Implementation Techniques for Petri Net Based Specifications of HumanComputer Dialogues

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Implementation Techniques for Petri Net Based
Specifications of Human-Computer Dialogues
Rémi Bastide, Philippe Palanque L.I.S Université
Toulouse I Toulouse (France)
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Goals of the ICO formalism

• Describe the dialogue structure of event-driven
  interfaces with high-level Petri nets
• Integrate cleanly with current software
  engineering practice
• UIMS to build the interface
• Object-oriented modelling for the design of the
  application

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From Specification to Implementation

• The specification work must not be thrown away at
  the implementation stage.
• First option generating code from the
  specification
• Advantage conventional implementation tools and
  languages.
• Second option directly interpret the
  specification (embedded models)
• Advantage rapid prototyping is made easier
• Use the embedded model at run-time, e.g. for
  debugging or for providing contextual help

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Event-driven vs. "hard" reactive systems

• Easier
• No true concurrency, "cooperative multitasking",
  no interrupts, critical sections, etc.
• Tougher
• The internal state must always be perceptible by
  the user
• Each user action must have an immediate feedback

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Why use Petri nets

• State and event on an equal basis
• Structuring mechanisms
• hierarchical refinement (macros)
• composition mechanisms (client-server protocol)
• Easy description of parallelism
• Timed-Petri nets for temporals aspects (timed
  places and timed transitions)
• Formal validation available

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Petri nets and objects formalism

• Places are the state variables of the system
• system state current marking
• marking of a place set of object references
• Arcs are labeled by variables, stating the flow
  of object references in the net
• Transitions are the state changing operators
• featuring a precondition, testing the values of
  the objects of the incoming places
• an action, triggering methods on the selected
  objects

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Petri nets in Interactive Applications
Events
Function calls
Methods calls
Application
Dialogue
Presentation
Non interactive
User
Interface
application kernel
Control
Component
Model
UIMS (interactors)
High-level Petri nets (ObCS)
Ctokens
Objectmodel
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Example The "Presentation" component
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Example the Dialogue
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Automatic code generation
ObCS of the ICOS
Marking tree
Transformation
Marking graph
of the OBCS
ATN
State / Transition matrix
State / Service matrix
Code
Application code
Generation
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ATN of the dialogue
state 1
Edit
state 4
Quit
Delete //n0
Add
Quit
States correspond to the following
markings of places
(default, selected, edited, list)
state 1 (1, 0, 0, 0)
state 2
Select // ngt0
Delete/n--/ ngt0
?
)
state 2 (0, 1, 0,
)
state 3 (0, 0, 1,
?
?
Edit
state 4 (0, 0, 0,
)
Registers
n number of tokens in the list place
Add / n
Reset
Replace
Edit
state 3
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Example of a generated event-handler

• Callback procedure ADD
• Switch (CurrentState) // test of the state
  variable
• case state1
• // no pre-condition to test
• // semantic action
• Create New Customer // add the tuple o to the
  table
• // state changing
• CurrentSate State2 // change the current
  state
• // feedback of the commands available in the
  new state
• disable(PushButtonAdd)
• disable(PushButtonReset)
• disable(PushButtonReplace)
• enable(PushButtonClose_Box)
• enable(PushButtonDelete)
• enable(PushButtonListBox)
• case state2
• ...

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The semi-interpreted environment
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Object model for the abstract interpreter
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The abstract class "Transition"
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A concrete transition and its corresponding
derived class
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Perspectives

• Evolution of the tool
• Enhance the coupling between the UIMS and the
  Petri net editor
• Take into account distributed (CSCW) interfaces
• Evolution of the method
• Model the user's behaviour (task) as well as the
  system's
• Task model and system model cross-Verification

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Thank you for your attention!