+/** A class that can return a Stateful object which is the subject of a MementoCommand.
+ *
+ * The existence of this class means that the undo record can refer to objects which
+ * don't exist in the session file. Currently this is used for
+ *
+ * 1. MIDI automation; when MIDI automation is edited, undo records are
+ * written for the AutomationList being changed. However this AutomationList
+ * is a temporary structure, built by a MidiModel, which doesn't get written
+ * to the session file. Hence we need to be able to go from a MidiSource and
+ * Parameter to an AutomationList. This Binder mechanism allows this through
+ * MidiAutomationListBinder; the undo record stores the source and parameter,
+ * and these are bound to an AutomationList by the Binder.
+ *
+ * 2. Crossfades; unlike regions, these are completely removed from a session
+ * when they are deleted. This means that the undo record can contain
+ * references to non-existant crossfades. To get around this, CrossfadeBinder
+ * can do `just-in-time' binding from the crossfade ID.
+ */
+template <class obj_T>
+class /*LIBPBD_API*/ MementoCommandBinder : public PBD::Destructible
+{
+public:
+ /** @return Stateful object to operate on */
+ virtual obj_T* get () const = 0;
+
+ /** @return Name of our type */
+ virtual std::string type_name () const {
+ return PBD::demangled_name (*get ());
+ }
+
+ /** Add our own state to an XMLNode */
+ virtual void add_state (XMLNode *) = 0;
+};
+
+/** A simple MementoCommandBinder which binds directly to an object */
+template <class obj_T>
+class /*LIBPBD_API*/ SimpleMementoCommandBinder : public MementoCommandBinder<obj_T>
+{
+public:
+ SimpleMementoCommandBinder (obj_T& o)
+ : _object (o)
+ {
+ _object.Destroyed.connect_same_thread (_object_death_connection, boost::bind (&SimpleMementoCommandBinder::object_died, this));
+ }
+
+ obj_T* get () const {
+ return &_object;
+ }
+
+ void add_state (XMLNode* node) {
+ node->add_property ("obj_id", _object.id().to_s());
+ }
+
+ void object_died () {
+ /* The object we are binding died, so drop references to ourselves */
+ this->drop_references ();
+ }
+
+private:
+ obj_T& _object;
+ PBD::ScopedConnection _object_death_connection;
+};
+