Add wxApp::GTKSuppressDiagnostics()

This allows to avoid GTK messages that are often more annoying and
confusing than useful.

Closes https://github.com/wxWidgets/wxWidgets/pull/2609

Closes #19347.

include/wx/gtk/app.h

@@ -58,6 +58,12 @@ public:
     // various bugs arising due to this.
     static bool GTKIsUsingGlobalMenu();
 
+    // Provide the ability to suppress GTK output. By default, all output
+    // will be suppressed, but the user can pass in a mask specifiyng the
+    // types of messages to suppress. Flags are defined by glib with the
+    // GLogLevelFlags enum.
+    static void GTKSuppressDiagnostics(int flags = -1);
+
     // implementation only from now on
     // -------------------------------
 

interface/wx/app.h

@@ -1002,6 +1002,37 @@ public:
     */
     void SetUseBestVisual(bool flag, bool forceTrueColour = false);
 
+    /**
+        @name GTK-specific functions
+    */
+    //@{
+
+    /**
+        Disables the printing of various GTK messages.
+
+        This function can be called to suppress GTK diagnostic messages that
+        are output on the standard error stream by default.
+
+        The default value of the argument disables all messages, but you
+        can pass in a mask flag to specifically disable only particular
+        categories of messages.
+
+        Note that this function only works when using glib 2.50 (released in
+        September 2016) or later and does nothing with the older versions of
+        the library.
+
+        @param flags
+           The mask for the types of messages to suppress. Refer to the
+           glib documentation for the @c GLogLevelFlags enum, which defines
+           the various message types.
+
+        @onlyfor{wxgtk}
+
+        @since 3.1.6
+    */
+    static void GTKSuppressDiagnostics(int flags = -1);
+
+    //@}
 
     /**
         @name Mac-specific functions

src/gtk/app.cpp

@@ -178,6 +178,49 @@ bool wxApp::DoIdle()
     return keepSource;
 }
 
+// Custom Glib log writer: setting it is only possible with glib 2.50 or later.
+#if GLIB_CHECK_VERSION(2, 50, 0)
+extern "C" {
+static GLogWriterOutput
+wx_log_writer(GLogLevelFlags   log_level,
+              const GLogField *fields,
+              gsize            n_fields,
+              gpointer         user_data)
+{
+    const wxUIntPtr log_mask = reinterpret_cast<wxUIntPtr>(user_data);
+
+    GLogWriterOutput result;
+    if (log_level & log_mask)
+    {
+        result = G_LOG_WRITER_HANDLED;
+    }
+    else
+    {
+        result = g_log_writer_default(log_level, fields, n_fields, NULL);
+    }
+    return result;
+}
+}
+
+/* static */
+void wxApp::GTKSuppressDiagnostics(int flags)
+{
+    if (glib_check_version(2, 50, 0) == 0)
+    {
+        g_log_set_writer_func(
+            wx_log_writer,
+            (wxUIntToPtr)(flags == -1 ? G_LOG_LEVEL_MASK : flags),
+            NULL);
+    }
+}
+#else // glib < 2.50
+/* static */
+void wxApp::GTKSuppressDiagnostics(int WXUNUSED(flags))
+{
+    // We can't do anything here.
+}
+#endif // glib >=/< 2.50
+
 //-----------------------------------------------------------------------------
 // wxApp
 //-----------------------------------------------------------------------------