Table of Contents

• General
  • Test Reports
  • License
  • Download
• Getting Started
• Advanced Topics
  • Embedding uSCXML
  • Extending uSCXML
• Miscellaneous
• Acknowledgments

• Building form Source
• Developer Overview

uSCXML is a SCXML interpreter written in C/C++. It is standards compliant and easily extended even in C# and Java. It runs on Linux, Windows, Raspberry Pi and Mac OSX, each 32- as well as 64Bits as well as iOS.

• Data Models
  • Full ECMAScript data model using Google's v8 (and JavaScriptCore on MacOSX and iOS)
    • Simplified support for Web Storage in document.localStorage
    • Support for binary data via TypedArrays (will not throw exceptions yet)
  • Full NULL data model with required In predicate
  • Prolog data model using SWI prolog
  • Experimental Promela data model for use with the SPIN model-checker
  • Early support for a Lua data model
  • Rudimentary support for XPath data model
• Invokers
  • scxml: Invoke a nested scxml interpreter
  • dirmon: Watches a directory for changes to files
  • scenegraph: Simplified 3D scenegraphs with custom markup
  • heartbeat: Periodically sends events
  • umundo: Subscribe to channels and publish events
  • Many others
• DOM
  • DOM Core Level 2 + XPath extensions available for ecmascript data model
  • Namespace aware to embed custom markup for special invokers
• Communication
  • Features the standard basichttp I/O processor
  • Features the required SCXML I/O processor
  • No DOM I/O processor
  • Early support for WebSockets
  • Can actually respond to HTTP requests with data via <response>
• Language Bindings
  • Java bindings
  • C# bindings
  • PHP module for apache and cli interpreter (discontinued)

• We continuously run the W3C IRP tests for SCXML.
• Have a look at the result for the various platforms.
• The manual and XPath specific tests are excluded.

uSCXML itself is distributed under the Simplified BSD license as in, do not sue us and do not misrepresent authorship. Please have a look at the licenses of the libraries we depend upon as well.

We do not yet feature installers. Please download the source and have a look at the build instructions.

In order to use the interpreter, you need to #include "uscxml/Interpreter.h" and instantiate objects of uscxml::Interpreter.

Interpreter scxml = Interpreter::fromURL("http://www.example.com/fancy.scxml");
scxml.start(); // non-blocking in own thread

There are some cases, i.e. with graphical invokers, where the main thread is required in order to react to UI events. You will have to deligate control flow from the main thread into the interpreter every now and then:

interpreter.runOnMainThread(25);

This will perform a single iteration on the invoked components with a maximum of 25 frames per seconds or return immediately. You will have to call this method every now and then if you are using e.g. the scenegraph invoker.

Note: Running the interpreter in its own thread via start is not exposed into the language bindings. Just use the threading concepts native to your language to call step or interpret as outlined below.

Interpreter scxml = Interpreter::fromXML("<scxml><final id="exit"/></scxml>");
scxml.interpret(); // blocking

When using blocking interpretation, it is assumed that it is running on the main thread and it will call runOnMainThread between stable configurations.

Interpreter scxml = Interpreter::fromXML("<scxml><final id="exit"/></scxml>");
InterpreterState state;
do {
  state = interpreter.step(ms);
} while(state != InterpreterState::USCXML_FINISHED)

Using step, you can run a single macrostep of the interpreter and interleave interpretation with the rest of your code. The step function will take an optional integer as the time in milliseconds it will block and wait if no more events are available, default is to block indefinitely until an event arrives or the interpreter finished.

You can register an InterpreterMonitor prior to start in order to receive control-flow upon various events in the Interpreter.

class StatusMonitor : public uscxml::InterpreterMonitor {
	void onStableConfiguration(...)
	void beforeCompletion(...)
	void afterCompletion(...)
	void beforeMicroStep(...)
	void beforeTakingTransitions(...)
	void beforeEnteringStates(...)
	void afterEnteringStates(...)
	void beforeExitingStates(...)
	void afterExitingStates(...)
};

StatusMonitor statMon;
Interpreter scxml = Interpreter::fromXML("<scxml><final id="exit"/></scxml>");
scxml.addMonitor(&statMon);
scxml.start();

This will cause the interpreter to invoke the callbacks from the monitor whenever the corresponding internal phase is reached.

There are bindings for Java and C# with some examples in the embedding directory. The bindings consist of two parts each

1. The C++ uscxml interpreter compiled as a loadable module for the target language and
2. A target language specific module (uscxml.jar / uscxmlCSharp.dll) with the wrapper classes.

The first one is loaded by the target language (System.loadLibrary / SetDLLDirectory) while the second is to be included in your actual project. Have a look at the examples in embedding and adapt the paths to reflect your setup. See the build instructions for details on how to build these.

