Provide monitor information.

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -44347,6 +44347,9 @@ ORIGIN: ../../../flutter/shell/platform/linux/fl_binary_messenger_test.cc + ../.
 ORIGIN: ../../../flutter/shell/platform/linux/fl_dart_project.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_dart_project_private.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_dart_project_test.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/linux/fl_display_monitor.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/linux/fl_display_monitor.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/linux/fl_display_monitor_test.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_engine.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_engine_private.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/linux/fl_engine_test.cc + ../../../flutter/LICENSE
@@ -47315,6 +47318,9 @@ FILE: ../../../flutter/shell/platform/linux/fl_binary_messenger_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_dart_project.cc
 FILE: ../../../flutter/shell/platform/linux/fl_dart_project_private.h
 FILE: ../../../flutter/shell/platform/linux/fl_dart_project_test.cc
+FILE: ../../../flutter/shell/platform/linux/fl_display_monitor.cc
+FILE: ../../../flutter/shell/platform/linux/fl_display_monitor.h
+FILE: ../../../flutter/shell/platform/linux/fl_display_monitor_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_engine.cc
 FILE: ../../../flutter/shell/platform/linux/fl_engine_private.h
 FILE: ../../../flutter/shell/platform/linux/fl_engine_test.cc

engine/src/flutter/shell/platform/linux/BUILD.gn

@@ -107,6 +107,7 @@ source_set("flutter_linux_sources") {
     "fl_binary_codec.cc",
     "fl_binary_messenger.cc",
     "fl_dart_project.cc",
+    "fl_display_monitor.cc",
     "fl_engine.cc",
     "fl_event_channel.cc",
     "fl_framebuffer.cc",
@@ -216,6 +217,7 @@ executable("flutter_linux_unittests") {
     "fl_binary_codec_test.cc",
     "fl_binary_messenger_test.cc",
     "fl_dart_project_test.cc",
+    "fl_display_monitor_test.cc",
     "fl_engine_test.cc",
     "fl_event_channel_test.cc",
     "fl_framebuffer_test.cc",

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

@@ -0,0 +1,120 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/shell/platform/linux/fl_display_monitor.h"
+#include "flutter/shell/platform/linux/fl_engine_private.h"
+
+struct _FlDisplayMonitor {
+  GObject parent_instance;
+
+  // Engine being updated.
+  GWeakRef engine;
+
+  // Display being monitored.
+  GdkDisplay* display;
+
+  // Mapping of GdkMonitor to display IDs.
+  GHashTable* display_ids_by_monitor;
+
+  // Next ID to assign to a new monitor.
+  FlutterEngineDisplayId next_display_id;
+};
+
+G_DEFINE_TYPE(FlDisplayMonitor, fl_display_monitor, G_TYPE_OBJECT)
+
+// Send the current monitor state to the engine.
+static void notify_display_update(FlDisplayMonitor* self) {
+  g_autoptr(FlEngine) engine = FL_ENGINE(g_weak_ref_get(&self->engine));
+  if (engine == nullptr) {
+    return;
+  }
+
+  int n_monitors = gdk_display_get_n_monitors(self->display);
+  g_autofree FlutterEngineDisplay* displays =
+      g_new0(FlutterEngineDisplay, n_monitors);
+  for (int i = 0; i < n_monitors; i++) {
+    FlutterEngineDisplay* display = &displays[i];
+
+    GdkMonitor* monitor = gdk_display_get_monitor(self->display, i);
+    FlutterEngineDisplayId display_id = GPOINTER_TO_INT(
+        g_hash_table_lookup(self->display_ids_by_monitor, monitor));
+    if (display_id == 0) {
+      display_id = self->next_display_id;
+      g_hash_table_insert(self->display_ids_by_monitor, g_object_ref(monitor),
+                          GINT_TO_POINTER(display_id));
+      self->next_display_id++;
+    }
+
+    GdkRectangle geometry;
+    gdk_monitor_get_geometry(monitor, &geometry);
+
+    display->struct_size = sizeof(FlutterEngineDisplay);
+    display->display_id = display_id;
+    display->single_display = false;
+    display->refresh_rate = gdk_monitor_get_refresh_rate(monitor) / 1000.0;
+    display->width = geometry.width;
+    display->height = geometry.height;
+    display->device_pixel_ratio = gdk_monitor_get_scale_factor(monitor);
+  }
+
+  fl_engine_notify_display_update(engine, displays, n_monitors);
+}
+
+static void monitor_added_cb(FlDisplayMonitor* self, GdkMonitor* monitor) {
+  notify_display_update(self);
+}
+
+static void monitor_removed_cb(FlDisplayMonitor* self, GdkMonitor* monitor) {
+  g_hash_table_remove(self->display_ids_by_monitor, monitor);
+  notify_display_update(self);
+}
+
+static void fl_display_monitor_dispose(GObject* object) {
+  FlDisplayMonitor* self = FL_DISPLAY_MONITOR(object);
+
+  g_weak_ref_clear(&self->engine);
+  g_clear_object(&self->display);
+  g_clear_pointer(&self->display_ids_by_monitor, g_hash_table_unref);
+
+  G_OBJECT_CLASS(fl_display_monitor_parent_class)->dispose(object);
+}
+
+static void fl_display_monitor_class_init(FlDisplayMonitorClass* klass) {
+  GObjectClass* object_class = G_OBJECT_CLASS(klass);
+  object_class->dispose = fl_display_monitor_dispose;
+}
+
+static void fl_display_monitor_init(FlDisplayMonitor* self) {
+  self->display_ids_by_monitor = g_hash_table_new_full(
+      g_direct_hash, g_direct_equal, g_object_unref, nullptr);
+  self->next_display_id = 1;
+}
+
+FlDisplayMonitor* fl_display_monitor_new(FlEngine* engine,
+                                         GdkDisplay* display) {
+  FlDisplayMonitor* self =
+      FL_DISPLAY_MONITOR(g_object_new(fl_display_monitor_get_type(), nullptr));
+  g_weak_ref_init(&self->engine, engine);
+  self->display = GDK_DISPLAY(g_object_ref(display));
+  return self;
+}
+
+void fl_display_monitor_start(FlDisplayMonitor* self) {
+  g_return_if_fail(FL_IS_DISPLAY_MONITOR(self));
+
+  g_signal_connect_object(self->display, "monitor-added",
+                          G_CALLBACK(monitor_added_cb), self,
+                          G_CONNECT_SWAPPED);
+  g_signal_connect_object(self->display, "monitor-removed",
+                          G_CALLBACK(monitor_removed_cb), self,
+                          G_CONNECT_SWAPPED);
+  notify_display_update(self);
+}
+
+FlutterEngineDisplayId fl_display_monitor_get_display_id(FlDisplayMonitor* self,
+                                                         GdkMonitor* monitor) {
+  g_return_val_if_fail(FL_IS_DISPLAY_MONITOR(self), 0);
+  return GPOINTER_TO_INT(
+      g_hash_table_lookup(self->display_ids_by_monitor, monitor));
+}

