Real-Time Systems,
COSC-4301-01,
Lecture 19
Stefan Andrei
7/28/2017
COSC-4301-01, Lecture 19
1
Reminder of the last lecture
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Model checking of finite-state systems
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COSC-4301-01, Lecture 19
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Overview of This Lecture
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Visual formalism, statecharts, and
STATEMATE
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Finite-state machine vs Statechart
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Lacks modularity
Exponential state
explosion
Cannot specify
absolute time and time
intervals
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Modular and
hierarchical features
Refinement: a state
decomposed into
lower-level states
Clustering: set of states
combined into a higherlevel state
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4
Applications of Statecharts

Specifying reactive systems.
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Reactive systems are complex-driven mechanisms
that interact with discrete occurrences in the
environment in which they are embedded.
Examples of reactive systems:
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Real-time computer systems, communication devices, control
plants, VLSI circuits, airplane avionics.
The reactive behavior of real-time systems cannot be
captured by specifying the outputs for every possible
set of inputs.
Instead, this behavior has to be described by
specifying the relationship of inputs, outputs, the
system state over time.
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Object Behavior

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The behavior of an object is defined by its reaction to
messages at any point in execution.
Some object behaviors can be studied by:
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Object Diagram, OR
Interaction Diagram
Not sufficient:
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Cannot model all possible scenarios, e.g., only specific
sequence of messages are studied.
Only model some legal (possible) states - show how an
object behaves in particular interactions.
We need to know about illegal or impossible states to plan for
them.
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Specifying Behaviour in UML

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Different notation is needed to summarize the overall
behaviour of objects.
UML defines a statechart for this purpose.
Complement interaction diagrams for understanding
the dynamic behaviour of system:
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Interaction Diagram:

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Models some inter-object messages with well defined order
during a short duration.
Statechart:
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Models the entire lifetime of a single object, specifying all
possible sequences of messages and responses.
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State-dependent Behaviour
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Objects respond differently to the same
stimulus/events at different times.
Can be modelled by defining a state machine:
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It has a set of states:
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It has a set of transitions:
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an object can be in one state at any time;
the state it is in determines how it responds to events.
an event can cause the object to move from one state to
another.
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8
CD Player: An Example
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Behaviour of a simple CD player:
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A drawer to hold the CD;
Control interface with 3 buttons:
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Load button: the drawer will open if it was shut and will
shut if it was open;
Stop button: the player will stop playing. If there is no
CD, there is no effect;
Play button: the CD will be played. If the drawer is open,
the drawer shuts before playing starts.
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UML Statechart Semantic
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A statechart defines the behaviour of instances of a
given class.
It shows:
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The possible states of an object;
The events it can detect;
Its response to those events.
An object is in one active state at a time:
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Events may be received at any time, which can trigger a
transition to the next active state;
If an Event only causes a loop on a state (i.e., no change of
an active state), the transition is known as self-transition.
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Statechart: Deciding States
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Identifying separate states:
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Informal principle:
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Syntax:
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States S1 and S2 are separate if an object in state S1
responds differently to at least one event from the way it
responds to that event in state S2.
States: rounded rectangles with the name of the
state ( StateName ).
Example:

There are 3 states for the CD player:
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Closed, Playing, Open.
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Statechart: Identifying Events
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Events are usually external stimulus:
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Events usually cause an object to change state:
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Moving from one state to another is known as transition;
Events that causes transition are known as trigger;
Events can optionally carry data (similar to message
parameter).
Syntax:
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E.g., messages that can be sent to an object;
Internal stimulus will be covered later in the lecture.
An arrow with the trigger attached (
trigger
).
Example:
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There are three events for the CD Player: load, play, stop.
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CD Player: Statechart Ver. 1
 Figure 10.1 from [Priestley; 2004], page 210
Transition:
From Closed state
to Open state
triggered by load
event.
State:
The Open state.
Statechart
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Initial and Final States
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To specify the first active state when an object is
created/initialized:
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Use Initial State:
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A transition leading from the initial state indicates the first active
state.
No event should be written on a transition from an initial state.
Syntax: a black disc ( ).
To model the destruction of an object:
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Use Final State:
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Represent the end of flow, i.e., the object no longer exists.
May correspond to actual destruction for software object.
Syntax: a circled black disc (
).
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CD Player: Statechart Ver. 2
Initial State:
CD Player
switched on
 Figure 10.2 from [Priestley; 2004], page 212
Final State:
Switched Off
the CD
Player
Statechart
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Non-Deterministic System
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When two or more transitions leading from a
state share the same trigger:
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No way to distinguish between them;
Randomly choose one of them;
The same history of events may end up in a
different state;
Known as non-deterministic system.
Can be removed by adding information to
distinguish between transitions with the same
trigger.
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CD Player: Non Deterministic Example
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In real-life, a CD player will not play if there is no CD
inside.
Statechart Ver. 2 does not describe this.
Correct Behaviour:
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When the play event is detected, the CD Player should
remain in the Closed state if there is no CD inside.
Resulting Statechart (partial), Figure 10.3 from [Priestley;
2004], page 213:
This transition
should be
used when
there is a CD
inside.
This transition
should be for
no CD inside.
Statechart
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Guard Condition
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Although the two transitions are meant for different
scenarios, the statechart does not distinguish among the
two, resulting in non-deterministic behavior.
The non-determinism can be removed if we can specify the
condition that determine the correct transition to use.
Syntax:
trigger [guard_condition]
Semantics:
 If a transition has a guard condition, it can only fire if that
transition is evaluated to true;
 If all guard conditions are false and there is no unguarded
transition, the event will be ignored.
 Usually, only one condition is true (determinism).
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CD Player: Statechart Ver. 3
Guard
Condition
 Figure 10.4 from [Priestley; 2004], page 213
Statechart

