Add a Linux Shell that uses GTK for rendering.

This is a start on a Linux shell, and only supports rendering of Flutter, no
input is currently provided. The API is expected to change as we flesh out
the shell. The shell doesn't build by default yet, to enable it you need to
set the build_linux_flag gn build argument.

This is the first stage of flutter/flutter#30729

The shell only works on X servers, the EGL code will need to be updated to cover
Wayland.

The Flutter engine currently crashes when the widget is destroyed, but this
seems to be an issue in the engine.

The code is designed to match GTK conventions.

A minimal GTK application that uses this looks like:

--------

int
main (int argc, char **argv)
{
    gtk_init (&argc, &argv);

    GtkWidget *window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    gtk_widget_show (window);

    g_autoptr(FlDartProject) project = fl_dart_project_new ("./build/flutter_assets", "./linux/flutter/ephemeral/icudtl.dat");
    g_autoptr(FlView) view = fl_view_new (project);

    gtk_widget_show (GTK_WIDGET (view));
    gtk_container_add (GTK_CONTAINER (window), GTK_WIDGET (view));

    gtk_main ();

    return 0;
}
--------

shell/platform/linux/BUILD.gn

@@ -5,15 +5,23 @@
 assert(is_linux)
 
 import("//flutter/shell/platform/glfw/config.gni")
+import("//flutter/shell/platform/linux/config.gni")
 
 group("linux") {
+  deps = []
   if (build_glfw_shell) {
-    deps = [
+    deps += [
       ":flutter_linux_glfw",
       "//flutter/shell/platform/glfw:publish_headers_glfw",
       "//flutter/shell/platform/glfw/client_wrapper:publish_wrapper_glfw",
     ]
   }
+  if (build_linux_shell) {
+    deps += [
+      ":flutter_linux_gtk",
+      ":publish_headers_linux",
+    ]
+  }
 }
 
 # Temporary workaround for the issue describe in
@@ -37,3 +45,45 @@ if (build_glfw_shell) {
     public_configs = [ "//flutter:config" ]
   }
 }
+
+if (build_linux_shell) {
+  source_set("flutter_linux") {
+    sources = [
+      "fl-dart-project.cc",
+      "fl-dart-project.h",
+      "fl-view.cc",
+      "fl-view.h",
+    ]
+
+    configs += [
+      "//flutter/shell/platform/linux/config:gtk",
+      "//flutter/shell/platform/linux/config:egl",
+    ]
+
+    # Set flag to stop headers being directly included (library users should not do this)
+    defines = [ "FLUTTER_GTK_COMPILATION" ]
+
+    deps = [
+      "//flutter/shell/platform/embedder:embedder_with_symbol_prefix",
+    ]
+  }
+
+  shared_library("flutter_linux_gtk") {
+    deps = [
+      ":flutter_linux",
+    ]
+
+    public_configs = [ "//flutter:config" ]
+  }
+
+  copy("publish_headers_linux") {
+    sources = [
+      "flutter-gtk.h",
+      "fl-dart-project.h",
+      "fl-view.h",
+    ]
+    outputs = [
+      "$root_out_dir/flutter-gtk/{{source_file_part}}",
+    ]
+  }
+}

shell/platform/linux/config.gni

@@ -0,0 +1,7 @@
+# Copyright 2020 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.
+
+declare_args() {
+  build_linux_shell = false
+}

shell/platform/linux/config/BUILD.gn

@@ -3,7 +3,17 @@
 # found in the LICENSE file.
 
 import("//build/config/linux/pkg_config.gni")
+import("//flutter/shell/platform/linux/config.gni")
 
 pkg_config("x11") {
   packages = [ "x11" ]
 }
+
+if (build_linux_shell) {
+  pkg_config("gtk") {
+    packages = [ "gtk+-3.0" ]
+  }
+  pkg_config("egl") {
+    packages = [ "egl" ]
+  }
+}

