Issue 443: Add typed sink allowing sinks to be reused across render p…

…asses.

When the UI does not update synchronously from a render pass, it may send
actions to the previous sink. If the same type of sink was declared on the
subsequent render pass, proxy the action to the new event pipe.

Fixes #443

swift/Workflow/Sources/SubtreeManager.swift

@@ -13,6 +13,9 @@ extension WorkflowNode {
         /// Sinks from the outside world (i.e. UI)
         private var eventPipes: [EventPipe] = []
 
+        /// Typed sinks from the previous render pass
+        private var previousSinks: [ObjectIdentifier:AnyTypedSink] = [:]
+
         /// The current array of children
         private (set) internal var childWorkflows: [ChildKey:AnyChildWorkflow] = [:]
 
@@ -34,6 +37,7 @@ extension WorkflowNode {
 
             /// Create a workflow context containing the existing children
             let context = Context(
+                previousSinks: previousSinks,
                 originalChildWorkflows: childWorkflows,
                 originalChildWorkers: childWorkers)
 
@@ -52,8 +56,12 @@ extension WorkflowNode {
             /// Merge all of the signals together from the subscriptions.
             self.subscriptions = Subscriptions(eventSources: context.eventSources, eventPipe: EventPipe())
 
+            /// Captured the typed sinks from this render pass to allow reuse.
+            self.previousSinks = context.sinkStore.usedSinks
+
             /// Capture all the pipes to be enabled after render completes.
             self.eventPipes = context.eventPipes
+            self.eventPipes.append(contentsOf: context.sinkStore.eventPipes())
             self.eventPipes.append(self.subscriptions.eventPipe)
 
             /// Set all event pipes to `pending`.
@@ -119,12 +127,16 @@ extension WorkflowNode.SubtreeManager {
 
 }
 
+// MARK: - Render Context
+
 extension WorkflowNode.SubtreeManager {
 
     /// The workflow context implementation used by the subtree manager.
     fileprivate final class Context: RenderContextType {
 
         private (set) internal var eventPipes: [EventPipe]
+
+        private (set) internal var sinkStore: SinkStore
 
         private let originalChildWorkflows: [ChildKey:AnyChildWorkflow]
         private (set) internal var usedChildWorkflows: [ChildKey:AnyChildWorkflow]
@@ -134,9 +146,11 @@ extension WorkflowNode.SubtreeManager {
 
         private (set) internal var eventSources: [Signal<AnyWorkflowAction<WorkflowType>, NoError>] = []
 
-        internal init(originalChildWorkflows: [ChildKey:AnyChildWorkflow], originalChildWorkers: [AnyChildWorker]) {
+        internal init(previousSinks: [ObjectIdentifier:AnyTypedSink], originalChildWorkflows: [ChildKey:AnyChildWorkflow], originalChildWorkers: [AnyChildWorker]) {
             self.eventPipes = []
 
+            self.sinkStore = SinkStore(previousSinks: previousSinks)
+
             self.originalChildWorkflows = originalChildWorkflows
             self.usedChildWorkflows = [:]
 
@@ -191,13 +205,12 @@ extension WorkflowNode.SubtreeManager {
 
         func makeSink<Action>(of actionType: Action.Type) -> Sink<Action> where Action : WorkflowAction, WorkflowType == Action.WorkflowType {
 
-            let eventPipe = EventPipe()
+            let typedSink = sinkStore.findOrCreate(actionType: Action.self)
 
             let sink = Sink<Action> { action in
-                let event = Output.update(AnyWorkflowAction(action), source: .external)
-                eventPipe.handle(event: event)
+                typedSink.handle(action: action)
             }
-            eventPipes.append(eventPipe)
+
             return sink
         }
 
