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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>Using Nasal with FlightGear</title>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<link rel="stylesheet" href="nasal.css">
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</head>
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<body>
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<h1>Using Nasal with FlightGear</h1>
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<p>This document is a tutorial on how to interface Nasal scripts
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with FlightGear. It is not an introduction to the Nasal language
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itself. For that, see Andy's <a
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href="http://www.plausible.org/nasal/">Nasal website</a> at <a
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href="http://www.plausible.org/nasal/">http://www.plausible.org/nasal</a>.
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The information there is sparse, but you should have it ready for
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reference while reading this document.
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<h2>Basic Nasal/FlightGear Integration</h2>
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<h3>Calling Nasal from Configuration File Bindings</h3>
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Nasal scripts can be used as FGBinding objects, and can therefore
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appear anywhere in a configuration file (keyboard, mouse and joystick
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bindings, etc...) that accepts a <code><binding></code> tag.
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The relevant command type is "nasal", and you place your Nasal code
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inside of the <code><script></code> tag:
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<pre>
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<binding>
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<command>nasal</command>
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<script>
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print("Binding Invoked!");
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</script>
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</binding>
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</pre>
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<p>The code above invokes the <code>print()</code> function. This is
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a simple extension function that simply prints out its arguments, in
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order, to the FlightGear console as a single-line log entry. It is
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useful for debugging, but little else.
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<p>Some command have SGPropertyNode arguments. This argument is
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available as a <code>props.Node</code> object and is returned from the
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built-in <code>cmdarg()</code> function. See below for full
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documentation, but as an example the following "joystick axis" binding
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will print the current axis value to the console:
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<pre>
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<binding>
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<command>nasal</command>
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<script>print(cmdarg().getNode("value").getValue());</script>
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</binding>
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</pre>
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<p>Note that the current implementation parses the Nasal code inside
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the <code><script></code> tag each time it is run. This means
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that you should avoid placing large code blocks inside a
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<code><script></code> tag for performance reasons. It also
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means that any local variables you set inside the script will be lost
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the next time it is run. See the discussion about namespaces and
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Nasal source files below for more information.
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<h3>Setting and Inspecting FlightGear Properties</h3>
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<h4>getprop() and setprop()</h4>
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<p>You can use <code>getprop()</code> and <code>setprop()</code>
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functions to interact with the global property tree. They work as you
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would expect. For example, the following Nasal code copies the value
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of the "/sim/foo" property to the "/sim/bar" property:
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<pre>setprop("/sim/bar", getprop("/sim/foo"));</pre>
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<p>Note that Nasal's notion of "type" is coarser than that of the
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SGPropertyNode class. Numers and strings come out of
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<code>getprop()</code> as Nasal scalars, of course, and boolean
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properties are converted to a numeric 1 or 0 by
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<code>getprop()</code>. But <b>all</b> non-string values passed to
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<code>setprop()</code> become doubles in the property tree. The
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props.Node interface (see below) provides the ability to set explicit
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types in the property tree.
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<p>A useful feature of getprop and setprop that you should be aware of
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is that they both accept variable numbers of arguments. These are
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concatenated together to form a property tree path internally within
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the function. This avoids the need for extensive string concatenation
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in the script for the common case where you are acting on a variable
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property root. That is, you can do:
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<pre>
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ThisAircraft = "/sim/my/aircraft/properties";
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setprop(ThisAircraft, "crashed", 0);
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</pre>
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to set the property "/sim/my/aircraft/properties/crashed" to false.
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This feature is useful for writing Nasal functions that will work on
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parameterized property trees.
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<h4>The props module</h4>
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<p>Think of the <code>getprop()</code> and <code>setprop()</code>
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functions as equivalents of the <code>fgGet*()</code> and
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<code>fgSet*()</code> C++ functions. They provide simple and easy
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access to the global property tree.
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<p>For some situations, however, you need finer control over the
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property nodes. For these situations you can use the props module,
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which provides a <code>Node</code> class similar to the SimGear
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<code>SGPropertyNode</code> class. The global tree is available as a
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<code>Node</code> object named <code>props.globals</code>. You can
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use the <code>getNode()</code> method on it to retrieve an arbitrary
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sub-node. The children of any given node can be inspected with
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<code>getChild()</code> and <code>getChildren()</code> methods. And
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of course its name, index, and value can be accessed with appropriate
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methods. See the reference below for complete documentation.
