -99,9 +99,14 @@ let package = Package(
         ),
         .target(
             name: "WorkflowTesting",
-            dependencies: ["Workflow"],
+            dependencies: ["Workflow", "WorkflowUI"],
             path: "WorkflowTesting/Sources"
         ),
+        .testTarget(
+            name: "WorkflowTestingTests",
+            dependencies: ["Workflow", "WorkflowTesting", "WorkflowUI"],
+            path: "WorkflowTesting/Tests"
+        ),
         .target(
             name: "WorkflowReactiveSwiftTesting",
             dependencies: ["WorkflowReactiveSwift", "WorkflowTesting"],

WorkflowTesting.podspec

@@ -19,6 +19,7 @@ Pod::Spec.new do |s|
   s.source_files = 'WorkflowTesting/Sources/**/*.swift'
 
   s.dependency 'Workflow', "#{s.version}"
+  s.dependency 'WorkflowUI', "#{s.version}"
   s.framework = 'XCTest'
 
   s.test_spec 'Tests' do |test_spec|

WorkflowTesting/Sources/Internal/RenderTester+TestContext.swift

@@ -19,6 +19,10 @@
     import XCTest
     @testable import Workflow
 
+    #if canImport(UIKit) && canImport(WorkflowUI)
+        import WorkflowUI
+    #endif
+
     extension RenderTester {
         internal final class TestContext: RenderContextType {
             var state: WorkflowType.State
@@ -68,10 +72,18 @@
 
                     // We can “recover” from missing Void-rendering workflows since there’s only one possible value to return
                     if Child.Rendering.self == Void.self {
-                        // Couldn’t find a nicer way to do this polymorphically
                         return () as! Child.Rendering
                     }
-                    fatalError("Unable to continue.")
+
+                    #if canImport(UIKit) && canImport(WorkflowUI)
+                        // We can "recover" from missing AnyScreen-rendering workflows since they render an opaque type that we can construct a value of
+                        if Child.Rendering.self == AnyScreen.self {
+                            return RenderTesterPlaceholderScreen().asAnyScreen() as! Child.Rendering
+                        }
+                    #endif
+
+                    // At this point, without an return value from an expectation, we have nothing to return from render and are unable to continue
+                    fatalError("Unable to continue; no expectation has given RenderTester a value to return from `render` (and cannot construct an arbitrary value of type \"\(Child.Rendering.self)\").")
                 }
                 let (inserted, _) = usedWorkflowKeys.insert(WorkflowKey(type: ObjectIdentifier(Child.self), key: key))
                 if !inserted {

WorkflowTesting/Sources/Internal/RenderTesterPlaceholderScreen.swift

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2020 Square Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#if DEBUG && canImport(UIKit) && canImport(WorkflowUI)
+
+    import WorkflowUI
+    import XCTest
+
+    /// Used as the stand-in value returned by RenderTester when an AnyScreen is expected but not provided
+    struct RenderTesterPlaceholderScreen: Screen {
+        func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+            ViewControllerDescription(
+                type: UIViewController.self,
+                build: {
+                    XCTFail("Unexpected construction of screen in RenderTester")
+                    return UIViewController()
+                },
+                update: { _ in }
+            )
+        }
+    }
+
+#endif

WorkflowTesting/Tests/WorkflowRenderTesterFailureTests.swift

@@ -16,8 +16,13 @@
 
 import Workflow
 import WorkflowTesting
+import WorkflowUI
 import XCTest
 
+#if canImport(UIKit)
+    import UIKit
+#endif
+
 /// Who tests the tester?
 ///
 /// WorkflowRenderTesterFailureTests does.
@@ -132,6 +137,42 @@ final class WorkflowRenderTesterFailureTests: XCTestCase {
         }
     }
 
+    #if canImport(UIKit)
+        func test_childWorkflow_unexpected_anyScreenRendering() {
+            struct MyScreen: Screen {
+                func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+                    return ViewControllerDescription(
+                        type: UIViewController.self,
+                        build: { UIViewController() },
+                        update: { _ in }
+                    )
+                }
+            }
+
+            struct MyChildWorkflow: Workflow {
+                typealias State = Void
+                func render(state: State, context: RenderContext<Self>) -> AnyScreen {
+                    return MyScreen().asAnyScreen()
+                }
+            }
+
+            struct MyWorkflow: Workflow {
+                typealias State = Void
+                func render(state: State, context: RenderContext<Self>) -> AnyScreen {
+                    let childRendering = MyChildWorkflow().rendered(in: context)
+                    return childRendering
+                }
+            }
+
+            let tester = MyWorkflow()
+                .renderTester()
+
+            expectingFailure(#"Unexpected workflow of type MyChildWorkflow with key """#) {
+                tester.render { _ in }
+            }
+        }
+    #endif
+
     func test_childWorkflowMultipleRenders_sameKey() {
         let tester = ParentWorkflow()
             .renderTester()