[feat]: give AnyWorkflow a Workflow conformance (#184)

### Motivation

the fact that `AnyWorkflow` is not itself a `Workflow` creates confusion, and some awkwardness in some of our testing facilities. we'd like to add such a conformance to address these issues, and prevent consumers from having to write such type-erasing wrappers themselves.

### Changes

- add a `Workflow` conformance to `AnyWorkflow`
- replace `RootWorkflow` with `AnyWorkflow` in `WorkflowHostingController`
- expose the wrapped workflow for use in observability code
- add/update tests

Author: jamieQ

Diff for: Workflow/Sources/AnyWorkflow.swift

@@ -18,17 +18,24 @@
 public struct AnyWorkflow<Rendering, Output> {
     private let storage: AnyStorage
 
+    /// The underlying erased workflow instance
+    public var base: Any { storage.base }
+
     private init(storage: AnyStorage) {
         self.storage = storage
     }
 
     /// Initializes a new type-erased wrapper for the given workflow.
     public init<T: Workflow>(_ workflow: T) where T.Rendering == Rendering, T.Output == Output {
-        self.init(storage: Storage<T>(
-            workflow: workflow,
-            renderingTransform: { $0 },
-            outputTransform: { $0 }
-        ))
+        if let workflow = workflow as? AnyWorkflow<Rendering, Output> {
+            self = workflow
+        } else {
+            self.init(storage: Storage<T>(
+                workflow: workflow,
+                renderingTransform: { $0 },
+                outputTransform: { $0 }
+            ))
+        }
     }
 
     /// The underlying workflow's implementation type.
@@ -37,7 +44,17 @@ public struct AnyWorkflow<Rendering, Output> {
     }
 }
 
-extension AnyWorkflow: AnyWorkflowConvertible {
+extension AnyWorkflow: Workflow {
+    public typealias Output = Output
+    public typealias State = Void
+    public typealias Rendering = Rendering
+
+    public func render(state: Void, context: RenderContext<AnyWorkflow<Rendering, Output>>) -> Rendering {
+        storage.render(context: context, key: "") {
+            AnyWorkflowAction(sendingOutput: $0)
+        }
+    }
+
     public func asAnyWorkflow() -> AnyWorkflow<Rendering, Output> {
         return self
     }
@@ -84,6 +101,8 @@ extension AnyWorkflow {
     ///
     /// This type is never used directly.
     fileprivate class AnyStorage {
+        var base: Any { fatalError() }
+
         func render<Parent, Action>(context: RenderContext<Parent>, key: String, outputMap: @escaping (Output) -> Action) -> Rendering where Action: WorkflowAction, Action.WorkflowType == Parent {
             fatalError()
         }
@@ -115,6 +134,8 @@ extension AnyWorkflow {
             self.outputTransform = outputTransform
         }
 
+        override var base: Any { workflow }
+
         override var workflowType: Any.Type {
             return T.self
         }

Diff for: Workflow/Tests/AnyWorkflowTests.swift

@@ -54,6 +54,34 @@ public class AnyWorkflowTests: XCTestCase {
         }
         wait(for: [renderingExpectation, outputExpectation], timeout: 1)
     }
+
+    func testOnlyWrapsOnce() {
+        // direct initializer
+        do {
+            let base = OnOutputWorkflow()
+            let wrappedOnce = AnyWorkflow(base)
+            let wrappedTwice = AnyWorkflow(wrappedOnce)
+
+            XCTAssertNotNil(wrappedOnce.base as? OnOutputWorkflow)
+            XCTAssertNotNil(wrappedTwice.base as? OnOutputWorkflow)
+        }
+
+        // method chaining
+        do {
+            let base = OnOutputWorkflow()
+            let wrappedOnce = base.asAnyWorkflow()
+            let wrappedTwice = base.asAnyWorkflow().asAnyWorkflow()
+
+            XCTAssertNotNil(wrappedOnce.base as? OnOutputWorkflow)
+            XCTAssertNotNil(wrappedTwice.base as? OnOutputWorkflow)
+        }
+    }
+
+    func testBaseValue() {
+        let erased = OnOutputWorkflow().asAnyWorkflow()
+
+        XCTAssertNotNil(erased.base as? OnOutputWorkflow)
+    }
 }
 
 /// Has no state or output, simply renders a reversed string

Diff for: WorkflowTesting/Tests/WorkflowRenderTesterTests.swift

@@ -109,6 +109,40 @@ final class WorkflowRenderTesterTests: XCTestCase {
             .assertNoOutput()
     }
 
