x11: Dynamically update the scale factor

If the text-scaling-factor setting is available via D-Bus, add a listener and update the content scale values for the displays if the value is changed during runtime.

Factors out the D-Bus message pump from the system theme detection code to the general D-Bus code, as it's now used for more purposes than just the system theme.

src/core/linux/SDL_dbus.c

@@ -498,4 +498,16 @@ SDL_DBus_ScreensaverInhibit(SDL_bool inhibit)
 
     return SDL_TRUE;
 }
+
+void SDL_DBus_PumpEvents(void)
+{
+    if (dbus.session_conn) {
+        dbus.connection_read_write(dbus.session_conn, 0);
+
+        while (dbus.connection_dispatch(dbus.session_conn) == DBUS_DISPATCH_DATA_REMAINS) {
+            /* Do nothing, actual work happens in DBus_MessageFilter */
+            SDL_DelayNS(SDL_US_TO_NS(10));
+        }
+    }
+}
 #endif

src/core/linux/SDL_dbus.h

@@ -94,6 +94,8 @@ extern SDL_bool SDL_DBus_QueryPropertyOnConnection(DBusConnection *conn, const c
 extern void SDL_DBus_ScreensaverTickle(void);
 extern SDL_bool SDL_DBus_ScreensaverInhibit(SDL_bool inhibit);
 
+extern void SDL_DBus_PumpEvents(void);
+
 #endif /* HAVE_DBUS_DBUS_H */
 
 #endif /* SDL_dbus_h_ */

src/core/linux/SDL_system_theme.c

@@ -158,18 +158,3 @@ SDL_SystemTheme_Get(void)
 {
     return system_theme_data.theme;
 }
-
-void
-SDL_SystemTheme_PumpEvents(void)
-{
-    SDL_DBusContext *dbus = system_theme_data.dbus;
-    DBusConnection *conn;
-    if (dbus == NULL) return;
-    conn = dbus->session_conn;
-    dbus->connection_read_write(conn, 0);
-
-    while (dbus->connection_dispatch(conn) == DBUS_DISPATCH_DATA_REMAINS) {
-        /* Do nothing, actual work happens in DBus_MessageFilter */
-        usleep(10);
-    }
-}

src/core/linux/SDL_system_theme.h

@@ -26,6 +26,5 @@
 
 extern SDL_bool SDL_SystemTheme_Init(void);
 extern SDL_SystemTheme SDL_SystemTheme_Get(void);
-extern void SDL_SystemTheme_PumpEvents(void);
 
 #endif /* SDL_system_theme_h_ */

src/video/wayland/SDL_waylandevents.c

@@ -391,7 +391,7 @@ int Wayland_WaitEventTimeout(SDL_VideoDevice *_this, Sint64 timeoutNS)
 #endif
 
 #ifdef SDL_USE_LIBDBUS
-    SDL_SystemTheme_PumpEvents();
+    SDL_DBus_PumpEvents();
 #endif
 
     /* If key repeat is active, we'll need to cap our maximum wait time to handle repeats */
@@ -464,7 +464,7 @@ void Wayland_PumpEvents(SDL_VideoDevice *_this)
 #endif
 
 #ifdef SDL_USE_LIBDBUS
-    SDL_SystemTheme_PumpEvents();
+    SDL_DBus_PumpEvents();
 #endif
 
 #ifdef HAVE_LIBDECOR_H

src/video/x11/SDL_x11events.c

@@ -1706,7 +1706,7 @@ int X11_WaitEventTimeout(SDL_VideoDevice *_this, Sint64 timeoutNS)
 #endif
 
 #ifdef SDL_USE_LIBDBUS
-    SDL_SystemTheme_PumpEvents();
+    SDL_DBus_PumpEvents();
 #endif
     return 1;
 }
@@ -1751,7 +1751,7 @@ void X11_PumpEvents(SDL_VideoDevice *_this)
 #endif
 
 #ifdef SDL_USE_LIBDBUS
-    SDL_SystemTheme_PumpEvents();
+    SDL_DBus_PumpEvents();
 #endif
 
     /* FIXME: Only need to do this when there are pending focus changes */

src/video/x11/SDL_x11modes.c

@@ -41,15 +41,22 @@
 
 #ifdef SDL_USE_LIBDBUS
 
+#define SCALE_FACTOR_NODE "org.freedesktop.portal.Desktop"
+#define SCALE_FACTOR_PATH "/org/freedesktop/portal/desktop"
+#define SCALE_FACTOR_INTERFACE "org.freedesktop.portal.Settings"
+#define SCALE_FACTOR_NAMESPACE "org.gnome.desktop.interface"
+#define SCALE_FACTOR_SIGNAL_NAME "SettingChanged"
+#define SCALE_FACTOR_KEY "text-scaling-factor"
+
 static DBusMessage *ReadDBusSetting(SDL_DBusContext *dbus, const char *key)
 {
-    static const char *iface = "org.gnome.desktop.interface";
+    static const char *iface = SCALE_FACTOR_NAMESPACE;
 
