INFO 355 Week #8 1

Systems Analysis IIExtending the requirements models

INFO 355Glenn Booker

Requirements?

This is to refine our initial development of system requirements New uses for the activity diagram State diagram

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Use Case descriptions

Recall that we defined ‘casual’ and ‘detailed’ use case documentation

The text has ‘brief’ and ‘fully developed’ use case descriptions Example of the latter on p. 123 Text’s ‘flow of activities’ is the main

success scenario ‘Exception conditions’ are alternate

scenarios or extensions

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Use Case descriptions

The text also adds Related use cases Stakeholders Preconditions Postconditions

Recall that Aleister Cockburn has many more characteristics that can be added to use case documentation

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Activity diagrams for use cases

We saw the activity diagram in week 1

It can also be used to document use cases, where the swimlanes (or partitions) are the primary actor and the System (and possibly lanes for external systems)

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System Sequence Diagram

As noted earlier in the course, we aren’t using the SSD

It is a high level sequence diagram which shows no details inside the :System object

The Loop, Alt, and Opt boxes are called ‘frames’ in the text

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State Machine Diagram

The “state diagram” is also known as Statechart diagram (Visio) State Machine diagram State Transition diagram

These all refer to the same thing

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State Diagrams

A state diagram is used to capture the details of a use case which involves some sort of processing where the system can change state

A state refers to a mode of operation or setting which will affect how the system responds to inputs

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State Diagrams

State diagrams are good for describing a single object’s behavior throughout several use cases If you have several objects interacting,

an interaction diagram is better Real-time systems use state

diagrams extensively

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Examples of State-based Systems

A telephone operates in two main states, “on hook” and “off hook” Within the latter, there are many

possible states, such as making a local call, making a long distance call, making an international call, making a 3-way call, etc.

The commands entered (via dialing) control which state the phone is in

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Examples of State-based Systems

A cruise control has basic states of on or off While on, it could be in states of

maintaining speed, accelerating, resuming previous speed, etc.

Various buttons control changes between states

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Examples of State-based Systems

Even Microsoft Word is somewhat state-based If I type a letter, normally it would

appear where the cursor is on a document

But if I press the Alt key first, for example, the interpretation of the next keystroke is quite different

Alt-f display the File menu The Esc key sends it back to normal state

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State Diagrams

So we want to use a state diagram if the interpretation of user actions depends on the history of previous actions Often a state corresponds to the status

or condition of an object Events occur which can change the

state of an object through a transition

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State Diagrams

The state diagram only has four main elements The starting point, shown by a big dot The ending point, a big dot inside

a circle States, shown by boxes Transitions between states, shown by lines

with arrows between the boxes A transition goes from an “origin state”

to a “destination state”

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Figure 10.1 The Safe Example

Starting point

Ending point

A state

A transition

Elements for state diagrams appear on the Activity Diagram shape menu in Visio

Wait Lock

Open

candle removed [door closed] / reveal lock

safe closed key turned [candle in] / open safe

key turned [candle out] / release killer rabbit

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The Safe Example

The labels on each line indicate an Event (trigger name), and if that occurs, the Action (action-expression) and state change occur Event [Guard] / Action is the syntax The Event is optional for each transition

(the state change might happen automatically, which is rare)

The Guard and Action are also optionalEventName [Guard] / ActionName

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The Safe Example

The syntax Event [Guard] / Action is Visio’s terminology

The text uses trigger-name[guard-condition] / action-expressionfor the same thing

Visio sample transition

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State1

State2

CallEvent1 [my guard condition] / CreateAction1,DestroyAction1

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The Safe Example

If no Action is specified, then just the state change occurs when that Event takes place (as is the case for “safe closed” between Open and Wait states) “When the safe is closed (the Event),

change to the Wait state”

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The Safe Example

An Event can have conditional statements, called a Guard, such as the [door closed] condition

So the transition from Wait to Lock means: “If the door is closed, and someone removes the candle, then reveal the lock and change to the Lock state”

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The Safe Example

If using a Guard condition, it generally makes sense to have mutually exclusive possible conditions [candle in] [candle out]

If there is no Guard condition, any given Event must have only one possible path out of a state

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The Safe Example

From the Lock state, “If the candle is in, and the key is turned, open the safe and change to the Open state”

The alternative transition is “If the candle is out, and the key is turned, exit and release the killer rabbit”

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Internal Activities

Sometimes events can happen in a state which results in some action, but not a change of state

These are internal activities (internal transitions in Visio)

The same Event / Action syntax is used, but these don’t have a change of state associated with them

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Internal Activities

The meaning is the same – when Event occurs, do Action

Events of ‘entry’ or ‘exit’ occur automatically when entering or exiting that state, but do not occur when other internal activities are triggered; e.g. for a text field: Entry / highlight all Exit / update field

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Activity States

It’s possible to have ongoing activity while in a state

This is shown by the activity state There’s an Action State in Visio, but no

way to show the activity except by using the Text Tool on the state name

Searching

Do / search for new HW

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Activity States

The activity which takes place during this state is preceded by “do/” It’s assumed that the activity takes

a noticeable amount of time When the activity is successful

or completed, then any transition which doesn’t have an Event is followed

Concurrent States

Some devices can have more than one state at a time, e.g. a printer can be On and Waiting This is concurrency

Can show it as an activity diagram with separate paths for multiple states, or use a composite state

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Concurrent States

Sometimes sets of states can be changing independently of each other, at the same time These are concurrent states

Separate concurrent states with a solid line (p. 137)

Concurrent states have no interaction with each other

Composite States

Show composite states by nesting states

For example, a printer that is On might change from Idle to Working (p. 136)

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History Pseudostate

We had pseudostates for the start and finish of a state diagram

Now add a History pseudostate A simple history pseudostate records

the last value of some state A deep history pseudostate records

many entries of history for a state

H

H*

Developing state diagrams

Identify use cases that have multiple status conditions

For each class affected, list the status conditions

Identify what transitions occur between states

Connect transitions into larger scale structures as possible

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Developing state diagrams

Look for independent concurrent paths and composite paths

Look for additional transitions Define event, guard, action for each

transition Review and test the diagram

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Integrate diagrams

The state diagram can help refine requirements for complex use cases

We now have many tools to understand our system from different points of view Use case diagram and documentation Activity and state diagrams Domain model (conceptual class

diagram)

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