Howto:Work with the Property Tree API

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Most FlightGear systems communicate internally using the so called Property Tree (see Property Tree Intro for an introduction).

This article is meant to introduce you to the C++ API that is used by FlightGear to work with the Property Tree.

Most of the high level API can be found in the $FG_SRC/Main folder, specifically the files fg_props.cxx and the corresponding fg_props.hxx header are relevant (this is the one, that you'll want to include when working with the property tree).

The property tree implementation itself is to be found in the SimGear sources ([1] see specifically SGPropertyNode).

Checking for Property Existence

bool fgHasNode (const char * path);

Getters

SGPropertyNode * fgGetNode (const char * path, bool create);
SGPropertyNode * fgGetNode (const char * path, int index, bool create);

Boolean Values

bool fgSetBool (const char * name, bool val);
bool fgGetBool (const char * name, bool defaultValue);

Integers

bool fgSetInt (const char * name, int val);
int fgGetInt (const char * name, int defaultValue);

Floats

bool fgSetFloat (const char * name, float val);
float fgGetFloat (const char * name, float defaultValue);

Doubles

bool fgSetDouble (const char * name, double val);
double fgGetDouble (const char * name, double defaultValue);

Longs

long fgGetLong (const char * name, long defaultValue);
fgSetLong (const char * name, long val);

Strings

bool fgSetString (const char * name, const char * val);
const char * fgGetString (const char * name, const char * defaultValue);

Using Listeners

void fgAddChangeListener (SGPropertyChangeListener * listener, const char * path);
void fgAddChangeListener (SGPropertyChangeListener * listener, const char * path, int index);

Attributes

Persistance

void fgSetArchivable (const char * name, bool state);

Access

void
fgSetReadable (const char * name, bool state)
{
 SGPropertyNode * node = globals->get_props()->getNode(name);
 if (node == 0)
   SG_LOG(SG_GENERAL, SG_DEBUG,

"Attempt to set read flag for non-existant property " << name);

 else
   node->setAttribute(SGPropertyNode::READ, state); 
}

void
fgSetWritable (const char * name, bool state)
{
 SGPropertyNode * node = globals->get_props()->getNode(name);
 if (node == 0)
   SG_LOG(SG_GENERAL, SG_DEBUG,

"Attempt to set write flag for non-existant property " << name);

 else
   node->setAttribute(SGPropertyNode::WRITE, state);
}

Tied Properties

void
fgUntie (const char * name)
{
 if (!globals->get_props()->untie(name))
   SG_LOG(SG_GENERAL, SG_WARN, "Failed to untie property " << name);
}