Note that guard conditions are also added to transitions
leading from the Open state. What do they represent?
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Actions
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A state can have actions triggered.
There are three ways an action can be triggered:
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By an event;
By entering a state;
By leaving a state.
An action triggered by an event takes place in
response to that event:
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Syntax:
Trigger [Guard_Condition] /action
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Example:
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The CD drawer will be closed when play is detected in the
Open state.
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CD Player: Statechart Ver. 4
 Figure 10.5 from [Priestley; 2004], page 215
Statechart
Action triggered
by Event
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Action: Entering/Exiting a State
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An action can be triggered as soon as the state is
entered.
Useful to capture actions that must be performed
regardless the transition used to arrive at the state.
Syntax:
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Write in a compartment in the state box: entry/action
Similarly, an action can be performed just before
leaving a state.
Syntax:
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Write in a compartment in the state box: exit/action
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CD Player: Statechart (partial)
 Figure 10.6 from [Priestley; 2004], page 215
Action triggered
by entering a
state.
Action triggered
by leaving a
state.
Statechart
Question:
What happened when the
play button is pressed
during CD playing?
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Action and Activity
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Properties of Actions:
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An “action” that does not conform to the above is
termed as activity instead.
Property of Activities:
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Short, self-contained processing;
Finished “instantaneously”;
Cannot be interrupted by events.
Performed during a state;
Carry out for an extended period of time;
Can be interrupted by events.
Syntax:
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Write in a compartment of the state box: do/activity
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CD Player: Activity Example
 Figure 10.7 from [Priestley; 2004], page 216
Activity performed
during the Playing
state.
Statechart (Partial)
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Question: When the play track activity is being
performed, what happen if the stop button is pressed?
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Completion Transition
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As well as being interrupted by events, some
activities will come to an end of their own accord.
If an activity completes uninterrupted, then it can
trigger a completion transition.
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these are transitions without event labels.
Multiple completion transitions can be distinguished
by guard conditions.
Example:
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When the play track activity is completed:
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If it is the last track, go to Closed state;
If it is not the last track, play the next track.
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CD Player: Completion Transition
Example  Figure 10.8 from [Priestley; 2004], page 217
Completion
Transition:
Play the next track
Completion
Transition:
No more track. Stop
playing
Statechart (Partial)
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Internal Transition
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These are transitions that leave the object in the
same state but does not trigger the entry and exit
actions:
A self-transition is not appropriate. Why?
Use an Internal Transition which does not trigger the
entry and exit action.
Syntax:
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Write in a compartment of the state box: event/action
Example:
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Suppose we add an info button for the CD Player to display
the remaining playing time for current track.
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CD Player: Internal Transition
 Figure 10.9 from [Priestley; 2004], page 218
Info /display time
Statechart – Self- transition(Partial)
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Statechart – Internal Transition (Partial)
Compare the two different ways to model the info
button.
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Composite State
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States that share similar behavior can be grouped
into a Composite State to simplify the statechart.
Composite State:
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Syntax similar to simple state;
Contains a number of substates;
When a composite state is active, exactly one of the
substate must be active;
Transitions can lead away from a composite state as well as
any of its substates.
Example:
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CD Player: the response to the play event is similar during
the Open or Closed states.
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CD Player: Composite State
 Figure 10.10 from [Priestley; 2004], page 219
Composite
State
Transition
shared by all
substates
Sub-State
Transition
specific to one
substate
Statechart
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Composite State: Additional Property
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A composite state is just like a simple state:
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A composite state is also like a mini-statechart:
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Can have Entry/Exit actions;
Can have extended activity.
Can have an Initial State to indicate the default substate if a
transition terminates at boundary of a composite state;
Can have a Final State, which is triggered when ongoing
activity within the state has finished.
Transitions:
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Leading away from a composite state apply to all substates;
Arrive at composite state go to the default/initial state;
Can cross composite state boundaries.
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CD Player: pause button
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Pressing the pause button causes playing to be interrupted;
When the button is pressed again, playing continues from the
position where it was paused, Figure 10.11 from [Priestley;
2004], page 220.
Only triggers if
the Composite
State is entered.
As there is no entry
action, reentering
the Playing
substate will not
restart the track.
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Statechart
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History State
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When a composite state is entered, it begins at the initial