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<p>As seen above in the discussion on <code>fgcommand()</code>, you
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can also create new, "rootless" <code>Node</code> objects using
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<code>props.Node.new()</code>.
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<p>The most powerful method on a Node object is
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<code>setValues()</code>. This takes a Nasal hash table as its only
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argument, and initializes properties under the node using the
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key/value pairs in the hash. This works with vectors (i.e. indexed
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properties) and recursively, essentially making a deep copy of a Nasal
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object in the property tree.
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<p>For debugging (and amusement) purposes, the <code>props</code>
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modules also defines a <code>dump()</code> function which recursively
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prints the state of a property node to the console. Try
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<code>props.dump(props.globals)</code> to see it walk the entire tree.
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<h3>Invoking FlightGear Commands from Nasal</h3>
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<p>Just as Nasal code can be run as a command binding from the
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property tree, existing FlightGear commands can be invoked by Nasal
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code using the <code>fgcommand()</code> function. The first argument
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to this function is a string, equivalent to what you would place
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inside the <code><command></code> tag in a property binding.
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The second argument specifies the property tree that will be passed as
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the "arguments" to the command. It can be a either a string
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specifying a path in the global property tree or a props.Node object.
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Example:
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<pre>
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# Use a temporary property in the global tree
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ShowDialog = func {
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setprop("/nasal/tmp/dialog-args/dialog-name", arg[0];
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fgcommand("dialog-show", "/nasal/tmp/dialog-args");
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}
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# Does the same thing, but with a rootless Node object
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ShowDialog = func {
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fgcommand("dialog-show", props.Node.new({dialog-name : arg[0]});
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}
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</pre>
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These both define a <code>ShowDialog()</code> function, which pops up
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a named dialog from the <fgroot>/gui/dialogs directory. Calling
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<code>ShowDialog("autopilot")</code> will therefore pop up the
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autopilot dialog just as if it had been bound to a key.
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<p>The first variant uses a "temporary" property tree in the global
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property space to hold the arguments. The second creates a new
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<code>props.Node</code> object instead. Note that
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<code>props.Node.new()</code> accepts a hash table as its argument and
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uses that to initialize the returned property tree.
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<h3>Writing extended Nasal code in source files</h3>
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<p>Nasal is a "real" language, with real namespaces and modules. What
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"really" happens when you run a script binding is that the script is
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treated as a function body and bound (lexically, in the functional
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programming sense) to a single global namespace. It is as if it were
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enclosed in a <code>func { ... }</code> expression and executed inside
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a "file" containing all the global symbols.
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<p>Some symbols in the global namespace are built-in extension
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functions, like the print/getprop/setprop we have already seen.
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Others are objects (or hash tables -- they are the same thing in
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Nasal). These objects act as namespaces and can contain code of their
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own. One such example is the math library. The built-in math
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functions live in their own namespace and are accessible as
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<code>math.sin()</code>, <code>math.cos()</code>, etc...
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<p>The global namespace itself is available as a module named
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"globals". This allows you to create new symbols in the global
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namespace if you desire (be careful!) and to otherwise inspect its
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contents. It's just a hash table, after all. The following code will
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print all the symbols found in the global namespace:
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<pre>
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print("These are the symbols found in the global namespace:");
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foreach(i; keys(globals)) { print(" ", i); }
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</pre>
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<p>You can write your own modules, too. The mechanism is very simple:
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merely create a file with a ".nas" extension in the Nasal directory of
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your FlightGear base package. FlightGear will read, parse and execute
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these files during initialization, and create a module of the same
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name for use by your scripts. So you can write, say, a "mouse.nas"
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script. Functions defined therein are available to your script
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bindings (and any other nasal code on the system) as members of the
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global "mouse" object. So you can define bindings that do things like
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<code>mouse.handleXAxis(offset)</code> to call functions defined in
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the mouse.nas file (remember that "offset" is an automatically
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initialized variable containing the binding's offset argument).
