diff --git a/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/CodeHost.kt b/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/CodeHost.kt
index 823a4b4bf0..461610cd76 100644
--- a/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/CodeHost.kt
+++ b/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/CodeHost.kt
@@ -44,12 +44,15 @@ import kotlinx.coroutines.launch
  *  * From `Starting` to `Running` when a `Zipline` finishes loading.
  *  * From `Running` to `Crashed` when a `Zipline` fails.
  *  * From `Running` to `Running` when the `Zipline` is replaced by a hot-reload.
+ *
+ * @param codeUpdatesFlow Returns a flow that emits a new [CodeSession] each time we should load fresh code.
  */
-internal abstract class CodeHost<A : AppService>(
+internal open class CodeHost<A : AppService>(
   private val dispatchers: TreehouseDispatchers,
   private val appScope: CoroutineScope,
   private val frameClockFactory: FrameClock.Factory,
   val stateStore: StateStore,
+  private val codeUpdatesFlow: Flow<CodeSession<A>>,
 ) {
   /** Contents that this app is currently responsible for. */
   private val listeners = mutableListOf<Listener<A>>()
@@ -76,9 +79,6 @@ internal abstract class CodeHost<A : AppService>(
   val codeSession: CodeSession<A>?
     get() = state.codeSession
 
-  /** Returns a flow that emits a new [CodeSession] each time we should load fresh code. */
-  abstract fun codeUpdatesFlow(): Flow<CodeSession<A>>
-
   fun start() {
     dispatchers.checkUi()
 
@@ -136,7 +136,7 @@ internal abstract class CodeHost<A : AppService>(
 
   private fun CoroutineScope.collectCodeUpdates() {
     launch(dispatchers.zipline) {
-      codeUpdatesFlow().collect {
+      codeUpdatesFlow.collect {
         codeSessionLoaded(it)
       }
     }
diff --git a/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/TreehouseApp.kt b/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/TreehouseApp.kt
index ca16e22a27..744d7b643c 100644
--- a/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/TreehouseApp.kt
+++ b/redwood-treehouse-host/src/commonMain/kotlin/app/cash/redwood/treehouse/TreehouseApp.kt
@@ -47,7 +47,7 @@ public class TreehouseApp<A : AppService> private constructor(
    * Continuously polls for updated code, and emits a new [LoadResult] instance when new code is
    * found.
    */
-  private fun ziplineFlow(): Flow<LoadResult> {
+  private val ziplineFlow: Flow<LoadResult> = run {
     var loader = ZiplineLoader(
       dispatcher = dispatchers.zipline,
       manifestVerifier = factory.manifestVerifier,
@@ -76,7 +76,7 @@ public class TreehouseApp<A : AppService> private constructor(
       }
     }
 
-    return loader.load(
+    loader.load(
       applicationName = spec.name,
       manifestUrlFlow = spec.manifestUrl,
       serializersModule = spec.serializersModule,
@@ -85,25 +85,22 @@ public class TreehouseApp<A : AppService> private constructor(
     }
   }
 
-  private val codeHost = object : CodeHost<A>(
+  private val codeHost = CodeHost(
     dispatchers = dispatchers,
     appScope = appScope,
     frameClockFactory = factory.frameClockFactory,
     stateStore = factory.stateStore,
-  ) {
-    override fun codeUpdatesFlow(): Flow<CodeSession<A>> {
-      return ziplineFlow().mapNotNull { loadResult ->
-        when (loadResult) {
-          is LoadResult.Failure -> {
-            null // EventListener already notified.
-          }
-          is LoadResult.Success -> {
-            createCodeSession(loadResult.zipline)
-          }
+    codeUpdatesFlow = ziplineFlow.mapNotNull { loadResult ->
+      when (loadResult) {
+        is LoadResult.Failure -> {
+          null // EventListener already notified.
+        }
+        is LoadResult.Success -> {
+          createCodeSession(loadResult.zipline)
         }
       }
     }
-  }
+  )
 
   /**
    * Returns the current zipline attached to this host, or null if Zipline hasn't loaded yet. The
@@ -167,7 +164,7 @@ public class TreehouseApp<A : AppService> private constructor(
   private fun createCodeSession(zipline: Zipline): ZiplineCodeSession<A> {
     val appService = spec.create(zipline)
 
-    // Extract the RealEventPublisher() created in ziplineFlow().
+    // Extract the RealEventPublisher() created in ziplineFlow.
     val eventListener = zipline.eventListener as RealEventPublisher.ZiplineEventListener
     val eventPublisher = eventListener.eventPublisher