state or directly transits into one of the substates.
Sometimes, it is useful to re-enter a composite state at a
point at which it was left.
 Re-enter the last active substate.
Indicate this using the History State.
 Transitions arriving at the History State activates the last
active substate.
If there is no last active substate, a default state can be
indicated by an unlabelled transition from History State.
Syntax:
 A circled ‘H’ ( H )
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CD Player: History State
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Suppose that re-pressing play button:
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During Playing: restart and play the current track;
During Paused: restart the current track but remain paused,
Figure 10.12 from [Priestley; 2004], page 221.
Make use of
History State to
model the behavior
correctly.
Statechart
Question: Does the statechart on slide 31 model this correctly?
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CD Player: Final Version Statechart
 Figure 10.13 from [Priestley; 2004], page 222
Statechart
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Statechart Notation Summary
State Name
Entry /action
Exit /action
Do /activity
Event /action
Event(param) [condition] /action
Transition and Trigger
State
H
Initial
State
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Final
State
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History
State
37
Steps for Constructing Statechart
1.
2.
3.
4.
5.
6.
7.
8.
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Identify the objects that have complex behavior;
Determine the initial and final states of the object;
Identify the events that affect the entity;
Working from the initial state, trace the impact of
events and identify the intermediate states;
Identify any entry and exit actions on the states;
Expand states into a composite state if necessary;
Check that actions in the state are supported by
operation (method);
Refine the class if necessary.
Another example of constructing Statechart: “Ticket
Machine”.
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Creating a Statechart
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How information from interaction diagrams
can be used to derive statecharts?
It can be hard to identify all necessary states.
Statecharts can be developed incrementally:
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consider individual sequences of events received
by an object;
these might be specified on interaction diagrams;
start with a statechart for one interaction;
add states as required by additional interactions.
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Ticket Machine
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Consider a ticket machine with two events:
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Basic interaction is to select a ticket and then enter
coins.
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select a ticket type;
enter a coin.
model this as a ‘linear’ statechart, Figure 10.14 from
[Priestley; 2004], page 224.
Problems:
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It defines only one transaction, whereas the ticket machine
is able to carry out repeated transactions;
It shows only 3 coins, but this number should be arbitrary.
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Refining the Statechart
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This can be improved by adding ‘loops’:
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the number of coins entered will vary: entry will
continue until the ticket is paid for;
the whole transaction can be repeated.
State ‘Idle’: no transaction is in progress,
Figure 10.15 from [Priestley; 2004], page 225.
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Adding Another Interaction
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Suppose the requirements allow the user to
enter a coin before selecting a ticket.
A ‘coin’ transition from the ‘Idle’ state is
needed to handle this event.
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
this transition can’t go to the ‘Paying for Ticket’
state as the ticket is not yet selected.
so a new state ‘Inserting Coins’ is required.
The statechart is thus built up step-by-step.
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Adding a Second Interaction
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If all coins are entered before ticket selected,
Figure 10.16 from [Priestley; 2004], page 226:
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Integrating the Interactions
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Suppose the requirements allow the user to enter some coins
before selecting a ticket and the rest after that.
In fact, events can occur in any sequence, Figure 10.17 from
[Priestley; 2004], page 227:
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Time Events
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Suppose the ticket machine times out after 30
seconds.
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we need to fire a transition that is not triggered by a
user-generated event;
UML defines time events to handle these cases.
Example: a transition will fire 30 seconds after the
‘no ticket selected’ state is entered, Figure 10.18
from [Priestley; 2004], page 228.
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Activity States
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Both ‘No Ticket Selected’ and ‘Ticket Selected’
should check if the machine is able to return
any change that is required.
Efficient solution: activity states!
Activity states defines periods of time when the
object is carrying out internal processing.
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unlike normal activities, these cannot be interrupted
by external events;
only completion transitions leading from them;
useful for simplifying the structure of complex
statecharts.
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Returning Change


Note that activity as a property of state (slide 23) is not the
same as activity states.
Use an activity state to calculate change, Figure 10.19
from [Priestley; 2004], page 229.
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Ticket Machine Statechart


It incorporates
‘Pressing Cancel’ in
the middle of a
transaction.
The composite state
‘Transaction’ reduces
the number of
statechart transitions,
Figure 10.20 from
[Priestley; 2004],
page 230.
Statechart
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Summary

Visual formalism, statecharts, and
STATEMATE:
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Statecharts
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Reading suggestions

Chapter 5 of [Cheng; 2002];

Chapter 10 of [Priestley; 2004], where this
is:

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M. Priestley: Practical Object-Oriented Design with
UML. Second Edition, 2004, ISBN: 978-0071239233
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Coming up next


Visual formalism and STATEMATE (next
lecture)
Chapter 5 of [Cheng; 2002]
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Thank you for your attention!
Questions?
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