     DBusMessage *reply = NULL;
-    DBusMessage *msg = dbus->message_new_method_call("org.freedesktop.portal.Desktop",  /* Node */
-                                                     "/org/freedesktop/portal/desktop", /* Path */
-                                                     "org.freedesktop.portal.Settings", /* Interface */
-                                                     "Read");                           /* Method */
+    DBusMessage *msg = dbus->message_new_method_call(SCALE_FACTOR_NODE,
+                                                     SCALE_FACTOR_PATH,
+                                                     SCALE_FACTOR_INTERFACE,
+                                                     "Read"); /* Method */
 
     if (msg) {
         if (dbus->message_append_args(msg, DBUS_TYPE_STRING, &iface, DBUS_TYPE_STRING, &key, DBUS_TYPE_INVALID)) {
@@ -85,11 +92,75 @@ static SDL_bool ParseDBusReply(SDL_DBusContext *dbus, DBusMessage *reply, int ty
     return SDL_TRUE;
 }
 
+static void UpdateDisplayContentScale(float scale)
+{
+    SDL_VideoDevice *viddevice = SDL_GetVideoDevice();
+    int i;
+
+    if (viddevice) {
+        for (i = 0; i < viddevice->num_displays; ++i) {
+            SDL_SetDisplayContentScale(&viddevice->displays[i], scale);
+        }
+    }
+}
+
+static DBusHandlerResult DBus_MessageFilter(DBusConnection *conn, DBusMessage *msg, void *data)
+{
+    SDL_DBusContext *dbus = SDL_DBus_GetContext();
+    double *scale_factor = (double *)data;
+    double new_scale = 0.0;
+
+    if (dbus->message_is_signal(msg, SCALE_FACTOR_INTERFACE, SCALE_FACTOR_SIGNAL_NAME)) {
+        DBusMessageIter signal_iter, variant_iter;
+        const char *namespace, *key;
+
+        dbus->message_iter_init(msg, &signal_iter);
+        /* Check if the parameters are what we expect */
+        if (dbus->message_iter_get_arg_type(&signal_iter) != DBUS_TYPE_STRING) {
+            goto not_our_signal;
+        }
+        dbus->message_iter_get_basic(&signal_iter, &namespace);
+        if (SDL_strcmp(SCALE_FACTOR_NAMESPACE, namespace) != 0) {
+            goto not_our_signal;
+        }
+        if (!dbus->message_iter_next(&signal_iter)) {
+            goto not_our_signal;
+        }
+        if (dbus->message_iter_get_arg_type(&signal_iter) != DBUS_TYPE_STRING) {
+            goto not_our_signal;
+        }
+        dbus->message_iter_get_basic(&signal_iter, &key);
+        if (SDL_strcmp(SCALE_FACTOR_KEY, key) != 0) {
+            goto not_our_signal;
+        }
+        if (!dbus->message_iter_next(&signal_iter)) {
+            goto not_our_signal;
+        }
+        if (dbus->message_iter_get_arg_type(&signal_iter) != DBUS_TYPE_VARIANT) {
+            goto not_our_signal;
+        }
+        dbus->message_iter_recurse(&signal_iter, &variant_iter);
+        if (dbus->message_iter_get_arg_type(&variant_iter) != DBUS_TYPE_DOUBLE) {
+            goto not_our_signal;
+        }
+        dbus->message_iter_get_basic(&variant_iter, &new_scale);
+
+        if (new_scale > 0.0) {
+            *scale_factor = new_scale;
+            UpdateDisplayContentScale(new_scale);
+        }
+
+        return DBUS_HANDLER_RESULT_HANDLED;
+    }
+
+not_our_signal:
+    return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
+}
+
 #endif
 
 static float GetGlobalContentScale()
 {
-    static const char *text_scaling_factor = "text-scaling-factor";
     static double scale_factor = 0.0;
 
     if (scale_factor <= 0.0) {
@@ -99,8 +170,16 @@ static float GetGlobalContentScale()
         SDL_DBusContext *dbus = SDL_DBus_GetContext();
 
         if (dbus) {
-            if ((reply = ReadDBusSetting(dbus, text_scaling_factor))) {
-                ParseDBusReply(dbus, reply, DBUS_TYPE_DOUBLE, &scale_factor);
+            if ((reply = ReadDBusSetting(dbus, SCALE_FACTOR_KEY))) {
+                if (ParseDBusReply(dbus, reply, DBUS_TYPE_DOUBLE, &scale_factor)) {
+                    /* If the setting exists, register a listener for scale changes. */
+                    dbus->bus_add_match(dbus->session_conn,
+                                        "type='signal', interface='"SCALE_FACTOR_INTERFACE"',"
+                                        "member='"SCALE_FACTOR_SIGNAL_NAME"', arg0='"SCALE_FACTOR_NAMESPACE"',"
+                                        "arg1='"SCALE_FACTOR_KEY"'", NULL);
+                    dbus->connection_add_filter(dbus->session_conn, &DBus_MessageFilter, &scale_factor, NULL);
+                    dbus->connection_flush(dbus->session_conn);
+                }
                 dbus->message_unref(reply);
             }
         }