revert meaning of third, optional argument, and add an optional fourth

argument:

  setlistener(<property>, <func> [, <initial=0> [, <persistent=1>]])

This definition defaults to the old behavior. The fourth argument can
be set to zero, in which case the function is only called when the
node value has actually changed. It should really default to zero, but
then all callers would have to be reviewed, which is a bit too dangerous
"shortly" before a release.

src/Scripting/NasalSys.cxx

@@ -913,13 +913,16 @@ void FGNasalSys::NasalTimer::timerExpired()
 
 int FGNasalSys::_listenerId = 0;
 
-// setlistener(property, func, bool) extension function.  The first argument
-// is either a ghost (SGPropertyNode_ptr*) or a string (global property
-// path), the second is a Nasal function, the optional third one an integer.
-// If it is 1, then the listener is executed initially. If it's 2, then
-// the listener is only triggered when the node value actually changed. The
-// setlistener() function returns a unique id number, that can be used
-// as argument to the removelistener() function.
+// setlistener(<property>, <func> [, <initial=0> [, <persistent=1>]])
+// Attaches a callback function to a property (specified as a global
+// property path string or a SGPropertyNode_ptr* ghost). If the third,
+// optional argument (default=0) is set to 1, then the function is also
+// called initially. If the fourth, optional argument is set to 0, then the
+// function is only called when the property node value actually changes.
+// Otherwise it's called independent of the value whenever the node is
+// written to (default). The setlistener() function returns a unique
+// id number, which is to be used as argument to the removelistener()
+// function.
 naRef FGNasalSys::setListener(naContext c, int argc, naRef* args)
 {
     SGPropertyNode_ptr node;
@@ -941,10 +944,12 @@ naRef FGNasalSys::setListener(naContext c, int argc, naRef* args)
         return naNil();
     }
 
-    int type = argc > 2 && naIsNum(args[2]) ? args[2].num : 0;
+    bool persistent = argc > 3 ? naTrue(args[3]) : true;
     FGNasalListener *nl = new FGNasalListener(node, handler, this,
-            gcSave(handler), _listenerId, type);
-    node->addChangeListener(nl, type != 0);
+            gcSave(handler), _listenerId, persistent);
+
+    bool initial = argc > 2 && naTrue(args[2]);
+    node->addChangeListener(nl, initial);
 
     _listener[_listenerId] = nl;
     return naNum(_listenerId++);
@@ -980,20 +985,19 @@ naRef FGNasalSys::removeListener(naContext c, int argc, naRef* args)
 
 FGNasalListener::FGNasalListener(SGPropertyNode_ptr node, naRef handler,
                                  FGNasalSys* nasal, int key, int id,
-                                 int type) :
+                                 bool persistent) :
     _node(node),
     _handler(handler),
     _gcKey(key),
     _id(id),
-    _type(type),
     _nas(nasal),
     _active(0),
     _dead(false),
     _first_call(true),
+    _persistent(persistent),
     _last_int(0L),
     _last_float(0.0)
 {
-    unchanged(node);
 }
 
 FGNasalListener::~FGNasalListener()
@@ -1007,22 +1011,21 @@ void FGNasalListener::valueChanged(SGPropertyNode* node)
     // drop recursive listener calls
     if(_active || _dead)
         return;
-    if(_type == 2 && !_first_call && unchanged(node))
-        return;
-
-    SG_LOG(SG_NASAL, SG_DEBUG, "trigger listener #" << _id);
-    _active++;
-    _nas->_cmdArg = node;
-    _nas->call(_handler, naNil());
-    _active--;
+    if(_persistent || changed(node) || _first_call) {
+        SG_LOG(SG_NASAL, SG_DEBUG, "trigger listener #" << _id);
+        _active++;
+        _nas->_cmdArg = node;
+        _nas->call(_handler, naNil());
+        _active--;
+    }
     _first_call = false;
 }
 
-bool FGNasalListener::unchanged(SGPropertyNode* node)
+bool FGNasalListener::changed(SGPropertyNode* node)
 {
     SGPropertyNode::Type type = node->getType();
-    if(type == SGPropertyNode::NONE) return true;
-    if(type == SGPropertyNode::UNSPECIFIED) return false;
+    if(type == SGPropertyNode::NONE) return false;
+    if(type == SGPropertyNode::UNSPECIFIED) return true;
 
     bool result;
     switch(type) {
@@ -1031,7 +1034,7 @@ bool FGNasalListener::unchanged(SGPropertyNode* node)
     case SGPropertyNode::LONG:
         {
             long l = node->getLongValue();
-            result = l == _last_int;
+            result = l != _last_int;
             _last_int = l;
             return result;
         }
@@ -1039,14 +1042,14 @@ bool FGNasalListener::unchanged(SGPropertyNode* node)
     case SGPropertyNode::DOUBLE:
         {
             double d = node->getDoubleValue();
-            result = d == _last_float;
+            result = d != _last_float;
             _last_float = d;
             return result;
         }
     default:
         {
             string s = node->getStringValue();
-            result = s == _last_string;
+            result = s != _last_string;
             _last_string = s;
             return result;
         }

src/Scripting/NasalSys.hxx

@@ -132,11 +132,11 @@ private:
 class FGNasalListener : public SGPropertyChangeListener {
 public:
     FGNasalListener(SGPropertyNode_ptr node, naRef handler,
-                    FGNasalSys* nasal, int key, int id, int type);
+                    FGNasalSys* nasal, int key, int id, bool persistent);
 
     ~FGNasalListener();
     void valueChanged(SGPropertyNode* node);
-    bool unchanged(SGPropertyNode* node);
+    bool changed(SGPropertyNode* node);
 
 private:
     friend class FGNasalSys;
@@ -144,11 +144,11 @@ private:
     naRef _handler;
     int _gcKey;
     int _id;
-    int _type;
     FGNasalSys* _nas;
     unsigned int _active;
     bool _dead;
     bool _first_call;
+    bool _persistent;
     long _last_int;
     double _last_float;
     string _last_string;