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Howto:Coding a simple Nasal Framework

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Revision as of 12:23, 26 February 2014 by Hooray (Talk | contribs) (Dealing with Properties)

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Illustrate the basic thought process required to come up with Nasal/Canvas code that is sufficiently generic to support the following requirements

  • support multiple independent instances (e.g. PFDs or NDs)
  • be aircraft/use-case agnostic (e.g. work without hard-coded properties)
  • configurable without touching back-end code (e.g. via configuration hashes)
  • modular (use separate files for splitting things up)

We'll be using the PFD/ND code as an example here, and won't be using any complicated techniques.


In order to support independent instances of each instrument, you need to use separate variables, so rather than having something like this at global scope:

var horizon = nil;
var markerBeacon = nil;
var markerBeaconText = nil;
var speedText = nil;
var machText = nil;
var altText = nil;
var selHdgText = nil;
var fdX = nil;
var fdY = nil;

You would instead use a hash, and populate it with your variables:

var PrimaryFlightDisplay= {
 new: func() { return {parents:[PrimaryFlightDisplay],}; },
 # set up fields
 horizon: nil,
 markerBeacon: nil,
 markerBeaconText: nil,
 speedText: nil,
 machText: nil,
 altText: nil,
 selHdgText: nil,
 fdX: nil,

The same thing can be accomplished by doing something like this in your constructor, using a temporary object:

var PrimaryFlightDisplay= {
 new: func() {
  var m = {parents:[PrimaryFlightDisplay]}; 
  m.horizon = nil;
  m.markerBeacon = nil;
  m.markerBeaconText = nil;
  m.speedText = nil;
  m.machText = nil;
  m.altText = "Hello World"; 
  m.selHdgText = nil;
  m.fdX = nil;
  m.fdY = nil;
  return m;

To create a new object, you would then simply have to call the .new() function:

 var myPFD = PrimaryFlightDisplay.new();
 print( myPDF.altText );

By using this method, you can easily create dozens of independent instances of your class:

 var PFDVector = [];
 forindex(var i=0;i<100;i+=1)
  append(PFDVector, PrimaryFlightDisplay.new() );

Dealing with Properties

The next complication is dealing with instrument-specific properties. Typically, code will have lots of getprop()/setprop() equivalent calls in many places. These need to be encapsulated, i.e. you don't want to use setprop/getprop (or prop.nas) directly for any properties that are specific to the instrument, otherwise your code would fail once several instances of it are active at the same time, because their property access may be competing/conflicting, such as overwriting properties.

One simple solution for this is to have your own setprop/getprop equivalent, as part of your class:

var PrimaryFlightDisplay = {

 set: func(property, value) {

 get: func(property, default) {

Your methods would then never use setprop/getprop directly, but instead use the set/get methods, by calling me.get() or me.set() respectively. The next step is identifying property that are instance-specific, i.e. that must not be shared with other instruments. One convenient way to accomplish this is using a numbered index for each instance, such as: /instrumentation/pfd[0], /instrumentation/pfd[1], /instrumentation/pfd[2] etc.

This would then be the place for your instance-specific properties.