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Nasal runtime re-loadable modules

Introduction and motivation

The embedded scripting language of FlightGear, Nasal comes with a limited number of core functions.

The language has been extended by C++ functions, as well as by libraries written in Nasal (for example props, io, math, debug etc).

The latter are stored in the FGData repository under /Nasal and loaded automatically by FlightGear.

Optional code, which is loaded only on demand, can be handled either by the legacy module system implemented in C++ or by modules.nas if run-time re-load support is required.

For more information on how and when these files are loaded, see Nasal Initialization.

Differences between add-ons and modules

While there are many similarities between add-ons and modules, there are also some differences:

Modules are distributed with FlightGear as part of FGData or they are aircraft specific and delivered with the aircraft.
Add-ons have to be downloaded separately by a FlightGear user from wherever the author of the add-on publishes the add-on.
Modules can be loaded for example by an aircraft if the aircraft developer wants to make use of the module.
Add-ons are selected by the user before launching FlightGear, thus they may or may not be available at runtime.

HOWTOs and examples

First some examples, the API documentation follows below.

Nasal modules in an aircraft

Nasal code for an aircraft (for example for development) is usually loaded by a XML declaration in the Aircraft-set.xml file like this:

        <foo> <!-- Nasal namespace foo -->
        <bar> <!-- Nasal namespace bar -->
        <myAircraft> <!-- Nasal namespace myAircraft-->

This is fine unless you are developing a Nasal subsystem for the aircraft and have to restart FlightGear every time you edit a few lines. For this case you could consider to use the Module class (defined in FGDATA/Nasal/modules.nas) and make your code re-loadable at runtime, which means you do not have to restart whole FlightGear just for a few lines of edited Nasal code.

Example Nasal/efis_module.nas:

#-- load EFIS as reloadable module
var my_efis ="myAircraft_EFIS"); # Module name
my_efis.setDebug(1); # 0=(mostly) silent; 1=print setlistener and maketimer calls to console; 2=print also each listener hit, be very careful with this! 

Menu item for reloading

Now add a menu item for easy reloading like this:

Note  The module name passed to the fgcommand must match the name passed to
        <menu n="100">
            <!-- normal aircraft menu -->
        <menu n="101">
            <label>Aircraft Development</label>
                <label>Reload EFIS</label>
Tip  You can also call myAircraft.my_efis.reload(); to trigger the reload. myAircraft is the namespace given in the Aircraft-set.xml file, my_efis is the module object (see above).

Nasal frameworks

Another use case for Modules.nas is frameworks that need reload support.

A framework usually provides more or less generic functionality that has to be adapted (configured) for a specific use case (for example a specific aircraft).

Reload support may become very convenient, if the customizing based on such framework involves many iterations of edit-reload-test.

First living example is the canvas_efis framework which can be included into an aircraft with a single line of code:

var efis_module = modules.load("canvas_efis");

(see also last chapter at the end of this article)

Structure of Nasal reloadable modules

A module must contain at least the file main.nas (default, other filename is possible) and should contain the functions main() and unload().

main() function

The main.nas file must contain a main() function that will be called on load with either the module object as parameter or the parameters passed to the module.load() function.

unload() function

If you want the module to be re-/un-loadable, you must make sure to track resources and remove them on onload. For this the main.nas shall contain a function unload() that removes any resources which were created by the module.

Example: any canvas created by the module should have called its del() method here.

Note  Calls to setlistener() and maketimer() are automatically tracked by the Module class for you, timers will be stopped automatically, listeners will be removed automatically on unload().

Rules for Module names

  1. Module names shall contain only letters (a-z, A-Z), numbers (0-9) and underscores (_).
  2. The name must contain at least one letter so it cannot be confused with a number
  3. The name shall not match any existing Nasal file (.nas) or directory in FGDATA/Nasal to avoid namespace clashes
Note  Reloadable modules shipped with FlightGear are stored in subdirectories of FGDATA/Nasal/modules/ as these subdirectories will not be loaded automatically by FlightGear on start.

However, modules.nas will scan this directory to create a list of available modules (list of subdirectories).

Each module corresponds to one subdirectory and the directory name is also the module name.


In modules.nas the class Module is defined.

A module object holds information about the path and filename of the Nasal script and supports unloading and reloading the code at runtime (for example without restarting FlightGear as a whole) by tracking some critical resources like listeners and timers.

