State machines provide a way to break complex behavior into well-defined states and transitions.
State machine
A state machine is a model for describing system behavior over time.
In a state machine:
- a task is broken down into a series of states
- the system moves through these states along defined transitions
- the system transitions from one state to another based on inputs or other triggers
Examples of systems controlled by state machines include:
- elevators
- traffic lights
- vending machines
Each of these systems:
- can only be in one state at any given time
- can perform different actions depending on inputs
States, Transitions, and Events
A state:
- describes the status of a system while it is waiting for a transition
- is one of a finite number of possible conditions
- for this reason, state machines are called finite state machines (FSMs)
A transition:
- is the action of moving from one state to another
- transitions are usually triggered by events
An event:
- is something that occurs, e.g., a button press or a timer expiring
- the same event might trigger a different transition depending on the current state of the state machine
- e.g., pressing the “5” button will make an elevator go down if it’s on the tenth floor, or go up if it is on the first floor
Designing a state machine entails:
- determining all possible states
- planning the transitions between states
- identifying the events that trigger those transitions
Joint