WIP persist tab state

This is just some tinkering to resolve #116 and after talking with @bryanstern. Wanna iterate on this more and see if we can make this simpler, and also possibly explore implementing an analogous `RetainedStateHolder`.

Another thing this highlights is the fact that we always initially show a progress indicator even when restoring from a loaded state, wonder what we can do to improve that.

circuit-foundation/src/commonMain/kotlin/com/slack/circuit/foundation/CircuitContent.kt

@@ -39,25 +39,7 @@ public fun CircuitContent(
 ) {
   val navigator =
     remember(onNavEvent) {
-      object : Navigator {
-        override fun goTo(screen: Screen) {
-          onNavEvent(NavEvent.GoTo(screen))
-        }
-
-        override fun resetRoot(newRoot: Screen): List<Screen> {
-          onNavEvent(NavEvent.ResetRoot(newRoot))
-          return emptyList()
-        }
-
-        override fun pop(): Screen? {
-          onNavEvent(NavEvent.Pop)
-          return null
-        }
-
-        override fun peek(): Screen {
-          return screen
-        }
-      }
+      Navigator.navEventNavigator(screen, onNavEvent)
     }
   CircuitContent(screen, navigator, modifier, circuit, unavailableContent)
 }

circuit-foundation/src/commonMain/kotlin/com/slack/circuit/foundation/NavEvent.kt

@@ -19,6 +19,31 @@ public fun Navigator.onNavEvent(event: NavEvent) {
   }
 }
 
+public fun Navigator.Companion.navEventNavigator(
+  screen: Screen,
+  onNavEvent: (event: NavEvent) -> Unit,
+): Navigator {
+  return object : Navigator {
+    override fun goTo(screen: Screen) {
+      onNavEvent(NavEvent.GoTo(screen))
+    }
+
+    override fun resetRoot(newRoot: Screen): List<Screen> {
+      onNavEvent(NavEvent.ResetRoot(newRoot))
+      return emptyList()
+    }
+
+    override fun pop(): Screen? {
+      onNavEvent(NavEvent.Pop)
+      return null
+    }
+
+    override fun peek(): Screen {
+      return screen
+    }
+  }
+}
+
 /** A sealed navigation interface intended to be used when making a navigation callback. */
 public sealed interface NavEvent : CircuitUiEvent {
   /** Corresponds to [Navigator.pop]. */

circuit-runtime/src/commonMain/kotlin/com/slack/circuit/runtime/Navigator.kt

@@ -43,6 +43,8 @@ public interface Navigator {
 
     override fun resetRoot(newRoot: Screen): List<Screen> = emptyList()
   }
+
+  public companion object
 }
 
 /** Calls [Navigator.pop] until the given [predicate] is matched or it pops the root. */

samples/star/src/commonMain/kotlin/com/slack/circuit/star/home/HomeScreen.kt

@@ -19,11 +19,14 @@ import androidx.compose.runtime.saveable.rememberSaveableStateHolder
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Color
-import com.slack.circuit.backstack.rememberSaveableBackStack
 import com.slack.circuit.codegen.annotations.CircuitInject
 import com.slack.circuit.foundation.CircuitContent
+import com.slack.circuit.foundation.LocalCircuit
 import com.slack.circuit.foundation.NavEvent
+import com.slack.circuit.foundation.navEventNavigator
 import com.slack.circuit.foundation.onNavEvent
+import com.slack.circuit.foundation.rememberPresenter
+import com.slack.circuit.foundation.rememberUi
 import com.slack.circuit.retained.rememberRetained
 import com.slack.circuit.runtime.CircuitUiEvent
 import com.slack.circuit.runtime.CircuitUiState
@@ -85,12 +88,28 @@ fun HomeContent(state: HomeScreen.State, modifier: Modifier = Modifier) {
       }
     },
   ) { paddingValues ->
-    val screen = state.navItems[state.selectedIndex].screen
-    CircuitContent(
-      screen,
-      modifier = Modifier.padding(paddingValues),
-      onNavEvent = { event -> state.eventSink(ChildNav(event)) },
-    )
+    val saveableStateHolder = rememberSaveableStateHolder()
+    val currentScreen = state.navItems[state.selectedIndex].screen
+    saveableStateHolder.SaveableStateProvider(currentScreen) {
+      val circuit = requireNotNull(LocalCircuit.current)
+      val ui = rememberUi(currentScreen, factory = circuit::ui)
+      val presenter =
+        rememberPresenter(
+          currentScreen,
+          navigator =
+            Navigator.navEventNavigator(currentScreen) { event ->
+              state.eventSink(ChildNav(event))
+            },
+          factory = circuit::presenter,
+        )
+
+      CircuitContent(
+        screen = currentScreen,
+        modifier = Modifier.padding(paddingValues),
+        presenter = presenter!!,
+        ui = ui!!,
+      )
+    }
     contentComposed = true
   }
   Platform.ReportDrawnWhen { contentComposed }