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Howto:Coding a simple Nasal Framework

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Revision as of 12:15, 26 February 2014 by Hooray (Talk | contribs)

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Objective

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.


Variables

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,
 fdY: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