[Embedder] Refactor how semantic updates are mapped (flutter#44553)

This refactors how engine semantic updates are mapped to embedder semantic updates. There are no behavioral changes.

Part of flutter/flutter#119970, flutter/flutter#98948

Next PR: flutter#44616

## Background
For flutter/flutter#119970, we will need to update the embedder API to add string attributes to semantic nodes' text values. There are multiple kinds of string attributes, and each text value can have multiple string attributes. This requires gnarly mapping code that's best kept out of `embedder.cc`.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: loic-sharma

ci/licenses_golden/licenses_flutter

@@ -2852,6 +2852,8 @@ ORIGIN: ../../../flutter/shell/platform/embedder/embedder_render_target_impeller
 ORIGIN: ../../../flutter/shell/platform/embedder/embedder_render_target_impeller.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/embedder/embedder_render_target_skia.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/embedder/embedder_render_target_skia.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/embedder/embedder_semantics_update.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/embedder/embedder_semantics_update.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/embedder/embedder_struct_macros.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/embedder/embedder_surface.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/embedder/embedder_surface.h + ../../../flutter/LICENSE
@@ -5597,6 +5599,8 @@ FILE: ../../../flutter/shell/platform/embedder/embedder_render_target_impeller.c
 FILE: ../../../flutter/shell/platform/embedder/embedder_render_target_impeller.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_render_target_skia.cc
 FILE: ../../../flutter/shell/platform/embedder/embedder_render_target_skia.h
+FILE: ../../../flutter/shell/platform/embedder/embedder_semantics_update.cc
+FILE: ../../../flutter/shell/platform/embedder/embedder_semantics_update.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_struct_macros.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_surface.cc
 FILE: ../../../flutter/shell/platform/embedder/embedder_surface.h

shell/platform/embedder/BUILD.gn

@@ -78,6 +78,8 @@ template("embedder_source_set") {
       "embedder_render_target_cache.h",
       "embedder_render_target_skia.cc",
       "embedder_render_target_skia.h",
+      "embedder_semantics_update.cc",
+      "embedder_semantics_update.h",
       "embedder_struct_macros.h",
       "embedder_surface.cc",
       "embedder_surface.h",

shell/platform/embedder/embedder.cc

@@ -56,6 +56,7 @@ extern const intptr_t kPlatformStrongDillSize;
 #include "flutter/shell/platform/embedder/embedder_platform_message_response.h"
 #include "flutter/shell/platform/embedder/embedder_render_target.h"
 #include "flutter/shell/platform/embedder/embedder_render_target_skia.h"
+#include "flutter/shell/platform/embedder/embedder_semantics_update.h"
 #include "flutter/shell/platform/embedder/embedder_struct_macros.h"
 #include "flutter/shell/platform/embedder/embedder_task_runner.h"
 #include "flutter/shell/platform/embedder/embedder_thread_host.h"
@@ -1494,214 +1495,11 @@ void PopulateAOTSnapshotMappingCallbacks(
   }
 }
 
