Cleanup how ComposableViewFactory handles initial rendering.

Duplicate of square/workflow#1146

Author: zach-klippenstein

core-compose/src/main/java/com/squareup/workflow/ui/compose/ComposeViewFactory.kt

@@ -25,6 +25,9 @@ import android.widget.FrameLayout
 import androidx.compose.Composable
 import androidx.compose.Recomposer
 import androidx.compose.StructurallyEqual
+import androidx.compose.frames.commit
+import androidx.compose.frames.inFrame
+import androidx.compose.frames.open
 import androidx.compose.mutableStateOf
 import androidx.ui.core.setContent
 import com.squareup.workflow.ui.ViewEnvironment
@@ -78,26 +81,50 @@ internal class ComposeViewFactory<RenderingT : Any>(
     // Composable function, so we need to use ViewGroup.setContent.
     val composeContainer = FrameLayout(contextForNewView)
 
-    // This model will be used to feed state updates into the composition.
+    // Create a single MutableState to feed state updates into the composition.
+    // We could also have two separate MutableStates, but using a Pair both makes it clear and
+    // enforces that both values are always updated together.
     val renderState = mutableStateOf<Pair<RenderingT, ViewEnvironment>?>(
+        // This will be updated immediately by bindShowRendering below.
         value = null,
         areEquivalent = StructurallyEqual
     )
 
-    // Entry point to the composition.
+    // If this is the first ever compose call in this process, we need to manually open a frame
+    // before updating the MutableState.
+    ensureComposeFrame {
+      // Update the state whenever a new rendering is emitted.
+      composeContainer.bindShowRendering(
+          initialRendering,
+          initialViewEnvironment
+      ) { rendering, environment ->
+        // This will be called synchronously before bindShowRendering returns.
+        renderState.value = Pair(rendering, environment)
+      }
+    }
+
+    // Entry point to the world of Compose.
     composeContainer.setContent(Recomposer.current()) {
-      // Don't compose anything until we have the first value (which should happen in the initial
-      // frame).
-      val (rendering, environment) = renderState.value ?: return@setContent
+      val (rendering, environment) = renderState.value!!
       showRendering(rendering, environment)
     }
 
-    composeContainer.bindShowRendering(
-        initialRendering,
-        initialViewEnvironment
-    ) { rendering, environment ->
-      renderState.value = Pair(rendering, environment)
-    }
     return composeContainer
   }
+
+  /**
+   * Currently, if we try setting a Model's value before any other Compose code has started running
+   * (e.g. by calling setContent), there will not be a frame open so the model set will throw. This
+   * function detects that situation and creates a temporary frame if necessary. To run [block] in.
+   * This should be removed once Compose gets support for global frames.
+   */
+  private fun ensureComposeFrame(block: () -> Unit) {
+    if (!inFrame) {
+      val frame = open(readOnly = false)
+      block()
+      commit(frame)
+    } else {
+      block()
+    }
+  }
 }