Statecharts are a well-known visual language for modeling the executable behavior of complex reactive event-based systems. They were invented in the 1980s by David Harel, and have gained a more widespread adoption since they became part of the UML modeling standard.
Statecharts offer more sophisticated modeling concepts than the more classical state diagrams of finite state machines. For example, they support hierarchical composition of states, orthogonal regions to express parallel execution, guarded transitions, and actions on transitions or states. Different flavours of executable semantics for statecharts have been proposed in the literature and in existing tools.
Because Sismic is supposed to be independent of a particular visual modeling tool, and easy to integrate in other programs without requiring the implementation of a visual notation, statecharts are expressed using YAML, a human-friendly textual notation (the alternative of using something like SCXML was discarded because its notation is too verbose and not really "human-readable").
This section explains how the elements that compose a valid statechart in Sismic can be defined using YAML. If you are not familiar with YAML, have a look at YAML official documentation.
While statecharts can be defined in YAML, they can be defined in pure Python too. Moreover, :py~sismic.model.Statechart instances exhibit several methods to query and manipulate statecharts (e.g.: :py~sismic.model.Statechart.rename_state, :py~sismic.model.Statechart.rotate_transition, :py~sismic.model.Statechart.copy_from_statechart, etc.). Consider looking at :py~sismic.model.Statechart API for more information.
Experimental import/export support for AMOLA specifications of statecharts is available as an extension of Sismic. AMOLA is notably used in ASEME IDE, which can be used to graphically create, edit and visualize statecharts. More information on extensions.
The root of the YAML file must declare a statechart:
statechart:
name: Name of the statechart
description: Description of the statechart
root state:
[...]The name and the root state keys are mandatory, the description is optional. The root state key contains a state definition (see below). If specific code needs to be executed during initialization of the statechart, this can be specified using preamble. In this example, the code is written in Python.
statechart:
name: statechart containing initialization code
preamble: x = 1Code can be written on multiple lines as follows:
preamble: |
x = 1
y = 2A statechart must declare a root state. Each state consist of at least a mandatory name. Depending on the state type, other optional fields can be declared.
statechart:
name: statechart with defined state
root state:
name: rootFor each declared state, the optional on entry and on exit fields can be used to specify the code that has to be executed when entering and leaving the state:
- name: s1
on entry: x += 1
on exit: |
x -= 1
y = 2A final state can be declared by specifying type: final:
- name: s1
type: finalHistory states can be declared as follows:
- type: shallow history to declare a shallow history state;
- type: deep history to declare a deep history state.
- name: history state
type: shallow historyA history state can optionally declare a default initial memory using memory. Importantly, the memory value must refer to a parent's substate.
- name: history state
type: deep history
memory: s1A state that is neither a final state nor a history state can contain nested states. Such a state is commonly called a composite state.
- name: composite state
states:
- name: nested state 1
- name: nested state 2
states:
- name: nested state 2aA composite state can define its initial state using initial.
- name: composite state
initial: nested state 1
states:
- name: nested state 1
- name: nested state 2
initial: nested state a2
states:
- name: nested state 2aNote
Unlike UML, but similarly to SCXML, Sismic does not explicitly represent the concept of region. A region is essentially a logical set of nested states, and thus can be viewed as a specialization of a composite state.
Orthogonal states (sometimes referred as parallel states) allow to specify multiple nested statecharts running in parallel. They must declare their nested states using parallel states instead of states.
statechart:
name: statechart containing multiple orthogonal states
root state:
name: processes
parallel states:
- name: process 1
- name: process 2Transitions between states, compound states and parallel states can be declared with the transitions field. Transitions typically specify a target state using the target field:
- name: state with transitions
transitions:
- target: other stateOther optional fields can be specified for a transition: a guard (a Boolean expression that will be evaluated to determine if the transition can be followed), an event (name of the event that will trigger the transition), an action (code that will be executed if the transition is processed). Here is a full example of a transition specification:
- name: state with an outgoing transition
transitions:
- target: some other state
event: click
guard: x > 1
action: print('Hello World!')One type of transition, called an internal transition, does not require to declare a target. Instead, it must either define an event or define a guard to determine when it should become active (otherwise, infinite loops would occur during simulation or execution).
Notice that such a transition does not trigger the on entry and on exit of its state, and can thus be used to model an internal action.
Priorities can be set for transitions using the priority property. By default, all transitions have a priority of 0. A priority can be any integer, or low (equivalent to -1) or high (equivalent to 1).
Note
Transition priorities are considered after the default semantics of Sismic, i.e., after the eventless transition first and inner-first/source state semantics. Because of this, transition priorities cannot be used to, e.g., give priority to a transition with event versus a transition without event.
The Elevator statechart is one of the running examples in this documentation. Its visual description generated from Sismic using PlantUML looks as follows:
The corresponding YAML description is given below.
/examples/elevator/elevator.yaml
Some other examples can be found in the Git repository of the project, in docs/examples.
The :py~sismic.model.Statechart class provides several methods to construct, to query and to manipulate a statechart. A YAML definition of a statechart can be easily imported to a :py~sismic.model.Statechart instance. The module :pysismic.io provides a convenient loader :py~sismic.io.import_from_yaml which takes a textual YAML definition of a statechart and returns a :py~sismic.model.Statechart instance.
sismic.io
For example:
python
from sismic.io import import_from_yaml from sismic.model import Statechart
- with open('examples/elevator/elevator.yaml') as f:
statechart = import_from_yaml(f) assert isinstance(statechart, Statechart)
The function also supports importing from a given filepath:
python
statechart = import_from_yaml(filepath='examples/elevator/elevator.yaml') assert isinstance(statechart, Statechart)
The parser performs several checks using :py~sismic.model.Statechart's :py~sismic.model.Statechart.validate method. It also does an automatic validation against some kind of schema to prevent erroneous keys. See schema library for more information about the semantics.
../sismic/io/yaml.py
Sismic is not bundle with any graphical tool that can be used to edit or even view a statechart. Module :pysismic.io contains routines that can be used to (import and) export statecharts to other formats, some of them being used by third-party tools that support visualising (or editing) statecharts.
Notably, module :pysismic.io contains a function :py~sismic.io.export_to_plantuml that exports a given statechart to PlantUML, a tool based on graphviz that can automatically render statecharts (to some extent). An online version of PlantUML can be found here.
For example, the elevator statechart can be exported to the following PlantUML file, which in turns can be used to generate the previously given representation of the elevator.
/examples/elevator/elevator.plantuml
PlantUML's rendering can be modified to some extent by adjusting the notation used for transitions. By default, --> transitions correspond to downward transitions of good length.
A transition can be shortened by using -> instead of -->, and the direction of a transition can be changed by using -up->, -right->, -down-> or -left->. Both changes can be applied at the same time using -u->, -r->, -d-> or -l->. See PlantUML documentation for more information.
If you already exported a statechart to PlantUML and made some changes to the direction or length of the transitions, it is likely that you want to keep these changes when exporting again the (possibly modified) statechart to PlantUML.
The :py~sismic.io.export_to_plantuml function accepts two optional (mutually exclusive) parameters based_on and based_on_filepath that can be used to provide an earlier version of a PlantUML text representation (or a path to such a version if based_on_filepath is used). This will then be used to incorporate as much as possible the changes made on transitions.
sismic.io.export_to_plantuml