+    func test_ignoredOutput_opaqueChild() {
+        OpaqueChildOutputIgnoringWorkflow(
+            childProvider: {
+                OutputWorkflow()
+                    .mapRendering { _ in "screen" }
+                    .asAnyWorkflow()
+            }
+        )
+        .renderTester()
+        .expectWorkflowIgnoringOutput(
+            type: AnyWorkflow<String, OutputWorkflow.Output>.self,
+            producingRendering: "test"
+        )
+        .render { rendering in
+            XCTAssertEqual(rendering, "test")
+        }
+    }
+
+    func test_opaqueChild() {
+        OpaqueChildWorkflow(
+            childProvider: {
+                MockChildWorkflow().asAnyWorkflow()
+            }
+        )
+        .renderTester()
+        .expectWorkflow(
+            type: MockChildWorkflow.self,
+            producingRendering: "test"
+        )
+        .render { rendering in
+            XCTAssertEqual(rendering, "test")
+        }
+    }
+
     func test_childWorkflow() {
         ParentWorkflow(initialText: "hello")
             .renderTester()
@@ -262,6 +296,41 @@ private struct OutputIgnoringWorkflow: Workflow {
     }
 }
 
+private struct MockChildWorkflow: Workflow {
+    typealias State = Void
+    typealias Rendering = String
+
+    func render(state: Void, context: RenderContext<MockChildWorkflow>) -> String {
+        XCTFail("should never be rendered")
+        return ""
+    }
+}
+
+private struct OpaqueChildWorkflow: Workflow {
+    typealias State = Void
+    typealias Rendering = String
+
+    var childProvider: () -> AnyWorkflow<String, Never>
+
+    func render(state: Void, context: RenderContext<Self>) -> Rendering {
+        childProvider()
+            .rendered(in: context)
+    }
+}
+
+private struct OpaqueChildOutputIgnoringWorkflow: Workflow {
+    typealias State = Void
+    typealias Rendering = String
+
+    var childProvider: () -> AnyWorkflow<String, OutputWorkflow.Output>
+
+    func render(state: Void, context: RenderContext<Self>) -> Rendering {
+        childProvider()
+            .ignoringOutput()
+            .rendered(in: context)
+    }
+}
+
 private struct TestSideEffectKey: Hashable {
     let key: String = "Test Side Effect"
 }

Diff for: WorkflowUI/Sources/Hosting/WorkflowHostingController.swift

@@ -29,7 +29,7 @@ public final class WorkflowHostingController<ScreenType, Output>: UIViewControll
 
     private(set) var rootViewController: UIViewController
 
-    private let workflowHost: WorkflowHost<RootWorkflow<ScreenType, Output>>
+    private let workflowHost: WorkflowHost<AnyWorkflow<ScreenType, Output>>
 
     private let (lifetime, token) = Lifetime.make()
 
@@ -45,7 +45,7 @@ public final class WorkflowHostingController<ScreenType, Output>: UIViewControll
         observers: [WorkflowObserver] = []
     ) where W.Rendering == ScreenType, W.Output == Output {
         self.workflowHost = WorkflowHost(
-            workflow: RootWorkflow(workflow),
+            workflow: workflow.asAnyWorkflow(),
             observers: observers
         )
 
@@ -74,7 +74,7 @@ public final class WorkflowHostingController<ScreenType, Output>: UIViewControll
 
     /// Updates the root Workflow in this container.
     public func update<W: AnyWorkflowConvertible>(workflow: W) where W.Rendering == ScreenType, W.Output == Output {
-        workflowHost.update(workflow: RootWorkflow(workflow))
+        workflowHost.update(workflow: workflow.asAnyWorkflow())
     }
 
     public required init?(coder aDecoder: NSCoder) {
@@ -150,25 +150,4 @@ public final class WorkflowHostingController<ScreenType, Output>: UIViewControll
     }
 }
 
-/// Wrapper around an AnyWorkflow that allows us to have a concrete
-/// WorkflowHost without WorkflowHostingController itself being generic
-/// around a Workflow.
-fileprivate struct RootWorkflow<Rendering, Output>: Workflow {
-    typealias State = Void
-    typealias Output = Output
-    typealias Rendering = Rendering
-
-    var wrapped: AnyWorkflow<Rendering, Output>
-
-    init<W: AnyWorkflowConvertible>(_ wrapped: W) where W.Rendering == Rendering, W.Output == Output {
-        self.wrapped = wrapped.asAnyWorkflow()
-    }
-
-    func render(state: State, context: RenderContext<RootWorkflow>) -> Rendering {
-        return wrapped
-            .mapOutput { AnyWorkflowAction(sendingOutput: $0) }
-            .rendered(in: context)
-    }
-}
-
 #endif