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<h3>Including Nasal Code from Configuration Files</h3>
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<p>Nasal modules can also be imported from the property tree at
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initialization. This is useful for applications like
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aircraft-specific scripts that need to be loaded only when that
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aircraft is active. The usage is simple: the Nasal interpreter
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creates a module for every property node child of "/nasal" that it
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finds at initialization time. Example:
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<pre>
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<nasal>
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<c172>
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<file>Aircraft/c172/c172.nas</file>
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</c172>
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</nasal>
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</pre>
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This creates a module named "c172" and loads the contents of the
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Aircraft/c172/c172.nas file into it. The module name is, by default,
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the same as the property node. But this can be overridden with the
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<code><module></code> tag. This trick can be useful if you need
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to load extra script source into a previously-initialized module.
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<p>You can also write literal Nasal scripts inside the property files
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by including it in a <code><script></code> tag. This sample
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uses the <code><module></code> tag to add an extra function to
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the math library.
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<pre>
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<nasal>
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<c172-tmp1> <!-- Use a unique, dummy name -->
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<module>math</module>
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<script><[CDATA[
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# The math library doesn't include this, because Andy is a pedant
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# and thinks it's dangerous. But the c172 code just *has* to have
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# it.
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atan = func { return atan2(arg[0], 1) }
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]]></script>
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</c172-tmp1>
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</nasal>
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</pre>
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Note the use of a CDATA declaration. This is required to properly
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escape XML special characters like "<code><</code>". As it
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happens, this code doesn't use them. But the CDATA is good practice
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nonetheless.
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<h3>Function Reference</h3>
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<p>These are the built-in extension functions available to all Nasal
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code in FlightGear. Be sure to examine the <a
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href="http://www.plausible.org/nasal/doc.html">core Nasal
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documentation</a> at the <a
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href="http://www.plausible.org/nasal">Nasal site</a> as well. Only
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FlightGear-specific library code is documented here:
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<h4>Global Functions</h4>
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<dl>
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<dt>rand()
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<dd>Returns a random number in the range [0:1) (that is, 0.0 is a
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possible return value, but 1.0 is not).
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<dt>getprop()
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<dd>The arguments are concatenated to form a path to a global
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property node. Returns the value of that node, or nil if it does
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not exist.
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<dt>setprop()
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<dd>The final argument specifies a value to set. The remaining
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arguments are concatenated to form a property path as in
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getprop().
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<dt>print()
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<dd>The arguments are printed, in order, to the FlightGear console.
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A newline is appended by the logging code, none is
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required.
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<dt>fgcommand()
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<dd>The first argument is a string specifying a FlightGear command to
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execute (e.g. "show-dialog"). The second is a property sub-tree
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(either a global path string or a props.Node object) which will be
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passed to the command as arguments.
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<dt>settimer()
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<dd>The first argument is a Nasal expression which evaluates to a
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Nasal function object (it can be either a symbol name for a
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function or a literal <code>func { ... }</code>
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expression. The second argument is a (floating point) number
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specifying a delta time in seconds. Some time after that delta
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time has elapsed, the specified function will be invoked. Exact
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timing will depend on the frame rate of the simulator.
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<dt>interpolate()
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<dd>The first argument specifies a property. It can be either a
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string representing a global property name or a
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<code>props.Node</code> object. The remaining arguments specify
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pairs of value/delta-time numbers. The property is interpolated
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smoothly from its current value to the new value over the
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specified time delta, in seconds. Multiple value pairs can be
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used to indicate successive values or to acheive a piecewise
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linear approximation to a non-linear function. This function
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cancels any preexisting interpolation for that property, so
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<code>interpolate("/sim/countdown", 0, 0)</code> has the
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effect of cancelling interpolation of "/sim/countdown" and setting
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its value to zero.
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</dl>
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<h4>Property Module</h4>
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<dl>
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<dt>Node
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<dd>The <code>props.Node</code> class wraps a SGPropertyNode object,
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either in or outside of the global property tree. It supports the
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following methods:
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<dl>
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<dt>getType()
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<dd>Returns the "type" of the SGPropertyNode object. The return value
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is a string; one of: NONE, ALIAS, BOOL, INT, LONG, FLOAT, DOUBLE,
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STRING or UNSPECIFIED.
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<dt>getName()
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<dd>Returns the name of the property node.
