Communications - July 2012

certain times. There are other ways to forbid things from occurring; one of which is done by indicating that events in the main chart must occur only in the specified (partial) order. For example, one can specify that when the main chart is active, events that appear in the chart but are not presently enabled cannot be triggered at that point in time by other charts.

A modest view of LSC considers it to be a requirements and specification language for making assertions about sequences of events. In this view, a system satisfies an LSC specification if all its runs satisfy all the universal charts in the specification, and for each existential chart, there is at least one run that satisfies it.

However, the play-out technique facilitates the execution of an LSC specification, that is, a collection of charts, just like any computer program. Play-out does this by tracking events that may be selected next in all lifelines in all charts, selecting and triggering events subject to the must/may/forbid-den modalities, and advancing affected charts accordingly. 21 As described in more detail below, play-out may be viewed as interpreting charts with modal events as threads of behavior with their requested, waited-for, and blocked events.

A dialect of LSC has been designed
to be compliant with UML 2.0, 22 and
can be defined as a profile therein.
Instead of precharts it uses solid and
dashed arrows to indicate whether an
event is to be executed or is only moni-
tored, while the red and blue color
retain their respective modalities of
must and may. The PlayGo tool18, 23 is
currently based on this version of the
language. Figure 6 depicts a PlayGo
example similar to the water-tap appli-
cation discussed earlier, with the addi-
tion of the user pressing a start button
to activate all scenarios.

This process is often referred to as naïve play-out. In a more advanced mechanism, called smart play-out19, 29 the Play-Engine uses either model-checking or AI planning algorithms to look ahead, in an attempt to select events in ways that do not eventually lead to violation of the specification or deadlock.

In addition to the interpreter-like approach of play-out, a compiler for LSC has been developed, which produces executable code by compiling the specification into Java and weaving the results with AspectJ. 38

One notable difference between LSC and the BPJ package is that BPJ benefits from the power of the Java host language. By contrast, the LSC language provides its own constructs for objects and properties, flow of control, exceptions, variables, symbolic objects and messages, a notion of time, sub-chart scope, access to functions in other languages, and external communication. 21

Play-in. An essential element of programming is the process by which programmers perform actual coding. In behavioral programming, it seems only natural to allow this activity to include walking through a scenario, generating events and sequences thereof, and using them in specifying what we want done or forbidden. Toward that purpose, the LSC language allows a new way of coding, called play-in, 15, 21 which captures scenarios as follows: whenever possible, the developer actually performs the event—

Figure 6. umL-compliant LsCs. each chart begins with the user pressing the start button. two charts request tap-turning events, and the third causes their interleaving by alternately waiting for these events. events can occur only when enabled for triggering in all charts in which they appear. the s YnC construct forces order between events in different lifelines.