-// Translates engine semantic nodes to embedder semantic nodes.
-FlutterSemanticsNode CreateEmbedderSemanticsNode(
-    const flutter::SemanticsNode& node) {
-  SkMatrix transform = node.transform.asM33();
-  FlutterTransformation flutter_transform{
-      transform.get(SkMatrix::kMScaleX), transform.get(SkMatrix::kMSkewX),
-      transform.get(SkMatrix::kMTransX), transform.get(SkMatrix::kMSkewY),
-      transform.get(SkMatrix::kMScaleY), transform.get(SkMatrix::kMTransY),
-      transform.get(SkMatrix::kMPersp0), transform.get(SkMatrix::kMPersp1),
-      transform.get(SkMatrix::kMPersp2)};
-
-  // Do not add new members to FlutterSemanticsNode.
-  // This would break the forward compatibility of FlutterSemanticsUpdate.
-  // All new members must be added to FlutterSemanticsNode2 instead.
-  return {
-      sizeof(FlutterSemanticsNode),
-      node.id,
-      static_cast<FlutterSemanticsFlag>(node.flags),
-      static_cast<FlutterSemanticsAction>(node.actions),
-      node.textSelectionBase,
-      node.textSelectionExtent,
-      node.scrollChildren,
-      node.scrollIndex,
-      node.scrollPosition,
-      node.scrollExtentMax,
-      node.scrollExtentMin,
-      node.elevation,
-      node.thickness,
-      node.label.c_str(),
-      node.hint.c_str(),
-      node.value.c_str(),
-      node.increasedValue.c_str(),
-      node.decreasedValue.c_str(),
-      static_cast<FlutterTextDirection>(node.textDirection),
-      FlutterRect{node.rect.fLeft, node.rect.fTop, node.rect.fRight,
-                  node.rect.fBottom},
-      flutter_transform,
-      node.childrenInTraversalOrder.size(),
-      node.childrenInTraversalOrder.data(),
-      node.childrenInHitTestOrder.data(),
-      node.customAccessibilityActions.size(),
-      node.customAccessibilityActions.data(),
-      node.platformViewId,
-      node.tooltip.c_str(),
-  };
-}
-
-// Translates engine semantic nodes to embedder semantic nodes.
-FlutterSemanticsNode2 CreateEmbedderSemanticsNode2(
-    const flutter::SemanticsNode& node) {
-  SkMatrix transform = node.transform.asM33();
-  FlutterTransformation flutter_transform{
-      transform.get(SkMatrix::kMScaleX), transform.get(SkMatrix::kMSkewX),
-      transform.get(SkMatrix::kMTransX), transform.get(SkMatrix::kMSkewY),
-      transform.get(SkMatrix::kMScaleY), transform.get(SkMatrix::kMTransY),
-      transform.get(SkMatrix::kMPersp0), transform.get(SkMatrix::kMPersp1),
-      transform.get(SkMatrix::kMPersp2)};
-  return {
-      sizeof(FlutterSemanticsNode2),
-      node.id,
-      static_cast<FlutterSemanticsFlag>(node.flags),
-      static_cast<FlutterSemanticsAction>(node.actions),
-      node.textSelectionBase,
-      node.textSelectionExtent,
-      node.scrollChildren,
-      node.scrollIndex,
-      node.scrollPosition,
-      node.scrollExtentMax,
-      node.scrollExtentMin,
-      node.elevation,
-      node.thickness,
-      node.label.c_str(),
-      node.hint.c_str(),
-      node.value.c_str(),
-      node.increasedValue.c_str(),
-      node.decreasedValue.c_str(),
-      static_cast<FlutterTextDirection>(node.textDirection),
-      FlutterRect{node.rect.fLeft, node.rect.fTop, node.rect.fRight,
-                  node.rect.fBottom},
-      flutter_transform,
-      node.childrenInTraversalOrder.size(),
-      node.childrenInTraversalOrder.data(),
-      node.childrenInHitTestOrder.data(),
-      node.customAccessibilityActions.size(),
-      node.customAccessibilityActions.data(),
-      node.platformViewId,
-      node.tooltip.c_str(),
-  };
-}
-
-// Translates engine semantic custom actions to embedder semantic custom
-// actions.
-FlutterSemanticsCustomAction CreateEmbedderSemanticsCustomAction(
-    const flutter::CustomAccessibilityAction& action) {
-  // Do not add new members to FlutterSemanticsCustomAction.
-  // This would break the forward compatibility of FlutterSemanticsUpdate.
-  // All new members must be added to FlutterSemanticsCustomAction2 instead.
-  return {
-      sizeof(FlutterSemanticsCustomAction),
-      action.id,
-      static_cast<FlutterSemanticsAction>(action.overrideId),
-      action.label.c_str(),
-      action.hint.c_str(),
-  };
-}
-
-// Translates engine semantic custom actions to embedder semantic custom
-// actions.
-FlutterSemanticsCustomAction2 CreateEmbedderSemanticsCustomAction2(
-    const flutter::CustomAccessibilityAction& action) {
-  return {
-      sizeof(FlutterSemanticsCustomAction2),
-      action.id,
-      static_cast<FlutterSemanticsAction>(action.overrideId),
-      action.label.c_str(),
-      action.hint.c_str(),
-  };
-}
-
-// Create a callback to notify the embedder of semantic updates
