From b7d22508fa7a6a4c31c2fca901566fe505193dfa Mon Sep 17 00:00:00 2001
From: Paul Davis <paul@linuxaudiosystems.com>
Date: Sun, 29 Nov 2020 12:46:01 -0700
Subject: [PATCH] Add blib event callback to BaseUI, with a virtual method to
 be overridden by derived classes.

---
 libs/pbd/base_ui.cc    | 12 ++++++++++++
 libs/pbd/pbd/base_ui.h |  9 +++++++++
 2 files changed, 21 insertions(+)

diff --git a/libs/pbd/base_ui.cc b/libs/pbd/base_ui.cc
index 34b972b3a4..e2f368527f 100644
--- a/libs/pbd/base_ui.cc
+++ b/libs/pbd/base_ui.cc
@@ -60,6 +60,7 @@ BaseUI::BaseUI (const string& loop_name)
 	, m_context(MainContext::get_default())
 	, run_loop_thread (0)
 	, request_channel (true)
+	, glib_event_callback (boost::bind (&BaseUI::event_loop_precall, this))
 {
 	base_ui_instance = this;
 	request_channel.set_receive_handler (sigc::mem_fun (*this, &BaseUI::request_handler));
@@ -120,6 +121,12 @@ BaseUI::run ()
 	_main_loop = MainLoop::create (m_context);
 	attach_request_source ();
 
+	/*
+	 * every time the main loop runs (i.e. before any actual event handling)
+	 */
+
+	glib_event_callback.attach (m_context);
+
 	Glib::Threads::Mutex::Lock lm (_run_lock);
 	run_loop_thread = Glib::Threads::Thread::create (mem_fun (*this, &BaseUI::main_thread));
 	_running.wait (_run_lock);
@@ -175,3 +182,8 @@ BaseUI::attach_request_source ()
 	DEBUG_TRACE (DEBUG::EventLoop, string_compose ("%1: attach request source\n", event_loop_name()));
 	request_channel.attach (m_context);
 }
+
+void
+BaseUI::event_loop_precall ()
+{
+}
diff --git a/libs/pbd/pbd/base_ui.h b/libs/pbd/pbd/base_ui.h
index af501fc666..0d10308392 100644
--- a/libs/pbd/pbd/base_ui.h
+++ b/libs/pbd/pbd/base_ui.h
@@ -33,6 +33,7 @@
 #include "pbd/libpbd_visibility.h"
 #include "pbd/crossthread.h"
 #include "pbd/event_loop.h"
+#include "pbd/glib_event_source.h"
 #include "pbd/pthread_utils.h"
 
 /** A BaseUI is an abstraction designed to be used with any "user
@@ -75,6 +76,12 @@ class LIBPBD_API BaseUI : public sigc::trackable, public PBD::EventLoop
 	 */
 	void quit ();
 
+	/* derived classes can override this in order to call code once per
+	   main loop iteration (i.e. before any event dispatching is done
+	   within the main loop)
+	*/
+	virtual void event_loop_precall ();
+
   protected:
 	bool _ok;
 
@@ -122,6 +129,8 @@ class LIBPBD_API BaseUI : public sigc::trackable, public PBD::EventLoop
 
 	int setup_request_pipe ();
 	void main_thread ();
+
+	GlibEventLoopCallback glib_event_callback;
 };
 
 #endif /* __pbd_base_ui_h__ */