[glibmm] Gio::Settings: Add connect_changed().

[Murray Cumming <murrayc src gnome org>]

commit 46fcd11eeae90247914499cfd58acdf295679849
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed May 6 09:40:14 2015 +0200

    Gio::Settings: Add connect_changed().
    
    This wraps the strange "somekeyname::changed" signal in GSettings,
    so we can get notification when a specific key has changed.
    I would welcome any suggestion to improve this API.

 gio/src/settings.ccg |   15 +++++++++++++++
 gio/src/settings.hg  |   13 +++++++++++++
 2 files changed, 28 insertions(+), 0 deletions(-)
---
diff --git a/gio/src/settings.ccg b/gio/src/settings.ccg
index d800aa9..8fa0860 100644
--- a/gio/src/settings.ccg
+++ b/gio/src/settings.ccg
@@ -18,6 +18,7 @@
 #include <gio/gio.h>
 #include <glibmm/exceptionhandler.h>
 #include <glibmm/vectorutils.h>
+#include <glibmm/propertyproxy_base.h> //For PropertyProxyConnectionNode
 
 namespace Gio
 {
@@ -70,4 +71,18 @@ std::vector<Glib::ustring> Settings::list_schemas()
 }
 _DEPRECATE_IFDEF_END
 
+sigc::connection Settings::connect_changed(const Glib::ustring& key, const SlotChanged& slot)
+{
+  // Create a proxy to hold our connection info
+  // This will be deleted by destroy_notify_handler.
+  Glib::PropertyProxyConnectionNode* pConnectionNode = new Glib::PropertyProxyConnectionNode(slot, 
G_OBJECT(gobj()));
+
+  const Glib::ustring signal_name = "changed::" + key;
+  g_signal_connect_data(gobj(),
+    signal_name.c_str(), (GCallback)(&Glib::PropertyProxyConnectionNode::callback), pConnectionNode,
+    &Glib::PropertyProxyConnectionNode::destroy_notify_handler,
+    G_CONNECT_AFTER);
+  return sigc::connection(pConnectionNode->slot_);
+}
+
 }
diff --git a/gio/src/settings.hg b/gio/src/settings.hg
index 75407d5..abb50e9 100644
--- a/gio/src/settings.hg
+++ b/gio/src/settings.hg
@@ -201,6 +201,19 @@ _DEPRECATE_IFDEF_END
 
   //TODO?: _WRAP_PROPERTY("backend", Glib::RefPtr<SettingsBackend>)
 
+  /** See connect_changed().
+   * @newin{2,46}
+   */
+  typedef sigc::slot<void> SlotChanged;
+
+  /** Connect a callback @a slot to the "changed" signal for the @a key.
+   * This will be emitted whenever the the value for that key has been changed.
+   * @result A sigc::connection
+   *
+   * @newin{2,46}
+   */
+  sigc::connection connect_changed(const Glib::ustring& key, const SlotChanged& slot);
+
   _WRAP_PROPERTY("delay-apply", bool)
   _WRAP_PROPERTY("has-unapplied", bool)
   _WRAP_PROPERTY("path", std::string)