@@ -227,6 +240,70 @@ extension WorkflowNode.SubtreeManager {
 }
 
 
+// MARK: - Reusable Sink
+
+extension WorkflowNode.SubtreeManager {
+    fileprivate struct SinkStore {
+        private var previousSinks: [ObjectIdentifier:AnyTypedSink]
+        private (set) var usedSinks: [ObjectIdentifier:AnyTypedSink]
+
+        init(previousSinks: [ObjectIdentifier:AnyTypedSink]) {
+            self.previousSinks = previousSinks
+            self.usedSinks = [:]
+        }
+
+        mutating func findOrCreate<Action: WorkflowAction>(actionType: Action.Type) -> TypedSink<Action> {
+            let key = ObjectIdentifier(actionType)
+
+            let typedSink: TypedSink<Action>
+
+            if let previousSink = previousSinks.removeValue(forKey: key) as? TypedSink<Action> {
+                // Reused a previous sink, creating a new event pipe to send the action through.
+                previousSink.eventPipe = EventPipe()
+                typedSink = previousSink
+            } else if let usedSink = usedSinks[key] as? TypedSink<Action> {
+                // Multiple sinks using the same backing sink.
+                typedSink = usedSink
+            } else {
+                // Create a new typed sink.
+                typedSink = TypedSink<Action>()
+            }
+
+            usedSinks[key] = typedSink
+
+            return typedSink
+        }
+
+        func eventPipes() -> [EventPipe] {
+            let eventPipes = usedSinks.values.map { typedSink -> EventPipe in
+                typedSink.eventPipe
+            }
+
+            return eventPipes
+        }
+    }
+
+    fileprivate class AnyTypedSink {
+        var eventPipe: EventPipe
+
+        init() {
+            eventPipe = EventPipe()
+        }
+    }
+
+    fileprivate final class TypedSink<Action: WorkflowAction>: AnyTypedSink where Action.WorkflowType == WorkflowType {
+
+        func handle(action: Action) {
+            let output = Output.update(AnyWorkflowAction(action), source: .external)
+
+            eventPipe.handle(event: output)
+        }
+    }
+}
+
+
+// MARK: - EventPipe
+
 extension WorkflowNode.SubtreeManager {
     fileprivate final class EventPipe {
 
@@ -295,6 +372,8 @@ extension WorkflowNode.SubtreeManager {
     }
 }
 
+// MARK: - ChildKey
+
 extension WorkflowNode.SubtreeManager {
 
     struct ChildKey: Hashable {
@@ -320,6 +399,8 @@ extension WorkflowNode.SubtreeManager {
 
 }
 
+// MARK: - Workers
+
 extension WorkflowNode.SubtreeManager {
 
     /// Abstract base class for running children in the subtree.
@@ -372,6 +453,8 @@ extension WorkflowNode.SubtreeManager {
 }
 
 
+// MARK: - Subscriptions
+
 extension WorkflowNode.SubtreeManager {
     fileprivate final class Subscriptions {
         private var (lifetime, token) = Lifetime.make()
@@ -395,6 +478,8 @@ extension WorkflowNode.SubtreeManager {
 }
 
 
+// MARK: - Child Workflows
+
 extension WorkflowNode.SubtreeManager {
 
     /// Abstract base class for running children in the subtree.
@@ -415,9 +500,9 @@ extension WorkflowNode.SubtreeManager {
         }
 
     }
-    
+
     fileprivate final class ChildWorkflow<W: Workflow>: AnyChildWorkflow {
-        
+
         private let node: WorkflowNode<W>
         private var outputMap: (W.Output) -> AnyWorkflowAction<WorkflowType>
 

swift/Workflow/Sources/WorkflowHost.swift

@@ -57,7 +57,7 @@ public final class WorkflowHost<WorkflowType: Workflow> {
     public func update(workflow: WorkflowType) {
         rootNode.update(workflow: workflow)
 
-        // Treat the update as an "output" from the workflow as an external event to force a render pass.
+        // Treat the update as an "output" from the workflow originating from an external event to force a render pass.
         let output = WorkflowNode<WorkflowType>.Output(
             outputEvent: nil,
             debugInfo: WorkflowUpdateDebugInfo(