add more context to action handling call & add observation hook for state updates

Author: jamieQ

Workflow/Sources/SubtreeManager.swift

@@ -47,7 +47,10 @@ extension WorkflowNode {
         }
 
         /// Performs an update pass using the given closure.
-        func render<Rendering>(_ actions: (RenderContext<WorkflowType>) -> Rendering) -> Rendering {
+        func render<Rendering>(
+            _ actions: (RenderContext<WorkflowType>) -> Rendering,
+            workflow: WorkflowType
+        ) -> Rendering {
             /// Invalidate the previous action handlers.
             for eventPipe in eventPipes {
                 eventPipe.invalidate()
@@ -58,6 +61,7 @@ extension WorkflowNode {
                 previousSinks: previousSinks,
                 originalChildWorkflows: childWorkflows,
                 originalSideEffectLifetimes: sideEffectLifetimes,
+                workflow: workflow,
                 session: session,
                 observer: observer
             )
@@ -146,14 +150,15 @@ extension WorkflowNode.SubtreeManager {
         private let originalSideEffectLifetimes: [AnyHashable: SideEffectLifetime]
         internal private(set) var usedSideEffectLifetimes: [AnyHashable: SideEffectLifetime]
 
-        private var session: WorkflowSession
-
-        private var observer: WorkflowObserver?
+        private let workflow: WorkflowType
+        private let session: WorkflowSession
+        private let observer: WorkflowObserver?
 
         internal init(
             previousSinks: [ObjectIdentifier: AnyReusableSink],
             originalChildWorkflows: [ChildKey: AnyChildWorkflow],
             originalSideEffectLifetimes: [AnyHashable: SideEffectLifetime],
+            workflow: WorkflowType,
             session: WorkflowSession,
             observer: WorkflowObserver?
         ) {
@@ -167,6 +172,7 @@ extension WorkflowNode.SubtreeManager {
             self.originalSideEffectLifetimes = originalSideEffectLifetimes
             self.usedSideEffectLifetimes = [:]
 
+            self.workflow = workflow
             self.session = session
             self.observer = observer
         }
@@ -231,13 +237,23 @@ extension WorkflowNode.SubtreeManager {
 
             let signpostRef = SignpostRef()
 
-            let sink = Sink<Action> { [observer, session] action in
+            // make the optional observer callback. doing this conditionally
+            // means we can avoid capturing values in the Sink closure unless
+            // they are actually needed.
+            let notifyObserver = observer.map { observer in
+                { [observer, workflow, session] (action: Action) in
+                    observer.workflowDidReceiveAction(
+                        action,
+                        workflow: workflow,
+                        session: session
+                    )
+                }
+            }
+
+            let sink = Sink<Action> { action in
                 WorkflowLogger.logSinkEvent(ref: signpostRef, action: action)
 
-                observer?.workflowDidReceiveAction(
-                    action,
-                    session: session
-                )
+                notifyObserver?(action)
 
                 reusableSink.handle(action: action)
             }

Workflow/Sources/WorkflowNode.swift

@@ -80,6 +80,13 @@ final class WorkflowNode<WorkflowType: Workflow> {
 
         switch subtreeOutput {
         case .update(let event, let source):
+            let actionObserverCompletion = observer?.workflowWillApplyAction(
+                event,
+                workflow: workflow,
+                state: state,
+                session: session
+            )
+
             /// Apply the update to the current state
             let outputEvent = event.apply(toState: &state)
 
@@ -92,6 +99,8 @@ final class WorkflowNode<WorkflowType: Workflow> {
                 )
             )
 
+            actionObserverCompletion?(state, outputEvent)
+
         case .childDidUpdate(let debugInfo):
             output = Output(
                 outputEvent: nil,
@@ -127,13 +136,13 @@ final class WorkflowNode<WorkflowType: Workflow> {
             WorkflowLogger.logWorkflowFinishedRendering(ref: self, isRootNode: isRootNode)
         }
 
-        rendering = subtreeManager.render { context in
+        rendering = subtreeManager.render({ context in
             workflow
                 .render(
                     state: state,
                     context: context
                 )
-        }
+        }, workflow: workflow)
 
         return rendering
     }

Workflow/Sources/WorkflowObserver.swift

@@ -75,8 +75,16 @@ public protocol WorkflowObserver {
     ///   - session: The `WorkflowSession` corresponding to the backing `WorkflowNode`.
     func workflowDidReceiveAction<Action: WorkflowAction>(
         _ action: Action,
+        workflow: Action.WorkflowType,
         session: WorkflowSession
     )
+
+    func workflowWillApplyAction<Action: WorkflowAction>(
+        _ action: Action,
+        workflow: Action.WorkflowType,
+        state: Action.WorkflowType.State,
+        session: WorkflowSession
+    ) -> ((Action.WorkflowType.State, Action.WorkflowType.Output?) -> Void)?
 }
 
 // MARK: - WorkflowSession
@@ -171,8 +179,16 @@ public extension WorkflowObserver {
 
     func workflowDidReceiveAction<Action: WorkflowAction>(
         _ action: Action,
+        workflow: Action.WorkflowType,
         session: WorkflowSession
     ) {}
+
+    func workflowWillApplyAction<Action: WorkflowAction>(
+        _ action: Action,
+        workflow: Action.WorkflowType,
+        state: Action.WorkflowType.State,
+        session: WorkflowSession
+    ) -> ((Action.WorkflowType.State, Action.WorkflowType.Output?) -> Void)? { nil }
 }
 
 // MARK: Chained Observer
@@ -240,13 +256,40 @@ final class ChainedWorkflowObserver: WorkflowObserver {
         }
     }
 
-    func workflowDidReceiveAction<Action>(_ action: Action, session: WorkflowSession) where Action: WorkflowAction {
+    func workflowDidReceiveAction<Action>(_ action: Action, workflow: Action.WorkflowType, session: WorkflowSession) where Action: WorkflowAction {
         for observer in observers {
             observer.workflowDidReceiveAction(
                 action,
+                workflow: workflow,
+                session: session
+            )
+        }
+    }
+
+    func workflowWillApplyAction<Action>(
+        _ action: Action,
+        workflow: Action.WorkflowType,
+        state: Action.WorkflowType.State,
+        session: WorkflowSession
+    ) -> ((Action.WorkflowType.State, Action.WorkflowType.Output?) -> Void)? where Action: WorkflowAction {
+        let callbacks = observers.compactMap {
+            $0.workflowWillApplyAction(
+                action,
+                workflow: workflow,
+                state: state,
                 session: session
             )
         }
+
+        guard !callbacks.isEmpty else {
+            return nil
+        }
+
+        return { state, output in
+            for callback in callbacks.reversed() {
+                callback(state, output)
+            }
+        }
     }
 }
 

Workflow/Tests/SubtreeManagerTests.swift

@@ -338,3 +338,11 @@ private extension WorkflowNode.SubtreeManager {
         )
     }
 }
+
+private extension WorkflowNode.SubtreeManager where WorkflowType == ParentWorkflow {
+    func render<Rendering>(
+        _ actions: (RenderContext<WorkflowType>) -> Rendering
+    ) -> Rendering {
+        render(actions, workflow: ParentWorkflow())
+    }
+}