-// using the deprecated embedder callback 'update_semantics_callback'.
-flutter::PlatformViewEmbedder::UpdateSemanticsCallback
-CreateNewEmbedderSemanticsUpdateCallback(
-    FlutterUpdateSemanticsCallback update_semantics_callback,
-    void* user_data) {
-  return [update_semantics_callback, user_data](
-             const flutter::SemanticsNodeUpdates& nodes,
-             const flutter::CustomAccessibilityActionUpdates& actions) {
-    std::vector<FlutterSemanticsNode> embedder_nodes;
-    for (const auto& value : nodes) {
-      embedder_nodes.push_back(CreateEmbedderSemanticsNode(value.second));
-    }
-
-    std::vector<FlutterSemanticsCustomAction> embedder_custom_actions;
-    for (const auto& value : actions) {
-      embedder_custom_actions.push_back(
-          CreateEmbedderSemanticsCustomAction(value.second));
-    }
-
-    FlutterSemanticsUpdate update{
-        .struct_size = sizeof(FlutterSemanticsUpdate),
-        .nodes_count = embedder_nodes.size(),
-        .nodes = embedder_nodes.data(),
-        .custom_actions_count = embedder_custom_actions.size(),
-        .custom_actions = embedder_custom_actions.data(),
-    };
-
-    update_semantics_callback(&update, user_data);
-  };
-}
-
-// Create a callback to notify the embedder of semantic updates
-// using the new embedder callback 'update_semantics_callback2'.
-flutter::PlatformViewEmbedder::UpdateSemanticsCallback
-CreateNewEmbedderSemanticsUpdateCallback2(
-    FlutterUpdateSemanticsCallback2 update_semantics_callback,
-    void* user_data) {
-  return [update_semantics_callback, user_data](
-             const flutter::SemanticsNodeUpdates& nodes,
-             const flutter::CustomAccessibilityActionUpdates& actions) {
-    std::vector<FlutterSemanticsNode2> embedder_nodes;
-    std::vector<FlutterSemanticsCustomAction2> embedder_custom_actions;
-
-    embedder_nodes.reserve(nodes.size());
-    embedder_custom_actions.reserve(actions.size());
-
-    for (const auto& value : nodes) {
-      embedder_nodes.push_back(CreateEmbedderSemanticsNode2(value.second));
-    }
-
-    for (const auto& value : actions) {
-      embedder_custom_actions.push_back(
-          CreateEmbedderSemanticsCustomAction2(value.second));
-    }
-
-    // Provide the embedder an array of pointers to maintain full forward and
-    // backward compatibility even if new members are added to semantic structs.
-    std::vector<FlutterSemanticsNode2*> embedder_node_pointers;
-    std::vector<FlutterSemanticsCustomAction2*> embedder_custom_action_pointers;
-
-    embedder_node_pointers.reserve(embedder_nodes.size());
-    embedder_custom_action_pointers.reserve(embedder_custom_actions.size());
-
-    for (auto& node : embedder_nodes) {
-      embedder_node_pointers.push_back(&node);
-    }
-
-    for (auto& action : embedder_custom_actions) {
-      embedder_custom_action_pointers.push_back(&action);
-    }
-
-    FlutterSemanticsUpdate2 update{
-        .struct_size = sizeof(FlutterSemanticsUpdate2),
-        .node_count = embedder_node_pointers.size(),
-        .nodes = embedder_node_pointers.data(),
-        .custom_action_count = embedder_custom_action_pointers.size(),
-        .custom_actions = embedder_custom_action_pointers.data(),
-    };
-
-    update_semantics_callback(&update, user_data);
-  };
-}
-
 // Create a callback to notify the embedder of semantic updates
 // using the legacy embedder callbacks 'update_semantics_node_callback' and
 // 'update_semantics_custom_action_callback'.
 flutter::PlatformViewEmbedder::UpdateSemanticsCallback
-CreateLegacyEmbedderSemanticsUpdateCallback(
+CreateEmbedderSemanticsUpdateCallbackV1(
     FlutterUpdateSemanticsNodeCallback update_semantics_node_callback,
     FlutterUpdateSemanticsCustomActionCallback
         update_semantics_custom_action_callback,
@@ -1710,20 +1508,20 @@ CreateLegacyEmbedderSemanticsUpdateCallback(
           update_semantics_custom_action_callback,
           user_data](const flutter::SemanticsNodeUpdates& nodes,
                      const flutter::CustomAccessibilityActionUpdates& actions) {
+    flutter::EmbedderSemanticsUpdate update{nodes, actions};
+    FlutterSemanticsUpdate* update_ptr = update.get();
+
     // First, queue all node and custom action updates.
     if (update_semantics_node_callback != nullptr) {
-      for (const auto& value : nodes) {
-        const FlutterSemanticsNode embedder_node =
-            CreateEmbedderSemanticsNode(value.second);
-        update_semantics_node_callback(&embedder_node, user_data);
+      for (size_t i = 0; i < update_ptr->nodes_count; i++) {
+        update_semantics_node_callback(&update_ptr->nodes[i], user_data);
       }
     }
 
