[platform_view] iOSP platformView composition optimize. (#8202)

Optimize the performance of the platform view composition.
When recomposition is required, we previously remove all the platform view related views (intercepting view, platform view) and re-add them for each frame. This is mentioned in flutter/flutter#23793.
This PR enhanced the performance by re-arrange the existing UIViews and only add the newly created Views.

As a result, it would also fix flutter/flutter#29427

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm

@@ -225,22 +225,24 @@ bool FlutterPlatformViewsController::SubmitFrame(bool gl_rendering,
     composition_order_.clear();
     return did_submit;
   }
+  DetachUnusedLayers();
+  active_composition_order_.clear();
   UIView* flutter_view = flutter_view_.get();
 
-  // This can be more efficient, instead of removing all views and then re-attaching them,
-  // we should only remove the views that has been completly removed from the layer tree, and
-  // reorder the views using UIView's bringSubviewToFront.
-  // TODO(amirh): make this more efficient.
-  // https://github.com/flutter/flutter/issues/23793
-  for (UIView* sub_view in [flutter_view subviews]) {
-    [sub_view removeFromSuperview];
-  }
-
-  active_composition_order_.clear();
   for (size_t i = 0; i < composition_order_.size(); i++) {
     int view_id = composition_order_[i];
-    [flutter_view addSubview:touch_interceptors_[view_id].get()];
-    [flutter_view addSubview:overlays_[view_id]->overlay_view.get()];
+    UIView* intercepter = touch_interceptors_[view_id].get();
+    UIView* overlay = overlays_[view_id]->overlay_view;
+    FML_CHECK(intercepter.superview == overlay.superview);
+
+    if (intercepter.superview == flutter_view) {
+      [flutter_view bringSubviewToFront:intercepter];
+      [flutter_view bringSubviewToFront:overlay];
+    } else {
+      [flutter_view addSubview:intercepter];
+      [flutter_view addSubview:overlay];
+    }
+
     active_composition_order_.push_back(view_id);
   }
 
@@ -248,6 +250,21 @@ bool FlutterPlatformViewsController::SubmitFrame(bool gl_rendering,
   return did_submit;
 }
 
+void FlutterPlatformViewsController::DetachUnusedLayers() {
+  std::unordered_set<int64_t> composition_order_set;
+
+  for (int64_t view_id : composition_order_) {
+    composition_order_set.insert(view_id);
+  }
+
+  for (int64_t view_id : active_composition_order_) {
+    if (composition_order_set.find(view_id) == composition_order_set.end()) {
+      [touch_interceptors_[view_id].get() removeFromSuperview];
+      [overlays_[view_id]->overlay_view.get() removeFromSuperview];
+    }
+  }
+}
+
 void FlutterPlatformViewsController::EnsureOverlayInitialized(int64_t overlay_id) {
   if (overlays_.count(overlay_id) != 0) {
     return;

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h

@@ -98,6 +98,7 @@ class FlutterPlatformViewsController {
   void OnAcceptGesture(FlutterMethodCall* call, FlutterResult& result);
   void OnRejectGesture(FlutterMethodCall* call, FlutterResult& result);
 
+  void DetachUnusedLayers();
   void EnsureOverlayInitialized(int64_t overlay_id);
   void EnsureGLOverlayInitialized(int64_t overlay_id,
                                   std::shared_ptr<IOSGLContext> gl_context,