Melchior FRANZ:

add optional arg to SGPropertyNode::addChangeListener that triggers
the listener function call initially. This is useful for cases where
a freshly installed listener wants to treat the current property
value as changed from 'unknown' to the actual value right away.

Examples can be found in the Nasal incarnation setlistener(),
where we have for example this (in $FG_ROOT/Nasal/gui.nas):

  INIT = func {
      ...
      setlistener("/sim/rendering/fps-display", fpsDisplay);
      if (getprop("/sim/rendering/fps-display")) {
          fgcommand("dialog-show", props.Node.new({"dialog-name": "fps"}));
      }
  }

That is: we first attach a listener that cares for changes to the FPS
display switch, but then we have to manually open the dialog initially.
That's a duplication of code and could be as simple as this
(INIT part only):

  INIT = func {
      ...
      setlistener("/sim/rendering/fps-display", fpsDisplay, 1);
  }

That is: the optional third arg makes fpsDisplay be called initially,
and then again with every write action. My first solution was in the
Nasal code only, but Andy (rightfully) says that this should rather
be in sg.

diff --git a/simgear/props/props.cxx b/simgear/props/props.cxx
index 673cde4cbf0aaa468b6064b83419df16eae1c18c..2f26f2a697648a12f9b154614f49b4f493b08eac 100644 (file)
--- a/simgear/props/props.cxx
+++ b/simgear/props/props.cxx
@@ -2030,12 +2030,15 @@ SGPropertyNode::untie (const char * relative_path)
 }
 
 void
-SGPropertyNode::addChangeListener (SGPropertyChangeListener * listener)
+SGPropertyNode::addChangeListener (SGPropertyChangeListener * listener,
+                                   bool initial)
 {
   if (_listeners == 0)
     _listeners = new vector<SGPropertyChangeListener*>;
   _listeners->push_back(listener);
   listener->register_property(this);
+  if (initial)
+    listener->valueChanged(this);
 }
 
 void

diff --git a/simgear/props/props.hxx b/simgear/props/props.hxx
index fb04b6bf2d091976ecdf0ec6d7aee69097a04082..332d47a29ad276facbbd61f5c6fb5148279f52c4 100644 (file)
--- a/simgear/props/props.hxx
+++ b/simgear/props/props.hxx
@@ -1097,9 +1097,11 @@ public:
 
 
   /**
-   * Add a change listener to the property.
+   * Add a change listener to the property. If "initial" is set call the
+   * listener initially.
    */
-  void addChangeListener (SGPropertyChangeListener * listener);
+  void addChangeListener (SGPropertyChangeListener * listener,
+                          bool initial = false);
 
 
   /**