engine/src/flutter/shell/platform/linux/fl_display_monitor.h

@@ -0,0 +1,56 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_SHELL_PLATFORM_LINUX_FL_DISPLAY_MONITOR_H_
+#define FLUTTER_SHELL_PLATFORM_LINUX_FL_DISPLAY_MONITOR_H_
+
+#include <gdk/gdk.h>
+
+#include "flutter/shell/platform/embedder/embedder.h"
+#include "flutter/shell/platform/linux/public/flutter_linux/fl_engine.h"
+
+G_BEGIN_DECLS
+
+G_DECLARE_FINAL_TYPE(FlDisplayMonitor,
+                     fl_display_monitor,
+                     FL,
+                     DISPLAY_MONITOR,
+                     GObject);
+
+/**
+ * fl_display_monitor_new:
+ * @engine: engine to update.
+ * @display: display to monitor.
+ *
+ * Creates a new object to keep the engine updated with the currently used
+ * displays. In GDK, a display is called a "monitor".
+ *
+ * Returns: a new #FlDisplayMontior.
+ */
+FlDisplayMonitor* fl_display_monitor_new(FlEngine* engine, GdkDisplay* display);
+
+/**
+ * fl_display_monitor_start:
+ * @monitor: an #FlDisplayMonitor.
+ *
+ * Start monitoring for display changes.
+ */
+void fl_display_monitor_start(FlDisplayMonitor* monitor);
+
+/**
+ * fl_display_monitor_get_display_id:
+ * @monitor: an #FlDisplayMonitor.
+ * @gdk_monitor: GDK monitor to get display ID for.
+ *
+ * Get the ID Flutter is using for a given monitor.
+ *
+ * Returns: an ID or 0 if unknown.
+ */
+FlutterEngineDisplayId fl_display_monitor_get_display_id(
+    FlDisplayMonitor* monitor,
+    GdkMonitor* gdk_monitor);
+
+G_END_DECLS
+
+#endif  // FLUTTER_SHELL_PLATFORM_LINUX_FL_DISPLAY_MONITOR_H_

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

@@ -0,0 +1,36 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/shell/platform/linux/fl_display_monitor.h"
+#include "flutter/shell/platform/embedder/test_utils/proc_table_replacement.h"
+#include "flutter/shell/platform/linux/fl_engine_private.h"
+
+#include "gtest/gtest.h"
+
+TEST(FlDisplayMonitorTest, Test) {
+  g_autoptr(FlDartProject) project = fl_dart_project_new();
+  g_autoptr(FlEngine) engine = fl_engine_new(project);
+
+  g_autoptr(GError) error = nullptr;
+  EXPECT_TRUE(fl_engine_start(engine, &error));
+  EXPECT_EQ(error, nullptr);
+
+  bool called = false;
+  fl_engine_get_embedder_api(engine)->NotifyDisplayUpdate = MOCK_ENGINE_PROC(
+      NotifyDisplayUpdate,
+      ([&called](auto engine, FlutterEngineDisplaysUpdateType update_type,
+                 const FlutterEngineDisplay* displays, size_t displays_length) {
+        called = true;
+
+        EXPECT_EQ(displays_length, 1u);
+
+        return kSuccess;
+      }));
+
+  g_autoptr(FlDisplayMonitor) monitor =
+      fl_display_monitor_new(engine, gdk_display_get_default());
+  EXPECT_FALSE(called);
+  fl_display_monitor_start(monitor);
+  EXPECT_TRUE(called);
+}