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<dt>getIndex()
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<dd>Returns the child index of the property node.
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<dt>getValue()
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<dd>Returns the current value of the node, or nil if it has none.
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<dt>setValue()
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<dd>Sets the current value as either a string or a double, depending
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on the internal type of the argument.
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<dt>setIntValue()
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<dd>Sets the current value, forcing the type to INT
|
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<dt>setBoolValue()
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<dd>Sets the current value, forcing the type to BOOL
|
||||||
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<dt>setDoubleValue()
|
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<dd>Sets the current value, forcing the type to DOUBLE
|
||||||
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<dt>getParent()
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<dd>Returns a Node object representing this node's parent, or nil if
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it has none.
|
||||||
|
<dt>getChild()
|
||||||
|
<dd>Returns a named child, or nil if it does not exist. If multiple
|
||||||
|
children with that name exist, returns the one with an index of zero.
|
||||||
|
<dt>getChildren()
|
||||||
|
<dd>Returns a vector containing all the node's children.
|
||||||
|
<dt>removeChild()
|
||||||
|
<dd>Removes a child by name (first argument) and index (second argument).
|
||||||
|
<dt>getNode()
|
||||||
|
<dd>Returns a Node specified by its "relative path" to this node, or
|
||||||
|
nil if none exists. The optional second argument, if true, causes the
|
||||||
|
node to be created if it does not exist.
|
||||||
|
<dt>setValues()
|
||||||
|
<dd>Takes a hash as argument, and sets all the key/value pairs in the
|
||||||
|
hash as property subnodes of the object. This works recursively, with
|
||||||
|
sub-hashes and vectors; thus making a deep copy of the Nasal hash in
|
||||||
|
the property tree.
|
||||||
|
</dl>
|
||||||
|
|
||||||
|
<dt>props.Node.new()
|
||||||
|
<dd>Static "constructor" function returning a new, rootless
|
||||||
|
<code>Node</code> object. Takes a hash argument to initialize the
|
||||||
|
new node via setValues().
|
||||||
|
|
||||||
|
<dt>props.globals
|
||||||
|
<dd>This is a <code>Node</code> object representing the root of the
|
||||||
|
global property tree; the Nasal equivalent of
|
||||||
|
<code>globals->get_props()</code>
|
||||||
|
|
||||||
|
<dt>props.dump()
|
||||||
|
<dd>This method prints out a "dump" of the state of a single
|
||||||
|
<code>Node</code> object and all of its children to the console.
|
||||||
|
Very useful for debugging and exploration.
|
||||||
|
|
||||||
|
</dl>
|
||||||
|
|
||||||
|
<h2>Integrating C++ code and Nasal</h2>
|
||||||
|
|
||||||
|
<h3>Calling Nasal from C++</h3>
|
||||||
|
|
||||||
|
<p>The FGNasalSys object has a <code>parseAndRun()</code> method to
|
||||||
|
which you can pass arbitrary Nasal source code for immediate
|
||||||
|
execution:
|
||||||
|
|
||||||
|
<pre>
|
||||||
|
FGNasalSys n = (FGNasalSys*)globals->get_subsystem("nasal");
|
||||||
|
if(! n->parseAndRun("print('This script was called from C++!')"))
|
||||||
|
SG_LOG(SG_GENERAL, SG_ALERT, "My Nasal code failed :(");
|
||||||
|
</pre>
|
||||||
|
|
||||||
|
<p>You can also use <code>parseScript()</code> to get a pointer to a
|
||||||
|
<code>FGNasalScript</code> object. This object supports a
|
||||||
|
<code>call()</code> method which you can use to invoke the script
|
||||||
|
later on, at a time of your choosing. If you will be invoking the
|
||||||
|
script multiple times, this mechanism can be more efficient because it
|
||||||
|
avoids the parsing and code generation overhead for the successive
|
||||||
|
calls.
|
||||||
|
|
||||||
|
<pre>
|
||||||
|
FGNasalSys n = (FGNasalSys*)globals->get_subsystem("nasal");
|
||||||
|
FGNasalScript* script = n->parseScript("print('Spam!')"))
|
||||||
|
if(!script) SG_LOG(SG_GENERAL, SG_ALERT, "My Nasal code failed :(");
|
||||||
|
|
||||||
|
...
