Delay the window until the first frame is received from the Flutter engine (flutter/engine#54703)

Fixes https://github.com/flutter/flutter/issues/151098

engine/src/flutter/shell/platform/linux/fl_application.cc

@@ -31,6 +31,16 @@ G_DEFINE_TYPE_WITH_CODE(FlApplication,
                         GTK_TYPE_APPLICATION,
                         G_ADD_PRIVATE(FlApplication))
 
+// Called when the first frame is received.
+static void first_frame_cb(FlApplication* self, FlView* view) {
+  GtkWidget* window = gtk_widget_get_toplevel(GTK_WIDGET(view));
+
+  // Show the main window.
+  if (window != nullptr && GTK_IS_WINDOW(window)) {
+    gtk_window_present(GTK_WINDOW(window));
+  }
+}
+
 // Default implementation of FlApplication::register_plugins
 static void fl_application_register_plugins(FlApplication* self,
                                             FlPluginRegistry* registry) {}
@@ -80,17 +90,20 @@ static void fl_application_activate(GApplication* application) {
       project, priv->dart_entrypoint_arguments);
 
   FlView* view = fl_view_new(project);
+  g_signal_connect_swapped(view, "first-frame", G_CALLBACK(first_frame_cb),
+                           self);
   gtk_widget_show(GTK_WIDGET(view));
 
   GtkWindow* window;
   g_signal_emit(self, fl_application_signals[kSignalCreateWindow], 0, view,
                 &window);
-  gtk_widget_show(GTK_WIDGET(window));
+
+  // Make the resources for the view so rendering can start.
+  // We'll show the view when we have the first frame.
+  gtk_widget_realize(GTK_WIDGET(view));
 
   g_signal_emit(self, fl_application_signals[kSignalRegisterPlugins], 0,
                 FL_PLUGIN_REGISTRY(view));
-
-  gtk_widget_grab_focus(GTK_WIDGET(view));
 }
 
 // Implements GApplication::local_command_line.

engine/src/flutter/shell/platform/linux/fl_view.cc

@@ -52,6 +52,9 @@ struct _FlView {
   // Background color.
   GdkRGBA* background_color;
 
+  // TRUE if have got the first frame to render.
+  gboolean have_first_frame;
+
   // Pointer button state recorded for sending status updates.
   int64_t button_state;
 
@@ -82,7 +85,9 @@ struct _FlView {
   FlViewAccessible* view_accessible;
 };
 
-enum { kPropFlutterProject = 1, kPropLast };
+enum { kSignalFirstFrame, kSignalLastSignal };
+
+static guint fl_view_signals[kSignalLastSignal];
 
 static void fl_view_plugin_registry_iface_init(
     FlPluginRegistryInterface* iface);
@@ -109,6 +114,15 @@ G_DEFINE_TYPE_WITH_CODE(
                 G_IMPLEMENT_INTERFACE(fl_text_input_view_delegate_get_type(),
                                       fl_view_text_input_delegate_iface_init))
 
+// Emit the first frame signal in the main thread.
+static gboolean first_frame_idle_cb(gpointer user_data) {
+  FlView* self = FL_VIEW(user_data);
+
+  g_signal_emit(self, fl_view_signals[kSignalFirstFrame], 0);
+
+  return FALSE;
+}
+
 // Signal handler for GtkWidget::delete-event
 static gboolean window_delete_event_cb(FlView* self) {
   fl_platform_handler_request_app_exit(self->platform_handler);
@@ -678,6 +692,16 @@ static void fl_view_dispose(GObject* object) {
   G_OBJECT_CLASS(fl_view_parent_class)->dispose(object);
 }
 
+// Implements GtkWidget::realize.
+static void fl_view_realize(GtkWidget* widget) {
+  FlView* self = FL_VIEW(widget);
+
+  GTK_WIDGET_CLASS(fl_view_parent_class)->realize(widget);
+
+  // Realize the child widgets.
+  gtk_widget_realize(GTK_WIDGET(self->gl_area));
+}
+
 // Implements GtkWidget::key_press_event.
 static gboolean fl_view_key_press_event(GtkWidget* widget, GdkEventKey* event) {
   FlView* self = FL_VIEW(widget);
@@ -702,9 +726,14 @@ static void fl_view_class_init(FlViewClass* klass) {
   object_class->dispose = fl_view_dispose;
 
