State Machines

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State Machines

• State diagrams

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The State Diagram can be a powerful design aid,
and one you may not have seen before
State Diagrams illustrate the dynamic behavior of
a systemwhere events trigger actions that cause
the system to changeits state.
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A State Diagram for a light bulb controlled with
a toggle switch
The event flip switch to 1 triggers the action
current flows and the transition to the on
state. The event flip switch to 0 triggers
the action current stops flowingand the
transition to the off state.
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A State Diagram for a light bulb controlled with
a pushbutton switch
The same event (button pressed) triggers
different actions depending upon its current
state.
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Different events may also cause transitions to
the same state
Different events may cause transitions to
different states
However, state transitions must be deterministic
that is, the same eventcannot cause a transition
from one given state to more than one subsequent
state.
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The same event can cause different states to
transition to the same state
These state transitions are still deterministic
since the same eventdoes not cause a transition
from one given state to more than one subsequent
state.
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In a complex system, different events may cause
transitions to different states or to the same
state
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It is common for an event to result in an action,
but no state transition
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Initial and Final states may be indicated as
follows
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A guard may be used to illustrate a condition
that must be satisfied before a state transition
can occur.
The guard appears in brackets ltguardgt
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State Diagrams can be used to help specify the
dynamic logic implemented in Controller objects
in response to events that the Controller receives
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Group Activity Draw the State Transition Diagram
for the iPod Shuffle

• Event sources are
• Power switch
• Pause/Play button
• Next button
• Previous button
• (ignore volume buttons they dont affect state)
• How many different states are there?
• What is the Initial state?
• What is the Final state?
• How many different transitions between states?