Optionally detect not filtered GTK log messages

In the future it might be useful to run the code with WXTRACE=gtklog to
see if any GTK log messages we're filtering don't need to be filtered
any longer.

include/wx/gtk/private/log.h

@@ -34,8 +34,8 @@ public:
     }
 
     // Function to call to install this filter as the active one.
-    // Does nothing if run-time glib version is too old.
+    // Does nothing and just returns false if run-time glib version is too old.
-    void Install();
+    bool Install();
 
 protected:
     // Function to override in the derived class to actually filter: return
@@ -104,10 +104,15 @@ public:
     explicit LogFilterByMessage(const char* message)
         : m_message(message)
     {
-        Install();
+        // We shouldn't warn about anything if Install() failed.
+        m_warnNotFiltered = Install();
     }
 
     // Remove this filter when the object goes out of scope.
+    //
+    // The dtor also checks if we actually filtered the message and logs a
+    // trace message with the "gtklog" mask if we didn't: this allows checking
+    // if the filter is actually being used.
     ~LogFilterByMessage();
 
 protected:
@@ -118,6 +123,8 @@ protected:
 private:
     const char* const m_message;
 
+    mutable bool m_warnNotFiltered;
+
     wxDECLARE_NO_COPY_CLASS(LogFilterByMessage);
 };
 

src/gtk/app.cpp

@@ -203,14 +203,14 @@ LogFilter::wx_log_writer(GLogLevelFlags   log_level,
     return g_log_writer_default(log_level, fields, n_fields, NULL);
 }
 
-void LogFilter::Install()
+bool LogFilter::Install()
 {
     if ( !ms_installed )
     {
         if ( glib_check_version(2, 50, 0) != 0 )
         {
             // No runtime support for log callback, we can't do anything.
-            return;
+            return false;
         }
 
         g_log_set_writer_func(LogFilter::wx_log_writer, NULL, NULL);
@@ -220,6 +220,8 @@ void LogFilter::Install()
     // Put this object in front of the linked list.
     m_next = ms_first;
     ms_first = this;
+
+    return true;
 }
 
 void LogFilter::Uninstall()
@@ -248,6 +250,7 @@ bool LogFilterByMessage::Filter(GLogLevelFlags WXUNUSED(log_level),
             if ( strcmp(static_cast<const char*>(f.value), m_message) == 0 )
             {
                 // This is the message we want to filter.
+                m_warnNotFiltered = false;
                 return true;
             }
         }
@@ -259,6 +262,11 @@ bool LogFilterByMessage::Filter(GLogLevelFlags WXUNUSED(log_level),
 LogFilterByMessage::~LogFilterByMessage()
 {
     Uninstall();
+
+    if ( m_warnNotFiltered )
+    {
+        wxLogTrace("gtklog", "Message \"%s\" wasn't logged.", m_message);
+    }
 }
 
 } // namespace wxGTKImpl