Howto:Exposing Subsystems to Nasal
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Document how existing FlightGear subsystems (i.e. those inheriting from SGSubsystem) can be exposed to Nasal space to make certain information available to Nasal space.
The main mechanism for triggering scripted Nasal code to be executed within the FlightGear main loop are so called listeners (Nasal callbacks that are stored in a property-specific vector/list, that get executed in a for-each loop whenever a property is modified), and timers, which represent "delays" (typically in seconds or milliseconds) after which a timer expires and fires its callback to be executed.
Unfortunately, these two mechanisms are rather low-level and tedious/error-prone to use correctly, so that code (Nasal callbacks) may be triggered accidently, and unnecessarily, which ultimately eats up considerable resources while creating a frame, sooner or later causing frame stuttering.
Even more unfortunate is the fact that FlightGear does not currently provide any mechanisms/APIs to look behind the scenes, i.e. to see how often a callback is actually executed, and where the callback is registered to be executed.
This article intends to document the steps required to expose meta information about timers and listeners to scripting space so that a Nasal/Canvas bsed UI (dialog) can be displayed with a list of active callbacks, and their runtime characteristics.
This will include patches to all 3 repositories of FlightGear, i.e. SimGear, FlightGear and fgdata.