skins[-qt]: Prompt to restart with XWayland

See accompanying change on core side:
"Enable XWayland only for skinned UI"

Author: jlindgren90

src/skins-qt/plugin.cc

@@ -176,10 +176,11 @@ bool QtSkins::init ()
         return false;
     }
 
-    if (QGuiApplication::platformName() == "wayland")
+    if (QGuiApplication::platformName () == "wayland" &&
+        qgetenv ("DISPLAY").isEmpty ())
     {
-        AUDERR ("The Winamp interface is not supported on Wayland. "
-                "Please run Audacious via XWayland.\n");
+        AUDERR ("The Winamp interface is not supported on Wayland, and "
+            "XWayland does not appear to be available on this system.\n");
         audqt::cleanup ();
         return false;
     }

src/skins-qt/view.cc

@@ -20,9 +20,15 @@
  */
 
 #include <libaudcore/hook.h>
+#include <libaudcore/i18n.h>
 #include <libaudcore/mainloop.h>
 #include <libaudcore/runtime.h>
+#include <libaudqt/libaudqt.h>
 
+#include <QGuiApplication>
+#include <QMessageBox>
+#include <QPointer>
+#include <QPushButton>
 #include <QWindow>
 
 #include "plugin.h"
@@ -38,8 +44,53 @@
 #include "view.h"
 #include "window.h"
 
+class QuitOnCloseMessageBox : public QMessageBox
+{
+protected:
+    void closeEvent (QCloseEvent *) override {
+        aud_quit ();
+    }
+};
+
 void view_show_player (bool show)
 {
+    static QPointer<QuitOnCloseMessageBox> dialog;
+
+    if (show && QGuiApplication::platformName () == "wayland")
+    {
+        if (! dialog)
+        {
+            dialog = new QuitOnCloseMessageBox;
+
+            auto restart = new QPushButton (audqt::translate_str (N_("Restart")), dialog);
+            auto quit = new QPushButton (audqt::translate_str (N_("Quit")), dialog);
+
+            restart->setIcon (QIcon::fromTheme ("view-refresh"));
+            quit->setIcon (QIcon::fromTheme ("application-exit"));
+
+            QObject::connect (restart, & QPushButton::clicked, aud_request_restart);
+            QObject::connect (quit, & QPushButton::clicked, aud_quit);
+
+            dialog->setIcon (QMessageBox::Warning);
+            dialog->setWindowTitle (_("Please Restart"));
+            dialog->setText
+                (_("The Winamp interface requires windowing system features not "
+                "supported by Wayland. Audacious will attempt to use XWayland "
+                "compatibility mode after restart."));
+
+            dialog->addButton (restart, QMessageBox::AcceptRole);
+            dialog->addButton (quit, QMessageBox::RejectRole);
+            dialog->setDefaultButton (restart);
+        }
+
+        dialog->show ();
+        show = false; // don't show main window
+    }
+    else if (dialog)
+    {
+        delete dialog.data ();
+    }
+
     if (show)
     {
         mainwin->show ();

src/skins/plugin.cc

@@ -169,10 +169,11 @@ bool SkinnedUI::init ()
     audgui_init ();
 
 #ifdef GDK_WINDOWING_WAYLAND
-    if (GDK_IS_WAYLAND_DISPLAY (gdk_display_get_default ()))
+    if (GDK_IS_WAYLAND_DISPLAY (gdk_display_get_default ()) &&
+        ! g_getenv ("DISPLAY"))
     {
-        AUDERR ("The Winamp interface is not supported on Wayland. "
-                "Please run Audacious via XWayland.\n");
+        AUDERR ("The Winamp interface is not supported on Wayland, and "
+            "XWayland does not appear to be available on this system.\n");
         audgui_cleanup ();
         return false;
     }

src/skins/view.cc

@@ -20,8 +20,15 @@
  */
 
 #include <libaudcore/hook.h>
+#include <libaudcore/i18n.h>
 #include <libaudcore/mainloop.h>
 #include <libaudcore/runtime.h>
+#include <libaudgui/libaudgui.h>
+#include <libaudgui/libaudgui-gtk.h>
+
+#ifdef GDK_WINDOWING_WAYLAND
+#include <gdk/gdkwayland.h>
+#endif
 
 #include "plugin.h"
 #include "plugin-window.h"
@@ -38,6 +45,37 @@
 
 void view_show_player (bool show)
 {
+#ifdef GDK_WINDOWING_WAYLAND
+    static GtkWidget * dialog;
+
+    if (show && GDK_IS_WAYLAND_DISPLAY (gdk_display_get_default ()))
+    {
+        if (! dialog)
+        {
+            auto restart = audgui_button_new (_("Restart"), "view-refresh",
+                [] (void *) { aud_request_restart (); }, nullptr);
+            auto quit = audgui_button_new (_("Quit"), "application-exit",
+                [] (void *) { aud_quit (); }, nullptr);
+
+            dialog = audgui_dialog_new (GTK_MESSAGE_WARNING, _("Please Restart"),
+                _("The Winamp interface requires windowing system features not "
+                "supported by Wayland. Audacious will attempt to use XWayland "
+                "compatibility mode after restart."),
+                restart, quit);
+
+            g_signal_connect (dialog, "delete-event", aud_quit, nullptr);
+            g_signal_connect (dialog, "destroy", [] () { dialog = nullptr; }, nullptr);
+        }
+
+        gtk_widget_show_all (dialog);
+        show = false; // don't show main window
+    }
+    else if (dialog)
+    {
+        gtk_widget_destroy (dialog);
+    }
+#endif
+
     if (show)
     {
         // "Move" the window to the position it's already at. This seems