Explain 0

shell/platform/darwin/macos/framework/Source/FlutterCompositor.mm

@@ -47,6 +47,8 @@
 
 void FlutterCompositor::InsertCALayerForIOSurface(const IOSurfaceRef& io_surface,
                                                   CATransform3D transform) {
+  // Always gets the first view, #0. After Flutter supports multi-view, it
+  // should get the view ID from somewhere.
   FlutterView* view = GetView(0);
   if (!view) {
     return;

shell/platform/darwin/macos/framework/Source/FlutterGLCompositor.mm

@@ -25,6 +25,8 @@
 
 bool FlutterGLCompositor::CreateBackingStore(const FlutterBackingStoreConfig* config,
                                              FlutterBackingStore* backing_store_out) {
+  // Always gets the first view, #0. After Flutter supports multi-view, it
+  // should get the view ID from somewhere.
   FlutterView* view = GetView(0);
   if (!view) {
     return false;

shell/platform/darwin/macos/framework/Source/FlutterMetalCompositor.mm

@@ -20,6 +20,8 @@
 
 bool FlutterMetalCompositor::CreateBackingStore(const FlutterBackingStoreConfig* config,
                                                 FlutterBackingStore* backing_store_out) {
+  // Always gets the first view, #0. After Flutter supports multi-view, it
+  // should get the view ID from somewhere.
   FlutterView* view = GetView(0);
   if (!view) {
     return false;
@@ -97,6 +99,8 @@
         break;
       }
       case kFlutterLayerContentTypePlatformView: {
+        // Always gets the first view, #0. After Flutter supports multi-view, it
+        // should get the view ID from somewhere.
         FlutterView* view = GetView(0);
         if (!view) {
           return false;