add new GSource-derived object that can be attached to a Glib::MainContext to execute code before other sources have their dispatch() method invoked

libs/pbd/glib_event_source.cc

@@ -0,0 +1,19 @@
+#include "pbd/glib_event_source.h"
+
+bool
+GlibEventLoopSource::prepare (int& timeout)
+{
+	return false;
+}
+
+bool
+GlibEventLoopSource::check ()
+{
+	return false;
+}
+
+bool
+GlibEventLoopSource::dispatch (sigc::slot_base*)
+{
+	return false;
+}

libs/pbd/pbd/glib_event_source.h

@@ -0,0 +1,33 @@
+#ifndef __libpbd_glib_event_source_h__
+#define __libpbd_glib_event_source_h__
+
+#include <boost/function.hpp>
+
+#include <glibmm/main.h>
+
+class GlibEventLoopSource : public Glib::Source
+{
+  public:
+	GlibEventLoopSource () {};
+
+	bool prepare (int& timeout);
+	bool check();
+	bool dispatch (sigc::slot_base*);
+};
+
+
+class GlibEventLoopCallback : public GlibEventLoopSource
+{
+  public:
+	GlibEventLoopCallback (boost::function<void()> callback) : _callback (callback) {}
+
+	bool check() {
+		_callback();
+		return false;
+	}
+
+  private:
+	boost::function<void()> _callback;
+};
+
+#endif /* __libpbd_glib_event_source_h__ */

libs/pbd/wscript

@@ -51,6 +51,7 @@ libpbd_sources = [
     'file_archive.cc',
     'file_utils.cc',
     'fpu.cc',
+    'glib_event_source.cc',
     'id.cc',
     'locale_guard.cc',
     'localtime_r.cc',