54 lines
2 KiB
Text
54 lines
2 KiB
Text
|
|
Internals
|
|
---------
|
|
|
|
The core of FlightGear is the property system. This is a tree like internal
|
|
representation of global variables. The property system is explained more
|
|
in detail later on.
|
|
|
|
FlightGear' way of doing things is breaking it up into small pieces. There is
|
|
(for example) animation code that reacts on property changes. There is also a
|
|
Flight Dynamics model (FDM) that (amongst other things) updates properties.
|
|
There is a menu system that can display and alter properties. Then we have
|
|
sound code that plays sound based on ... properties.
|
|
|
|
Maybe you see a pattern evolve by now.
|
|
|
|
All subsystems are almost self containing. Most of the time they only read the
|
|
values of some properties, and sometimes they alter other properties. This is
|
|
the basic way of communicating between subsystems.
|
|
|
|
|
|
Property System
|
|
---------------
|
|
|
|
The property system is best described as an in-memory LDAP database which holds
|
|
the state of global variables. The system has a tree like hierarchy (like a
|
|
file system) and has a root node, sub nodes (like subdirectories) and end-nodes
|
|
(variables).
|
|
|
|
All variables are kept internally as raw values and can be converted to any
|
|
other supported type (boolean, int, float double and string).
|
|
|
|
Like a file system, every node can be accessed relative to the current node, or
|
|
absolute to the root node.
|
|
|
|
The property system also allows aliasing nodes to other nodes (like symbolic
|
|
linking files or directories to other files or directories) and may be assigned
|
|
read-only or read-write.
|
|
|
|
If necessary it would be possible for parts of the program to hold it's own
|
|
property tree, which is inaccessible from the global property tree, by keeping
|
|
track of it's own root-node.
|
|
|
|
Property I/O code allows one to easily read the tree from, or write the tree to
|
|
an XML file.
|
|
|
|
|
|
Scripting
|
|
---------
|
|
|
|
The scripting langage Nasal can also read and modify properties but it can also
|
|
be incorporated into the menu system. The documentation for Nasal can be found
|
|
here: http://www.plausible.org/nasal/flightgear.html
|
|
|