     if (update_semantics_custom_action_callback != nullptr) {
-      for (const auto& value : actions) {
-        const FlutterSemanticsCustomAction embedder_action =
-            CreateEmbedderSemanticsCustomAction(value.second);
-        update_semantics_custom_action_callback(&embedder_action, user_data);
+      for (size_t i = 0; i < update_ptr->custom_actions_count; i++) {
+        update_semantics_custom_action_callback(&update_ptr->custom_actions[i],
+                                                user_data);
       }
     }
 
@@ -1747,26 +1545,62 @@ CreateLegacyEmbedderSemanticsUpdateCallback(
   };
 }
 
+// Create a callback to notify the embedder of semantic updates
+// using the deprecated embedder callback 'update_semantics_callback'.
+flutter::PlatformViewEmbedder::UpdateSemanticsCallback
+CreateEmbedderSemanticsUpdateCallbackV2(
+    FlutterUpdateSemanticsCallback update_semantics_callback,
+    void* user_data) {
+  return [update_semantics_callback, user_data](
+             const flutter::SemanticsNodeUpdates& nodes,
+             const flutter::CustomAccessibilityActionUpdates& actions) {
+    flutter::EmbedderSemanticsUpdate update{nodes, actions};
+
+    update_semantics_callback(update.get(), user_data);
+  };
+}
+
+// Create a callback to notify the embedder of semantic updates
+// using the new embedder callback 'update_semantics_callback2'.
+flutter::PlatformViewEmbedder::UpdateSemanticsCallback
+CreateEmbedderSemanticsUpdateCallbackV3(
+    FlutterUpdateSemanticsCallback2 update_semantics_callback,
+    void* user_data) {
+  return [update_semantics_callback, user_data](
+             const flutter::SemanticsNodeUpdates& nodes,
+             const flutter::CustomAccessibilityActionUpdates& actions) {
+    flutter::EmbedderSemanticsUpdate2 update{nodes, actions};
+
+    update_semantics_callback(update.get(), user_data);
+  };
+}
+
 // Creates a callback that receives semantic updates from the engine
 // and notifies the embedder's callback(s). Returns null if the embedder
 // did not register any callbacks.
 flutter::PlatformViewEmbedder::UpdateSemanticsCallback
 CreateEmbedderSemanticsUpdateCallback(const FlutterProjectArgs* args,
                                       void* user_data) {
-  // The embedder can register the new callback, or the legacy callbacks, or
-  // nothing at all. Handle the case where the embedder registered the 'new'
-  // callback.
+  // There are three variants for the embedder API's semantic update callbacks.
+  // Create a callback that maps to the embedder's desired semantic update API.
+  //
+  // Handle the case where the embedder registered the callback
+  // 'updated_semantics_callback2'
   if (SAFE_ACCESS(args, update_semantics_callback2, nullptr) != nullptr) {
-    return CreateNewEmbedderSemanticsUpdateCallback2(
+    return CreateEmbedderSemanticsUpdateCallbackV3(
         args->update_semantics_callback2, user_data);
   }
 
+  // Handle the case where the embedder registered the deprecated callback
+  // 'update_semantics_callback'.
   if (SAFE_ACCESS(args, update_semantics_callback, nullptr) != nullptr) {
-    return CreateNewEmbedderSemanticsUpdateCallback(
+    return CreateEmbedderSemanticsUpdateCallbackV2(
         args->update_semantics_callback, user_data);
   }
 
-  // Handle the case where the embedder registered 'legacy' callbacks.
+  // Handle the case where the embedder registered the deprecated callbacks
+  // 'update_semantics_node_callback' and
+  // 'update_semantics_custom_action_callback'.
   FlutterUpdateSemanticsNodeCallback update_semantics_node_callback = nullptr;
   if (SAFE_ACCESS(args, update_semantics_node_callback, nullptr) != nullptr) {
     update_semantics_node_callback = args->update_semantics_node_callback;
@@ -1782,7 +1616,7 @@ CreateEmbedderSemanticsUpdateCallback(const FlutterProjectArgs* args,
 
   if (update_semantics_node_callback != nullptr ||
       update_semantics_custom_action_callback != nullptr) {
-    return CreateLegacyEmbedderSemanticsUpdateCallback(
+    return CreateEmbedderSemanticsUpdateCallbackV1(
         update_semantics_node_callback, update_semantics_custom_action_callback,
         user_data);
   }