Note  Parts of this functionality were added to the addon manager earlier and have now been extracted to avoid code duplication.
Caution  Within a module setlistener() is overloaded and the default value for the 4th argument (runtime) is changed to 0, so the listener will run only if the property value has changed.

library functions



This function returns true, if there is a module (subdirectory) in FGDATA/Nasal/modules/ with the given name.

The name of the module (in the subdirectory in Nasal/modules) to load.


if (modules.isAvailable("foo_bar")) {


modules.setDebug(module_name, [debug=1]);

This function enables debugging for a module. It must be called before load()!

The name of the module (in the subdirectory in Nasal/modules) to load.
Defaults to 1 (true), use 0 to disable debug. If debug > 1, each listener hit will be printed, be careful not to listen to rapidly changing props, do not set runtime=1 in setlistener().


var debug = 1;
modules.setDebug("foo_bar", debug);


modules.load(module_name, [namespace_name]);

This function attempts to load a module from FGDATA/Nasal/modules/

The name of the module (in the subdirectory in Nasal/modules) to load.
Optional, load module to a different namespace.



Methods of class Module



Configure where to look for the main file, for example in the aircraft (sub-)directory.

File path where the module is stored.



Configure the nasal file to load.

File that will be loaded by load().


# if you develop a new nasal system for your aircraft, it might be handy to implement it as module
# so you can reload the file quickly without restarting FlightGear
var my_foo_sys ="my_aircraft_foo");



Configure the Nasal namespace to use. Be really careful when using existing namespaces! unload() or reload() will destroy them!

The Nasal namespace the module code will be loaded into.



This changes the default setlistener behaviour for this module regarding 4th ('runtime') argument. FG default is 1 (=run listener every time the prop is written to), but you might want a default of 0 for your module (= run listener only, if value has changed). This is kind of a convenience function. It is better to explicitly specify the desired parameters when calling setlistener().

integer, 0..2 define the default value to setlister 4th parameter if not specified explicitly



Activate debugging for this module. Must be called before calling load()!

0: no debugging;

1 (default if no argument given): print calls to redirected setlister() and maketimer();

2: listeners print property path when hit (Use with caution! Do not call setlistener() with runtime=1.)



This function attempts to load the module into its namespace.

optional args
Arguments are passed to the main() function of the module. If empty, the module object will be passed to main().



This function attempts to remove tracked resources and remove the module by killing its namespace.



Shorthand, calls unload() and load().



Returns a variable from the modules namespace.

The variable to get.


var foo = modules.load("foo");
var bar = foo.get("bar"); # get variable "bar" defined in FGDATA/Nasal/modules/foo/main.nas (or a file included by this file)

Property tree interface for modules.nas

In the property tree there is a subtree /nasal to control modules and get some statistics.

The properties available depend on the type of module ("load-once" or "reloadable", see Nasal Initialization for more information on the differences).

Reloadable modules / frameworks

Modules handled by modules.nas will have their properties in /nasal/modules/<moduleName> where <moduleName> is given by the developer when calling either"<moduleName>") or modules.load("<moduleName>").

In the latter case <moduleName> specifies the subdirectory FGDATA/Nasal/modules/<moduleName> in which some framework is stored.

property type content
loaded bool true if module was loaded without errors
reload bool to trigger reload from the property browser. For menues, XML bindings and scripts the fgcommand is the prefered way to trigger reload.
listeners int Number of tracked listeners
listener-hits int If debugging is enabled, this prop shows the total number of hits to all tracked listeners.
timers int Number of tracked timers (maketimer).

Legacy load-once modules

A legacy load-once module is a direct (1st level) subdirectory of FGDATA/Nasal/ and its corresponding property tree is /nasal/<moduleName>/ where <moduleName> equals the name of the subdirectory.

It is handled by C++ code and must have a corresponding entry in FGDATA/defaults.xml which defines a property "enabled" and optionally a property "module".

If enabled is set to false in defaults.xml, the C++ code will setup a listener and load the module as soon as enabled is set to true.

The property name "module" is a bit misleading, it is used to define into which namespace the files shall be loaded.

For each Nasal file in the subdirectory a file[i] property is created holding the full path+filename.

The bool loaded property shows the status of the module.

Existing modules with reload support

Stable Nasal frameworks which support reloading can be added to FGDATA/Nasal/modules/<module_name>. This allows an aircraft developer to configure the framework for a specific aircraft and make use of the reload magic while developing the configuration.

Module name Desctiption time added
canvas_efis framework to manage canvas based EFIS screens 02/2020