Modified some code

src/text-viewer-application.c

@@ -117,4 +117,7 @@ text_viewer_application_init (TextViewerApplication *self)
 	gtk_application_set_accels_for_action (GTK_APPLICATION (self),
 	                                       "app.quit",
 	                                       (const char *[]) { "<primary>q", NULL });
+        gtk_application_set_accels_for_action (GTK_APPLICATION (self),
+                                               "win.open",
+                                               (const char *[]) { "<Ctrl>o", NULL });
 }

src/text-viewer-window.c

@@ -28,11 +28,102 @@ struct _TextViewerWindow
 
 	/* Template widgets */
 	AdwHeaderBar        *header_bar;
-	GtkLabel            *label;
+	GtkTextView         *main_text_view;
+        GtkButton           *open_button;
 };
 
 G_DEFINE_FINAL_TYPE (TextViewerWindow, text_viewer_window, ADW_TYPE_APPLICATION_WINDOW)
 
+static void
+open_file_complete (GObject          *source_object,
+                    GAsyncResult     *result,
+                    TextViewerWindow *self)
+{
+        GFile *file = G_FILE (source_object);
+
+        g_autofree char *contents = NULL;
+        gsize length = 0;
+
+        g_autoptr (GError) error = NULL;
+
+        g_file_load_contents_finish (file,
+                                     result,
+                                     &contents,
+                                     &length,
+                                     NULL,
+                                     &error);
+        g_autofree char *display_name = NULL;
+        g_autoptr (GFileInfo) info = g_file_query_info(file,
+                                                       "standard::display-name",
+                                                       G_FILE_QUERY_INFO_NONE,
+                                                       NULL,
+                                                       NULL);
+        if (info != NULL) {
+                display_name = g_strdup (g_file_info_get_attribute_string (info, "standard::display-name"));
+        } else {
+                display_name = g_file_get_basename (file);
+        }
+
+        if (error != NULL) {
+                g_printerr ("Unable to open '%': %s\n",
+                            g_file_peek_path (file),
+                            error->message);
+                return;
+        }
+
+        if (!g_utf8_validate (contents, length, NULL)) {
+                g_printerr ("Unable to load the contents of '%s': "
+                            "the file is not encoded with UTF-8\n",
+                            g_file_peek_path (file));
+                return;
+        }
+
+        GtkTextBuffer *buffer = gtk_text_view_get_buffer (self->main_text_view);
+
+        gtk_text_buffer_set_text (buffer, contents, length);
+
+        GtkTextIter start;
+        gtk_text_buffer_get_start_iter (buffer, &start);
+        gtk_text_buffer_place_cursor (buffer, &start);
+}
+
+static void
+open_file (TextViewerWindow *self,
+           GFile            *file)
+{
+        g_file_load_contents_async (file,
+                                    NULL,
+                                    (GAsyncReadyCallback) open_file_complete,
+                                    self);
+}
+
+static void on_open_response (GObject       *source,
+                              GAsyncResult  *result,
+                              gpointer      user_data)
+{
+        GtkFileDialog *dialog = GTK_FILE_DIALOG (source);
+        TextViewer *self = user_data;
+
+        g_autoptr (GFile) file = gtk_file_dialog_open_finish (dialog, result, NULL);
+
+        if (file != NULL)
+          open_file (self, file);
+}
+
+static void
+text_viewer_window__open_file_dialog (GAction          *action G_GNUC_UNUSED,
+                                      GVariant         *parameter G_GNUC_UNUSED,
+                                      TextViewerWindow *self)
+{
+        g_autoptr (GtkFileDialog) dialog = gtk_file_dialog_new ();
+
+        gtk_file_dialog_open (dialog,
+                              GTK_WINDOW (self),
+                              NULL,
+                              on_open_response,
+                              self);
+}
+
 static void
 text_viewer_window_class_init (TextViewerWindowClass *klass)
 {
@@ -40,11 +131,17 @@ text_viewer_window_class_init (TextViewerWindowClass *klass)
 
 	gtk_widget_class_set_template_from_resource (widget_class, "/com/example/TextViewer/text-viewer-window.ui");
 	gtk_widget_class_bind_template_child (widget_class, TextViewerWindow, header_bar);
-	gtk_widget_class_bind_template_child (widget_class, TextViewerWindow, label);
+	gtk_widget_class_bind_template_child (widget_class, TextViewerWindow, main_text_view);
+        gtk_widget_class_bind_template_child (widget_class, TextViewerWindow, open_button);
 }
 
 static void
 text_viewer_window_init (TextViewerWindow *self)
 {
 	gtk_widget_init_template (GTK_WIDGET (self));
+
+        g_autoptr (GSimpleAction) open_action = g_simple_action_new ("open", NULL);
+        g_signal_connect (open_action, "activate", G_CALLBACK (text_viewer_window__open_file_dialog), self);
+        g_action_map_add_action (G_ACTION_MAP (self), G_ACTION (open_action));
+
 }

src/text-viewer-window.ui

@@ -13,6 +13,12 @@
       <object class="AdwToolbarView">
         <child type="top">
           <object class="AdwHeaderBar" id="header_bar">
+            <child type="start">
+              <object class="GtkButton" id="open_button">
+                <property name="label">Open</property>
+                <property name="action-name">win.open</property>
+              </object>
+            </child>
             <child type="end">
               <object class="GtkMenuButton">
                 <property name="primary">True</property>