Issue 351: Update input to root workflow in WorkflowHost

Resolves #351

swift/Workflow/Sources/WorkflowHost.swift

@@ -53,6 +53,19 @@ public final class WorkflowHost<WorkflowType: Workflow> {
 
     }
 
+    /// Update the input for the workflow. Will cause a render pass.
+    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.
+        let output = WorkflowNode<WorkflowType>.Output(
+            outputEvent: nil,
+            debugInfo: WorkflowUpdateDebugInfo(
+                workflowType: "\(WorkflowType.self)",
+                kind: .didUpdate(source: .external)))
+        handle(output: output)
+    }
+
     private func handle(output: WorkflowNode<WorkflowType>.Output) {
         mutableRendering.value = rootNode.render()
 

swift/Workflow/Tests/WorkflowHostTests.swift

@@ -0,0 +1,43 @@
+import XCTest
+import Workflow
+
+
+final class WorkflowHostTests: XCTestCase {
+
+    func test_updatedInputCausesRenderPass() {
+        let host = WorkflowHost(workflow: TestWorkflow(step: .first))
+
+        XCTAssertEqual(1, host.rendering.value)
+
+        host.update(workflow: TestWorkflow(step: .second))
+
+        XCTAssertEqual(2, host.rendering.value)
+    }
+
+    fileprivate struct TestWorkflow: Workflow {
+        var step: Step
+        enum Step {
+            case first
+            case second
+        }
+
+        struct State {}
+        func makeInitialState() -> State {
+            return State()
+        }
+
+        func workflowDidChange(from previousWorkflow: TestWorkflow, state: inout State) {
+        }
+
+        typealias Rendering = Int
+
+        func render(state: State, context: RenderContext<TestWorkflow>) -> Rendering {
+            switch self.step {
+            case .first:
+                return 1
+            case .second:
+                return 2
+            }
+        }
+    }
+}