   GtkWidgetClass* widget_class = GTK_WIDGET_CLASS(klass);
+  widget_class->realize = fl_view_realize;
   widget_class->key_press_event = fl_view_key_press_event;
   widget_class->key_release_event = fl_view_key_release_event;
 
+  fl_view_signals[kSignalFirstFrame] =
+      g_signal_new("first-frame", fl_view_get_type(), G_SIGNAL_RUN_LAST, 0,
+                   NULL, NULL, NULL, G_TYPE_NONE, 0);
+
   gtk_widget_class_set_accessible_type(GTK_WIDGET_CLASS(klass),
                                        fl_socket_accessible_get_type());
 }
@@ -810,7 +839,15 @@ G_MODULE_EXPORT void fl_view_set_background_color(FlView* self,
 
 void fl_view_redraw(FlView* self) {
   g_return_if_fail(FL_IS_VIEW(self));
+
   gtk_widget_queue_draw(GTK_WIDGET(self->gl_area));
+
+  if (!self->have_first_frame) {
+    self->have_first_frame = TRUE;
+    // This is not the main thread, so the signal needs to be done via an idle
+    // callback.
+    g_idle_add(first_frame_idle_cb, self);
+  }
 }
 
 GHashTable* fl_view_get_keyboard_state(FlView* self) {

engine/src/flutter/shell/platform/linux/fl_view_test.cc

@@ -3,31 +3,57 @@
 // found in the LICENSE file.
 
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_view.h"
+#include "flutter/shell/platform/linux/fl_view_private.h"
 #include "flutter/shell/platform/linux/testing/fl_test_gtk_logs.h"
 
 #include "gtest/gtest.h"
 
+static void first_frame_cb(FlView* view, gboolean* first_frame_emitted) {
+  *first_frame_emitted = TRUE;
+}
+
 TEST(FlViewTest, GetEngine) {
   flutter::testing::fl_ensure_gtk_init();
   g_autoptr(FlDartProject) project = fl_dart_project_new();
-  g_autoptr(FlView) view = fl_view_new(project);
+  FlView* view = fl_view_new(project);
 
   // Check the engine is immediately available (i.e. before the widget is
   // realized).
   FlEngine* engine = fl_view_get_engine(view);
   EXPECT_NE(engine, nullptr);
-
-  g_object_ref_sink(view);
 }
 
 TEST(FlViewTest, StateUpdateDoesNotHappenInInit) {
   flutter::testing::fl_ensure_gtk_init();
   g_autoptr(FlDartProject) project = fl_dart_project_new();
-  g_autoptr(FlView) view = fl_view_new(project);
+  FlView* view = fl_view_new(project);
   // Check that creating a view doesn't try to query the window state in
   // initialization, causing a critical log to be issued.
   EXPECT_EQ(
       flutter::testing::fl_get_received_gtk_log_levels() & G_LOG_LEVEL_CRITICAL,
       (GLogLevelFlags)0x0);
-  g_object_ref_sink(view);
+
+  (void)view;
+}
+
+TEST(FlViewTest, FirstFrameSignal) {
+  flutter::testing::fl_ensure_gtk_init();
+
+  g_autoptr(FlDartProject) project = fl_dart_project_new();
+  FlView* view = fl_view_new(project);
+  gboolean first_frame_emitted = FALSE;
+  g_signal_connect(view, "first-frame", G_CALLBACK(first_frame_cb),
+                   &first_frame_emitted);
+
+  EXPECT_FALSE(first_frame_emitted);
+
+  fl_view_redraw(view);
+
+  // Signal is emitted in idle, clear the main loop.
+  while (g_main_context_iteration(g_main_context_default(), FALSE)) {
+    // Repeat until nothing to iterate on.
+  }
+
+  // Check view has detected frame.
+  EXPECT_TRUE(first_frame_emitted);
 }