CONDITIONS IN FLIGHTGEAR PROPERTY FILES

Written by David Megginson, david@megginson.com
Last modified: $Date$

This document is in the Public Domain and comes with NO WARRANTY!


1. Introduction
---------------

Some FlightGear property files contain conditions, affecting whether
bindings or animations are applied.  For example, the following
binding will apply only when the /sim/input/selected/engine[0]
property is true:

  <binding>
   <condition>
    <property>/sim/input/selected/engine[0]</property>
   </condition>
   <command>property-assign</command>
   <property>/controls/starter[0]</property>
   <value type="bool">true</value>
  </binding>

Conditions always occur within a property subtree named "condition",
which is equivalent to an "and" condition.


2. Comparison Operators
-----------------------

The simplest condition is "property".  It resolves as true when the
specified property has a boolean value of true (i.e. non-zero, etc.)
and false otherwise.  Here is an example:

  <condition>
   <property>/sim/input/selected/engine[0]</property>
  </condition>

For more sophisticated tests, you can use the "less-than",
"less-than-equals", "greater-than", "greater-than-equals", "equals",
and "not-equals" comparison operators.  These all take two operands,
either two "property" operands or one "property" and one "value"
operand, and return true or false depending on the result of the
comparison.  The value of the second operand is always forced to the
type of the first; for example, if you compare a string and a double,
the double will be forced to a string and lexically compared.  If one
of the operands is a property, it is always assumed to be first.  Here
is an example of a comparison that is true only if the RPM of the
engine is less than 1500:

  <condition>
   <less-than>
    <property>/engines/engine[0]/rpm</property>
    <value>1500</value>
   </less-than>
  </condition>


3. Boolean Operators
--------------------

Finally, there are the regular boolean operators "and", "or", and
"not".  Each one surrounds a group of other conditions, and these can
be nested to arbitrary depths.  Here is an example:

  <condition>
   <and>
    <or>
     <less-than>
      <property>/engines/engine[0]/rpm</property>
      <value>1500</value>
     </less-than>
     <greater-than>
      <property>/engines/engine[0]/rpm</property>
      <value>2500</value>
     </greater-than>
    </or>
    <property>/engines/engine[0]/running</property>
   </and>
  </condition>

The top-level "condition" is an implicit "and".


4. Approximating if...else
--------------------------

There is no equivalent to the regular programming 'else' statement in
FlightGear conditions; instead, each condition separately must take
the others into account.  For example, the equivalent of

  if (x == 3) ... else if (y == 5) ... else ...

in FlightGear conditions is

  <condition>
   <equals>
    <property>/x</property>
    <value>3</value>
   </equals>
   <not>
    <equals>
     <property>/y</property>
     <value>5</value>
    </equals>
   </not>
  </condition>

and then

  <condition>
   <equals>
    <property>/y</property>
    <value>5</value>
   </equals>
   <not>
    <equals>
     <property>/x</property>
     <value>3</value>
    </equals>
   </not>
  </condition>

and then

  <condition>
   <not>
    <equals>
     <property>/x</property>
     <value>3</value>
    </equals>
   </not>
   <not>
    <equals>
     <property>/y</property>
     <value>5</value>
    </equals>
   </not>
  </condition>

It's verbose, but it works nicely within existing property-based
formats and provides a lot of flexiblity.


5. Syntax Summary
-----------------

Here's a quick syntax summary:

* <and>...</and>

  Contains one or more subconditions, all of which must be true.

* <condition>...</condition>

  The top-level container for conditions, equivalent to an "and" group

* <equals>...</equals>

  Contains two properties or a property and value, and is true if the
  properties have equivalent values.

* <not-equals>...</not-equals>

  Contains two properties or a property and value, and is true if the
  properties have different values.

* <greater-than>...</greater-than>

  Contains two properties or a property and a value, and is true if
  the second property or the value has a value greater than the first
  property.

* <greater-than-equals>...</greater-than-equals>

  Contains two properties or a property and a value, and is true if
  the second property or the value has a value greater than or equal
  to the first property.

* <less-than>...</less-than>

  Contains two properties or a property and a value, and is true if
  the second property or the value has a value less than the first
  property.

* <less-than-equals>...</less-than-equals>

  Contains two properties or a property and a value, and is true if
  the second property or the value has a value less than or equal
  to the first property.

* <not>...</not>

  Contains one subcondition, which must not be true.

* <not-equals>...</not-equals>

  Contains two properties or a property and value, and is true if the
  properties do not have equivalent values.

* <or>...</or>

  Contains one or more subconditions, at least one of which must be
  true.

* <property>...</property>

  The name of a property to test.

* <value>...</value>

  A literal value in a comparison.