Howto:Create animation XML files from Nasal

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Objective: Use Nasal scripting to dynamically create an XML file by using the setprop() API call to set up your XML file's PropertyList-encoded structure and then writing the result to the file system using the write_properties() wrapper found in $FG_ROOT/Nasal/io.nas.

There are two wrappers provided: read_properties() and write_properties. These are wrappers for the corresponding fgcommands "load-xml" and "save-xml".

So, you don't need to look into the low level XML processing routines, you can directly use these PropertyList-wrappers - these allow you to directly dump a property tree sub branch to an XML file. So you can create your structure dynamically using setprop() and at the end use the write_properties() wrapper to write the resulting XML file to disk. That should save you some time.

Background

In general, this shouldn't really be necessary - because XML files are meant to be created and edited manually. On the other hand, due to the lack of a simple 2D drawing API accessible from Nasal, more and more complex "glass cockpit"-style instruments are getting implemented using the XML-based approach. These files (often more than 100-200 kbytes in size) contain fairly repetitive markup that could (and should) be created procedurally in an automated fashion. So, the approach suggested here is to use a separate Nasal script to create the required XML markup dynamically, rather than having to copy/paste and customize hundreds of XML animations manually.

Getting started

The following piece of Nasal creates a new XML file:

 var location= "/temp/test/foo"; # location in the global FG property tree
 var filename="test.xml"; # location in the local file system (will be OVERWRITTEN/DELETED!)
 setprop(location, "hello world");
 io.write_properties(filename, location);

The created output is:

 <?xml version="1.0"?>
 <PropertyList>
  <temp>
   <test>
    <foo>hello world</foo>
   </test>
  </temp>

Once you understand how this works, you can dynamically create PropertyList-encoded XML files for your instruments. Consider the following XML file containing a single animation:

       <path>vsd.ac</path>
       <animation>
          <type>select</type>
          <object-name>vsd</object-name>
          <condition>
             <greater-than-equals>
                <property>systems/electrical/outputs/efis</property>
                <value>9</value>
             </greater-than-equals>
          </condition>
       </animation>

Now, to create this snippet of XML procedurally, you can use this piece of Nasal:

    var location = "/temp/test/";
    var filename="xmltest.xml";
    setprop(location~"path", "vsd.ac");
    setprop(location~"animation/type", "select");
    setprop(location~"animation/object-name", "vsd");
    setprop(location~"animation/condition/greather-than-equals/property", "systems/electrical/outputs/efis");
    setprop(location~"animation/condition/greather-than-equals/value", "9");
    io.write_properties(filename, location);

Next, you could add a handful of helpers (or even a class wrapping animations) to make things a bit easier. Anybody interested in coming up with an OOP wrapper for this, should look into using the props.nas module available in $FG_ROOT/Nasal.