replace theme heartbeat as the implementation was flawed. instead check window object for existence of required theme functions and give user option to switch themes if check fails.

src/greeter.c

@@ -69,17 +69,12 @@ static int
 
 static gint config_timeout;
 
-static gboolean
+static gboolean debug_mode;
-	debug_mode,
-	heartbeat,
-	heartbeat_exit;
 
 
 static void
 initialize_web_extensions_cb(WebKitWebContext *context, gpointer user_data) {
-
 	webkit_web_context_set_web_extensions_directory(context, WEBEXT_DIR);
-
 }
 
 
@@ -104,7 +99,6 @@ context_menu_cb(WebKitWebView *view,
 				GdkEvent *event,
 				WebKitHitTestResult *hit_test_result,
 				gpointer user_data) {
-
 	/* Returning true without creating a custom context menu results in no context
 	 * menu being shown. Thus, we are returning the opposite of debug_mode to get
 	 * desired result (which is only show menu when debug_mode is enabled.
@@ -113,50 +107,6 @@ context_menu_cb(WebKitWebView *view,
 }
 
 
-static gboolean
-check_theme_heartbeat(void) {
-	if (! heartbeat && ! heartbeat_exit) {
-		/* Theme heartbeat not received. We assume that an error has occurred
-		 * which broke script execution. We will fallback to the simple theme
-		 * so the user won't be stuck with a broken login screen.
-		 */
-		g_warning("[ERROR] :: A problem was detected with the current theme. Falling back to simple theme...");
-		webkit_web_view_load_uri(
-			WEBKIT_WEB_VIEW(web_view),
-			g_strdup_printf("file://%ssimple/index.html", THEME_DIR)
-		);
-	}
-
-	heartbeat = FALSE;
-
-	return heartbeat;
-}
-
-
-/**
- * Callback for Theme Heartbeat.
- */
-static void
-theme_heartbeat_handler(void) {
-	if (! heartbeat) {
-		/* Setup g_timeout callback for theme heartbeat check */
-		g_timeout_add_seconds(8, (GSourceFunc) check_theme_heartbeat, NULL);
-		heartbeat = TRUE;
-		heartbeat_exit = FALSE;
-	}
-}
-
-
-/**
- * Before starting the user's session, the web process will send a signal to exit the hearbeat
- * in order to prevent a race condition while the greeter is shutting down.
- */
-static void
-theme_heartbeat_exit_handler(void) {
-	heartbeat_exit = TRUE;
-}
-
-
 /**
  * Makes the greeter behave a bit more like a screensaver if it was launched as
  * a lock-screen by blanking the screen.
@@ -210,17 +160,11 @@ message_received_cb(WebKitUserContentManager *manager,
 	if (strcmp(message_str, "LockHint") == 0) {
 		lock_hint_enabled_handler();
 
-	} else if (strcmp(message_str, "Heartbeat") == 0) {
-		theme_heartbeat_handler();
-
-	} else if (strcmp(message_str, "Heartbeat::Exit") == 0) {
-		theme_heartbeat_exit_handler();
 	} else {
 		printf("UI PROCESS - message_received_cb(): no match!");
 	}
 
 	g_free(message_str);
-
 }
 
 
@@ -275,6 +219,89 @@ javascript_bundle_injection_setup() {
 }
 
 
+static void
+theme_function_exists_cb(GObject *object,
+						 GAsyncResult *result,
+						 gpointer user_data) {
+
+	WebKitJavascriptResult *js_result;
+	GError                 *error = NULL;
+	JSValueRef              value;
+	JSGlobalContextRef      context;
+	gboolean                result_as_bool;
+
+	js_result = webkit_web_view_run_javascript_finish(WEBKIT_WEB_VIEW(object), result, &error);
+
+	if (NULL != error) {
+		g_warning ("Error running javascript: %s", error->message);
+		g_error_free(error);
+		return;
+
+	} else {
+		context = webkit_javascript_result_get_global_context(js_result);
+		value = webkit_javascript_result_get_value(js_result);
+
+		result_as_bool = JSValueToBoolean(context, value);
+	}
+
+	if (FALSE == result_as_bool) {
+		GtkWidget *dialog, *label, *content_area, *button, *button_box;
+
+		dialog = gtk_dialog_new_with_buttons(
+			_("Greeter Theme Error Detected"),
+			GTK_WINDOW(window),
+			GTK_DIALOG_MODAL | GTK_DIALOG_DESTROY_WITH_PARENT,
+			_("_Load Default Theme"),
+			GTK_RESPONSE_ACCEPT,
+			_("_Load Fallback Theme"),
+			GTK_RESPONSE_OK,
+			_("_Cancel"),
+			GTK_RESPONSE_REJECT,
+			NULL
+		);
+		content_area = gtk_dialog_get_content_area(GTK_DIALOG(dialog));
+		button = gtk_dialog_get_widget_for_response(GTK_DIALOG(dialog), GTK_RESPONSE_REJECT);
+		button_box = gtk_widget_get_parent(button);
+		label = gtk_label_new(_("An error was detected in the current theme that could interfere with the system login process."));
+
+		gtk_button_box_set_layout(GTK_BUTTON_BOX(button_box), GTK_BUTTONBOX_EXPAND);
+		gtk_container_add(GTK_CONTAINER(content_area), label);
+		gtk_widget_show_all(content_area);
+
+		gint response = gtk_dialog_run(GTK_DIALOG(dialog));
+		gchar *log_msg = "[ERROR] :: A problem was detected with the current theme. Falling back to simple theme...";
+
+		if (GTK_RESPONSE_ACCEPT == response) {
+			g_warning(log_msg);
+			webkit_web_view_load_uri(WEBKIT_WEB_VIEW(web_view), g_strdup_printf("file://%s/antergos/index.html", THEME_DIR));
+
+		} else if (GTK_RESPONSE_OK == response) {
+			g_warning(log_msg);
+			webkit_web_view_load_uri(WEBKIT_WEB_VIEW(web_view), g_strdup_printf("file://%s/simple/index.html", THEME_DIR));
+		}
+
+		gtk_widget_destroy(dialog);
+	}
+
+	webkit_javascript_result_unref(js_result);
+}
+
+
+gboolean
+maybe_show_theme_fallback_dialog(void) {
+	/* Check for existence of a function that themes must add to window object */
+	webkit_web_view_run_javascript(
+		WEBKIT_WEB_VIEW(web_view),
+		"(() => 'authentication_complete' in window)()",
+		NULL,
+		(GAsyncReadyCallback) theme_function_exists_cb,
+		NULL
+	);
+
+	return FALSE;
+}
+
+
 int
 main(int argc, char **argv) {
 	GdkScreen *screen;
@@ -355,9 +382,6 @@ main(int argc, char **argv) {
 