shell/platform/linux/fl-dart-project.cc

@@ -0,0 +1,149 @@
+// Copyright 2020 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 <gmodule.h>
+
+#include "fl-dart-project.h"
+
+struct _FlDartProject
+{
+    GObject parent_instance;
+
+    gchar *assets_path;
+    gchar *icu_data_path;
+};
+
+enum
+{
+    PROP_ASSETS_PATH = 1,
+    PROP_ICU_DATA_PATH,
+    PROP_LAST
+};
+
+G_DEFINE_TYPE (FlDartProject, fl_dart_project, G_TYPE_OBJECT)
+
+static void
+fl_dart_project_set_property (GObject *object, guint prop_id, const GValue *value, GParamSpec *pspec)
+{
+    FlDartProject *self = FL_DART_PROJECT (object);
+
+    switch (prop_id) {
+    case PROP_ASSETS_PATH:
+        g_free (self->assets_path);
+        self->assets_path = g_strdup (g_value_get_string (value));
+        break;
+    case PROP_ICU_DATA_PATH:
+        g_free (self->icu_data_path);
+        self->icu_data_path = g_strdup (g_value_get_string (value));
+        break;
+    default:
+        G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
+        break;
+    }
+}
+
+static void
+fl_dart_project_get_property (GObject *object, guint prop_id, GValue *value, GParamSpec *pspec)
+{
+    FlDartProject *self = FL_DART_PROJECT (object);
+
+    switch (prop_id) {
+    case PROP_ASSETS_PATH:
+        g_value_set_string (value, self->assets_path);
+        break;
+    case PROP_ICU_DATA_PATH:
+        g_value_set_string (value, self->icu_data_path);
+        break;
+    default:
+        G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
+        break;
+    }
+}
+
+static void
+fl_dart_project_dispose (GObject *object)
+{
+    FlDartProject *self = FL_DART_PROJECT (object);
+
+    g_clear_pointer (&self->assets_path, g_free);
+    g_clear_pointer (&self->icu_data_path, g_free);
+
+    G_OBJECT_CLASS (fl_dart_project_parent_class)->dispose (object);
+}
+
+static void
+fl_dart_project_class_init (FlDartProjectClass *klass)
+{
+    G_OBJECT_CLASS (klass)->set_property = fl_dart_project_set_property;
+    G_OBJECT_CLASS (klass)->get_property = fl_dart_project_get_property;
+    G_OBJECT_CLASS (klass)->dispose = fl_dart_project_dispose;
+
+    g_object_class_install_property (G_OBJECT_CLASS (klass),
+                                     PROP_ASSETS_PATH,
+                                     g_param_spec_string ("assets-path",
+                                                          "assets-path",
+                                                          "Path to Flutter assets",
+                                                          NULL,
+                                                          (GParamFlags) (G_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY | G_PARAM_STATIC_STRINGS)));
+    g_object_class_install_property (G_OBJECT_CLASS (klass),
+                                     PROP_ICU_DATA_PATH,
+                                     g_param_spec_string ("icu-data-path",
+                                                          "icu-data-path",
+                                                          "Path to ICU data",
+                                                          NULL,
+                                                          (GParamFlags) (G_PARAM_READWRITE | G_PARAM_CONSTRUCT_ONLY | G_PARAM_STATIC_STRINGS)));
+}
+
+static void
+fl_dart_project_init (FlDartProject *self)
+{
+}
+
+/**
+ * fl_dart_project_new:
+ * @assets_path: a file path, e.g. "build/assets"
+ * @icu_data_path: a file path, e.g. "build/icudtl.dat"
+ *
+ * Create a Flutter project.
+ *
+ * Returns: a new #FlDartProject
+ */
+G_MODULE_EXPORT FlDartProject *
+fl_dart_project_new (const gchar *assets_path, const gchar *icu_data_path)
+{
+    return (FlDartProject *) g_object_new (fl_dart_project_get_type (),
+                                           "assets-path", assets_path,
+                                           "icu-data-path", icu_data_path,
+                                           NULL);
+}
+
+/**
+ * fl_dart_project_get_assets_path:
+ * @view: a #FlDartProject
+ *
+ * Get the path to the directory containing the assets used in the Flutter application.
+ *
+ * Returns: a file path, e.g. "build/assets"
+ */
+G_MODULE_EXPORT const gchar *
+fl_dart_project_get_assets_path (FlDartProject *self)
+{
+    g_return_val_if_fail (FL_IS_DART_PROJECT (self), NULL);
+    return self->assets_path;
+}
+
+/**
+ * fl_dart_project_get_icu_data_path:
+ * @view: a #FlDartProject
+ *
+ * Get the path to the ICU data file in the Flutter application.
+ *
+ * Returns: a file path, e.g. "build/icudtl.dat"
+ */
+G_MODULE_EXPORT const gchar *
+fl_dart_project_get_icu_data_path (FlDartProject *self)
+{
+    g_return_val_if_fail (FL_IS_DART_PROJECT (self), NULL);
+    return self->icu_data_path;
+}

shell/platform/linux/fl-dart-project.h

@@ -0,0 +1,23 @@
+// Copyright 2020 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.
+
+#pragma once
+
+#include <glib-object.h>
+
+#if !defined(__FLUTTER_GTK_INSIDE__) && !defined(FLUTTER_GTK_COMPILATION)
+#error "Only <flutter-gtk/flutter-gtk.h> can be included directly."
+#endif
+
+G_BEGIN_DECLS
+
+G_DECLARE_FINAL_TYPE (FlDartProject, fl_dart_project, FL, DART_PROJECT, GObject)
+
+FlDartProject *fl_dart_project_new               (const gchar *assets_path, const gchar *icu_data_path);
+
+const gchar   *fl_dart_project_get_assets_path   (FlDartProject *project);
+
+const gchar   *fl_dart_project_get_icu_data_path (FlDartProject *project);
+
+G_END_DECLS