The uSCXML interpreter can be extended by introducing new

1. Data models as embedded scripting languages (e.g. ECMAScript, Prolog and XPath)
2. Invokers to represent external components that deliver and accept events (e.g. iCal, SceneGraph, DirectoryMonitor)
3. I/O-Processors to provide communication with external systems (e.g. BasicHTTP, SCXML).
4. Elements for Executable Content (e.g. <respond>, <fetch>, <postpone>).
5. Data model extionsions to establish callbacks from the data model into the host language.

The basic approach to extend the interpreter is the same in all cases:

1. Write a class inheriting the abstract base class (e.g. DataModelImpl, InvokerImpl, IOProcessorImpl, ExecutableContentImpl).
2. Instantiate your class and register it as a prototype at the Factory via one of its static register* methods.
   1. You can register at the global Factory Singleton via Factory::register*(prototypeInstance)
   2. Or provide a new Factory instance to selected interpreters as an in-between.
3. Write an interpreter using your new functionality.

Note: Within the language bindings, you will have to inherit the base classes without the Impl suffix. Have a look at the examples in embedding for examples.

Sometimes, it is more suited to provide an interpreter with an already instantiated extension (e.g. an IOProcessor with an existing connection). In this case, it is somewhat awkward to register a prototype and have all initialization in its create(Interpreter interpreter) method. While you can still dispatch over the interpreter instance and access information from some global Interpreter->Data map, there is a more straight-forward approach, e.g. in Java:

Interpreter interpreter = Intepreter.fromURI(uri);
AdhocIOProcessor ioProc = new AdhocIOProcessor(Whatever youLike);
ioProc.setParameter1(something);
interpreter.addIOProcessor(ioProc);

This will cause the interpreter to use the given instance for all send requests targeting one of the types returned by ioProc.getNames() and not instantiate an instance via the factory. The instance can deliver events into the interpreter via returnEvent(Event e, boolean toInternalQueue = false). The same approach can be used for invokers:

Interpreter interpreter = Intepreter.fromURI(uri);
TestAdhocInvoker invoker1 = new TestAdhocInvoker(Whatever youLike);
invoker1.setParameter1(something);
interpreter.setInvoker("invokeId", invoker1);

This will cause the interpreter to use the given instance for a given invokeId and not instantiate via the factory. Similarly, data models can be registered via interpreter.setDataModel(DataModel dm).

Note: Providing ad-hoc extensions is only supported before the interpreter is started. If you change instances with a running interpreter, the behavior is undefined.

If you register any ad-hoc extension with an interpreter, be it in C++ or a language binding, this extension's instance belongs to the interpreter. This means i.e. that (i) the interpreter's destructor will deallocate the extension instance, (ii) you cannot reuse it for another interpreter and (iii) you may not call its destructor.

For the language bindings, this means furthermore that you have to call swigReleaseOwnership() on the extension instance to prevent the target language's memory managment form calling the instances C++ native destructor. The destructor can only be called once and the interpreter's destructor will do it.

If allocating additional extension instances per interpreter is expensive, consider using aggregation as a "has a" relationship with the expensive part.

Only exceptions raised during the following methods are propagated into the target language:

Interpreter::fromXML
Interpreter::fromURI
Interpreter::step
Interpreter::interpret

If you dig around in the exposed APIs, there are other methods that may raise exceptions (e.g. interpreter.getDataModel().eval()). Be careful when calling these. Ultimately, all exceptions ought to be propagated into the target language to be handled accordingly. We are currently evaluating different approaches to do so.

When I originally tried to compile the required libraries for uSCXML on Android (libevent, curl, libxml2), it would not work out of the box and I postponed. If there is a demand for an Android port, I can have another look. uSCXML itself is written in a subset of C++99 and ought to compile just fine.

Currently, we use std::string to represent all strings. This is not a problem as e.g. the ECMAScript data models will just interpret these as character arrays and handle Unicode respectively. Though it is a problem if you like to use non-ASCII characters e.g. in the id attribute of states.

The performance of uSCXML depends on many things like the employed data model and the platform it runs on. Using a MacBook Pro with an Intel i7 @2.4Ghz and the ECMAScript data model (test/uscxml/test-performance.scxml), we achieve about 20.000 events/sec. On a Raspberry Pi, however, only 350 events/sec are achieved.

If performance ought to be increased further, the first place to look would be most likely the employed DOM implementation, which uses the rather expensive dynamic_cast somewhat too liberally. For a real performance boost, the explicit SCXML DOM representation at runtime might be dropped in favor of some simple structs representing the states and transitions. This has been no priority for us so far as even 350 events/sec is plenty for our use-cases.

Up until recently, the APIs of uSCXML were still subject to rather substantial changes. If there is one thing worse than no documentation, it is wrong documentation, so we did not document the source. Another stumbling block was the fact that documentation would not show up in the language bindings.

Both issues are resolved by now: The APIs have not changed substantially in about 8 month and the new version of SWIG will allow doxygen comments to be show up in various target languages; so we will document somewhen soon.

This SCXML interpreter is developed at the Telekooperation Group of the Technical University of Darmstadt as part of the SmartVortex project funded by the 7th European framework program.