 	gdk_window_set_cursor(root_window, gdk_cursor_new_for_display(default_display, GDK_LEFT_PTR));
 
-	/* Set the GTK theme to Adwaita */
-	g_object_set(gtk_settings_get_default(), "gtk-theme-name", "Adwaita", NULL);
-
 	/* Setup the main window */
 	window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
 	screen = gtk_window_get_screen(GTK_WINDOW(window));
@@ -426,6 +450,9 @@ main(int argc, char **argv) {
 	/* Maybe disable the context (right-click) menu. */
 	g_signal_connect(WEBKIT_WEB_VIEW(web_view), "context-menu", G_CALLBACK(context_menu_cb), NULL);
 
+	/* Register callback to check if theme loaded successfully */
+	g_timeout_add_seconds(5, (GSourceFunc) maybe_show_theme_fallback_dialog, NULL);
+
 	/* There's no turning back now, let's go! */
 	gtk_container_add(GTK_CONTAINER(window), web_view);
 	webkit_web_view_load_uri(WEBKIT_WEB_VIEW(web_view),

src/gresource/css/style.css

@@ -2,3 +2,8 @@ window,
 GtkWindow {
 	background: #000000;
 }
+
+dialog .dialog-vbox > label,
+GtkDialog .dialog-vbox > GtkLabel {
+	margin: 20px 15px;
+}

src/gresource/js/ThemeHeartbeat.js

@@ -1,82 +0,0 @@
-/*
- * ThemeHeartbeat.js
- *
- * Copyright © 2016 Antergos Developers <dev@antergos.com>
- *
- * This file is part of lightdm-webkit2-greeter.
- *
- * lightdm-webkit2-greeter is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; either version 3 of the License, or
- * (at your option) any later version.
- *
- * lightdm-webkit2-greeter is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- * GNU General Public License for more details.
- *
- * The following additional terms are in effect as per Section 7 of the license:
- *
- * The preservation of all legal notices and author attributions in
- * the material or in the Appropriate Legal Notices displayed
- * by works containing it is required.
- *
- * You should have received a copy of the GNU General Public License
- * along with lightdm-webkit2-greeter; If not, see <http://www.gnu.org/licenses/>.
- */
-
-
-/**
- * Singleton class for the greeter theme heartbeat.
- *
- * Once instantiated, the instance will begin sending a heartbeat message to the greeter
- * every 5 seconds. Upon receiving the first heartbeat message, the greeter will schedule checks
- * to ensure that subsequent heartbeat messages are received.
- *
- * If a heartbeat message is not received by the time any of the greeter's
- * subsequent checks run, it will assume that there has been an error in the theme's script
- * execution and fall back to the simple theme.
- */
-class ThemeHeartbeat  {
-
-	constructor() {
-		if ( '__heartbeat' in window ) {
-			return __heartbeat;
-		}
-
-		window.__heartbeat = theme_utils.bind_this(this);
-		this.heartbeat = '';
-		this.heartbeats = 0;
-
-		this.initialize_theme_heartbeat();
-
-		return window.__heartbeat;
-	}
-
-	initialize_theme_heartbeat() {
-		if ( '' !== this.heartbeat ) {
-			console.log( 'Heartbeat has already been initialized!' );
-			return;
-		}
-
-		console.log('Initializing theme heartbeat.');
-
-		this.send_heartbeat();
-		this.heartbeat = setInterval(this.send_heartbeat, 5000);
-	}
-
-	send_heartbeat() {
-		++this.heartbeats;
-
-		if ( true === lightdm.session_starting ) {
-			clearInterval( this.heartbeat );
-			return;
-		}
-
-		if ( this.heartbeats < 2 ) {
-			console.log('Sending heartbeat...');
-		}
-
-		window.webkit.messageHandlers.GreeterBridge.postMessage('Heartbeat');
-	}
-}

src/gresource/js/ThemeUtils.js

@@ -227,8 +227,6 @@ const __theme_utils = new Promise( (resolve, reject) => {
 __theme_utils.then( result => {
 	window.theme_utils = result;
 
-	new ThemeHeartbeat();
-
 	/**
 	 * @deprecated
 	 * @type {LightDM.ThemeUtils}