|
||||||
|
|
||||||
|
for(int i=0; i<1000; i++)
|
||||||
|
script->call(); // Spam the console
|
||||||
|
</pre>
|
||||||
|
|
||||||
|
<p>Note that there is no way to inspect the return value of the
|
||||||
|
function that you called. It simply returns a boolean indicating
|
||||||
|
successful execution. Handling of "native" Nasal data structures has
|
||||||
|
to be done via the Nasal extension API. See below.
|
||||||
|
|
||||||
|
<h3>Calling C++ from Nasal</h3>
|
||||||
|
|
||||||
|
<p>You have three options for invoking C++ code from Nasal. The first
|
||||||
|
two take advantage of pre-existing FlightGear mechanisms for
|
||||||
|
registering "callback" handlers for specific events.
|
||||||
|
|
||||||
|
<p>If your task is sufficiently general, you should consider defining
|
||||||
|
it as a new FGCommand using the existing interface. This can be
|
||||||
|
invoked efficiently from both Nasal code (using the fgcommand()
|
||||||
|
function) and existing property bindings, and is very easy to do.
|
||||||
|
Simply define a handler function which takes a property tree as an
|
||||||
|
argument and returns a bool (to indicate successful execution), and
|
||||||
|
register it during initialization with the global command manager:
|
||||||
|
|
||||||
|
<pre>
|
||||||
|
// Define your handler function:
|
||||||
|
bool my_new_command(SGPropertyNode* arg) { ... }
|
||||||
|
|
||||||
|
...
|
||||||
|
// And register it in your initialization code:
|
||||||
|
globals->get_commands()->addCommand("my-new-command", my_new_command);
|
||||||
|
...
|
||||||
|
</pre>
|
||||||
|
|
||||||
|
<p>This mechanism works well when your C++ code is a "global" function
|
||||||
|
that you will want to call from many locations with potentially
|
||||||
|
differing data. For some applications, however, you want to register
|
||||||
|
a handler that will be called only by code involved with computations
|
||||||
|
on a single data set.
|
||||||
|
|
||||||
|
<p>For this, there is the property listener interface. You can create
|
||||||
|
a subclass of SGPropertyChangeListener which implements the
|
||||||
|
valueChange, childAdded and/or childRemoved methods and associate it
|
||||||
|
with a specific property node in the global tree. You can then
|
||||||
|
"invoke" this handler from Nasal code (or from anywhere else) by
|
||||||
|
simply setting the property value.
|
||||||
|
|
||||||
|
<p>I haven't tested the property listener interface, and it requires
|
||||||
|
somewhat more typing to implement; so I will include no example here.
|
||||||
|
It is also rather rarely used by existing FlightGear code (the
|
||||||
|
property picker GUI is the only significant application I could find).
|
||||||
|
|
||||||
|
<h3>Extending Nasal</h3>
|
||||||
|
|
||||||
|
<p>This is the third mechanism for invoking C++ (strictly C, in this
|
||||||
|
case) code from Nasal. This is the API you must use if you want to
|
||||||
|
inspect and/or modify Nasal data structures, or create a function
|
||||||
|
object that will be visible to Nasal scripts as a callable function.
|
||||||
|
Unfortunately, there really isn't space here to document this API
|
||||||
|
fully. For now, examin the <code>nasal.h</code> header which defines
|
||||||
|
it, and the <code>lib.c</code> and <code>mathlib.c</code> source files
|
||||||
|
which implement the existing built-in library functions. The
|
||||||
|
FlightGear-specific extension functions in <code>NasalSys.cxx</code>
|
||||||
|
and <code>nasal-props.cxx</code> are also good examples.
|
||||||
|
|
||||||
|
<p>But for most purposes, consider the first two mechanisms instead.
|
||||||
|
FlightGear's general inter-module communication mechanism is the
|
||||||
|
property tree, which is exposed from both Nasal and C++ code already.
|
||||||
|
A Nasal extension function, by definition, is useful only to Nasal
|
||||||
|
code. Even worse, data structures definied by a Nasal interface are
|
||||||
|
completely invisible to the C++ world.
|
||||||
|
</body>
|
||||||
|
</html>
|
||||||
Loading…
Reference in a new issue