``<condition>``
===============

The ``<condition>`` element is used to define logical conditions in FlightGear using XML. A
number of child elements can be used inside of an opening ``<condition>`` and closing
``</condition>``, and chained together to create complex conditional statements.

Simple Conditions
-----------------

Before covering complex condition statements, it is worth discussing the usage of simplistic condition statements can be created using consisting only of the ``<condition>``
element, with a ``<property>`` element enclosed.




.. code-block:: xml

    <!-- Evaluates to either true or false based on the value of the property -->
    <condition>
        <property>engines/engine[0]/running</property>
    </condition>

For those with programming experience, this concept is similar to an expression that is implicitly
checking if the value is ``true``, or ``false``

.. code-block:: cpp

    if (engine_running) { // engine_running is true
        // do something
    }

These child elements include:

.. toctree::
   :maxdepth: 3

   and
   equals
   false
   greater-than-equals
   greater-than
   not-equals
   less-than-equals
   less-than
   not
   or
   true

Usage
-----

.. code-block:: xml

    <condition>
        <equals>
            <property>/my/p
            <value>my-value</value>
        </equals>
    </condition>

Sub-tags
--------

Examples
--------

Because conditional logic is such a foundational building block, it can be used in a tremendous
number of different ways. Imagine an aircraft developer wants to simulation a failing engine.
Part of this engine failure simulation will include a particle effect, so that when the engine is
failing, it begins to emit smoke. In order to prevent the engine from smoking all of the time, a
conditional statement can be used alongside the particle effect, to ensure that it is only active
under the right conditions.

.. code-block:: xml

    <condition>
        <and>
            <!-- If the engine is smoking.. -->
            <equals>
                <property>engines/engine[0]/running</property>
                <value>1</value>
            </equals>

            <!-- and the engine is running.. -->
            <equals>
                <property>engines/engine[0]/smoking</property>
                <value>1</value>
            </equals>
        </and>
    </condition>