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

@@ -12,6 +12,7 @@
 #include "flutter/shell/platform/embedder/embedder.h"
 #include "flutter/shell/platform/linux/fl_binary_messenger_private.h"
 #include "flutter/shell/platform/linux/fl_dart_project_private.h"
+#include "flutter/shell/platform/linux/fl_display_monitor.h"
 #include "flutter/shell/platform/linux/fl_engine_private.h"
 #include "flutter/shell/platform/linux/fl_pixel_buffer_texture_private.h"
 #include "flutter/shell/platform/linux/fl_platform_handler.h"
@@ -41,6 +42,9 @@ struct _FlEngine {
   // The project this engine is running.
   FlDartProject* project;
 
+  // Watches for monitors changes to update engine.
+  FlDisplayMonitor* display_monitor;
+
   // Renders the Flutter app.
   FlRenderer* renderer;
 
@@ -532,6 +536,11 @@ FlRenderer* fl_engine_get_renderer(FlEngine* self) {
   return self->renderer;
 }
 
+FlDisplayMonitor* fl_engine_get_display_monitor(FlEngine* self) {
+  g_return_val_if_fail(FL_IS_ENGINE(self), nullptr);
+  return self->display_monitor;
+}
+
 gboolean fl_engine_start(FlEngine* self, GError** error) {
   g_return_val_if_fail(FL_IS_ENGINE(self), FALSE);
 
@@ -642,23 +651,9 @@ gboolean fl_engine_start(FlEngine* self, GError** error) {
     g_warning("Failed to enable accessibility features on Flutter engine");
   }
 
-  gdouble refresh_rate = fl_renderer_get_refresh_rate(self->renderer);
-  // FlutterEngineDisplay::refresh_rate expects 0 if the refresh rate is
-  // unknown.
-  if (refresh_rate <= 0.0) {
-    refresh_rate = 0.0;
-  }
-  FlutterEngineDisplay display = {};
-  display.struct_size = sizeof(FlutterEngineDisplay);
-  display.display_id = 0;
-  display.single_display = true;
-  display.refresh_rate = refresh_rate;
-
-  result = self->embedder_api.NotifyDisplayUpdate(
-      self->engine, kFlutterEngineDisplaysUpdateTypeStartup, &display, 1);
-  if (result != kSuccess) {
-    g_warning("Failed to notify display update to Flutter engine: %d", result);
-  }
+  self->display_monitor =
+      fl_display_monitor_new(self, gdk_display_get_default());
+  fl_display_monitor_start(self->display_monitor);
 
   return TRUE;
 }
@@ -667,6 +662,19 @@ FlutterEngineProcTable* fl_engine_get_embedder_api(FlEngine* self) {
   return &(self->embedder_api);
 }
 
+void fl_engine_notify_display_update(FlEngine* self,
+                                     const FlutterEngineDisplay* displays,
+                                     size_t displays_length) {
+  g_return_if_fail(FL_IS_ENGINE(self));
+
+  FlutterEngineResult result = self->embedder_api.NotifyDisplayUpdate(
+      self->engine, kFlutterEngineDisplaysUpdateTypeStartup, displays,
+      displays_length);
+  if (result != kSuccess) {
+    g_warning("Failed to notify display update to Flutter engine: %d", result);
+  }
+}
+
 FlutterViewId fl_engine_add_view(FlEngine* self,
                                  size_t width,
                                  size_t height,
@@ -681,11 +689,16 @@ FlutterViewId fl_engine_add_view(FlEngine* self,
   FlutterViewId view_id = self->next_view_id;
   self->next_view_id++;
 
+  // We don't know which display this view will open on, so set to zero and this
+  // will be updated in a following FlutterWindowMetricsEvent
+  FlutterEngineDisplayId display_id = 0;
+
   FlutterWindowMetricsEvent metrics;
   metrics.struct_size = sizeof(FlutterWindowMetricsEvent);
   metrics.width = width;
   metrics.height = height;
   metrics.pixel_ratio = pixel_ratio;
+  metrics.display_id = display_id;
   metrics.view_id = view_id;
   FlutterAddViewInfo info;
   info.struct_size = sizeof(FlutterAddViewInfo);
@@ -881,6 +894,7 @@ GBytes* fl_engine_send_platform_message_finish(FlEngine* self,
 }
 
 void fl_engine_send_window_metrics_event(FlEngine* self,
+                                         FlutterEngineDisplayId display_id,
                                          FlutterViewId view_id,
                                          size_t width,
                                          size_t height,
@@ -896,6 +910,7 @@ void fl_engine_send_window_metrics_event(FlEngine* self,
   event.width = width;
   event.height = height;
   event.pixel_ratio = pixel_ratio;
+  event.display_id = display_id;
   event.view_id = view_id;
   self->embedder_api.SendWindowMetricsEvent(self->engine, &event);
 }