sketch out CompositionLocal propagation

Author: zach-klippenstein

workflow-core/src/commonMain/kotlin/com/squareup/workflow1/compose/WorkflowComposables.kt

@@ -46,6 +46,8 @@ fun <ChildPropsT, ChildOutputT, ChildRenderingT> renderWorkflow(
     }
   }
 
+// region Convenience overloads for specific type arguments
+
 /**
  * Renders a child [Workflow] that has no output (`OutputT` is [Nothing]).
  * For more documentation see [renderWorkflow].
@@ -81,3 +83,5 @@ inline fun <ChildOutputT, ChildRenderingT> renderWorkflow(
 inline fun <ChildRenderingT> renderWorkflow(
   workflow: Workflow<Unit, Nothing, ChildRenderingT>,
 ): ChildRenderingT = renderWorkflow(workflow, Unit, onOutput = null)
+
+// endregion

workflow-runtime/src/commonMain/kotlin/com/squareup/workflow1/internal/ComposedWorkflowChild.kt

@@ -1,6 +1,6 @@
 package com.squareup.workflow1.internal
 
-import androidx.compose.runtime.CompositionContext
+import androidx.compose.runtime.CompositionLocalContext
 import androidx.compose.runtime.RecomposeScope
 import androidx.compose.runtime.RememberObserver
 import com.squareup.workflow1.WorkflowAction
@@ -10,8 +10,8 @@ import kotlinx.coroutines.CoroutineScope
 internal class ComposedWorkflowChild<ChildOutputT, ParentPropsT, ParentOutputT, ParentRenderingT>(
   compositeHashKey: Int,
   private val coroutineScope: CoroutineScope,
-  private val compositionContext: CompositionContext,
-  private val recomposeScope: RecomposeScope
+  private val recomposeScope: RecomposeScope,
+  private val localsContext: CompositionLocalContext,
 ) : RememberObserver {
   val workflowKey: String = "composed-workflow:${compositeHashKey.toString(radix = 16)}"
   private var disposed = false

workflow-runtime/src/commonMain/kotlin/com/squareup/workflow1/internal/SubtreeManager.kt

@@ -4,15 +4,15 @@ package com.squareup.workflow1.internal
 
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.Composition
+import androidx.compose.runtime.CompositionLocalContext
 import androidx.compose.runtime.CompositionLocalProvider
 import androidx.compose.runtime.MonotonicFrameClock
 import androidx.compose.runtime.Recomposer
 import androidx.compose.runtime.currentCompositeKeyHash
+import androidx.compose.runtime.currentCompositionLocalContext
 import androidx.compose.runtime.currentRecomposeScope
-import androidx.compose.runtime.key
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
-import androidx.compose.runtime.rememberCompositionContext
 import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.saveable.LocalSaveableStateRegistry
 import androidx.compose.runtime.saveable.SaveableStateRegistry
@@ -176,6 +176,7 @@ internal class SubtreeManager<PropsT, StateT, OutputT>(
     val recomposer = Recomposer(coroutineContext)
     val composition = Composition(UnitApplier, recomposer)
     val saveableStateRegistry: SaveableStateRegistry // TODO
+    val localsContext: CompositionLocalContext? // TODO
 
     // TODO I think we need more than a simple UNDISPATCHED start to make this work – we have to
     //  pump the dispatcher until the composition is finished.
@@ -188,7 +189,7 @@ internal class SubtreeManager<PropsT, StateT, OutputT>(
     }
 
     val rendering = mutableStateOf<ChildRenderingT?>(null)
-    composition.setContent {
+    val wrappedContent = @Composable {
       CompositionLocalProvider(
         LocalWorkflowCompositionHost provides this,
         LocalSaveableStateRegistry provides saveableStateRegistry,
@@ -197,6 +198,19 @@ internal class SubtreeManager<PropsT, StateT, OutputT>(
       }
     }
 
+    composition.setContent {
+      // Must provide the locals from the parent composition first so we can override the ones we
+      // need. If it's null then there's no parent, but the CompositionLocalProvider API has no nice
+      // way to pass nothing in this overload. I believe it's safe to actually call content through
+      // two different code paths because whether there's a parent composition cannot change for an
+      // existing workflow session – they can't move.
+      if (localsContext == null) {
+        wrappedContent()
+      } else {
+        CompositionLocalProvider(localsContext, wrappedContent)
+      }
+    }
+
     // TODO prime the first frame to generate the initial rendering
 
     @Suppress("UNCHECKED_CAST")
@@ -218,14 +232,14 @@ internal class SubtreeManager<PropsT, StateT, OutputT>(
 
     val key = currentCompositeKeyHash
     val coroutineScope = rememberCoroutineScope()
-    val compositionContext = rememberCompositionContext()
+    val localsContext = currentCompositionLocalContext
     val recomposeScope = currentRecomposeScope
     val child = remember {
-      ComposedWorkflowChild(
-        key,
-        coroutineScope,
-        compositionContext,
-        recomposeScope
+      ComposedWorkflowChild<ChildOutputT, PropsT, OutputT, Unit /* TODO */>(
+        compositeHashKey = key,
+        coroutineScope = coroutineScope,
+        recomposeScope = recomposeScope,
+        localsContext = localsContext,
       )
     }
     child.onOutput = onOutput
@@ -236,7 +250,7 @@ internal class SubtreeManager<PropsT, StateT, OutputT>(
     // tracking. After the composition frame is finished, we can update the WorkflowNode state as
     // required.
     // TODO don't call render, it's not powerful enough for what we need.
-    render(
+    return render(
       child = workflow,
       props = props,
       key = child.workflowKey,