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— 8 minute read

Introduction to state machines and statecharts

Laura Kalbag

Statecharts are a visual language used to describe the states in a process.

You may have used similar diagrams in the past to design user flows, plan databases or map app architecture. Statecharts are another way of using boxes and arrows to represent flows, but with XState these flows are also executable code that can be used to control the logic in your applications.

This guide covers the basics of statecharts in a beginner-friendly way, including:

States​

The states are represented by rounded rectangle boxes. To draw a statechart for the process of a dog, there are two states that would first come to mind:

A dog is always asleep or awake. The dog can’t be asleep and awake at the same time, and it’s impossible for the dog to be neither asleep nor awake. There’s only these two states, a precisely limited, finite number of states.

Transitions and events​

How the dog goes between asleep and awake is through transitions, which are symbolised by an arrow pointing from one state to the next state in the process’s sequence.

A transition is caused by an event that results in the change of state. Transitions are labelled with their events.

Transitions and events are deterministic. Deterministic means that each transition and event always points to the same next state, and always produces the same result from their given starting condition, every time the process is run. Dogs never wake up to become asleep or fall asleep to become awake.

This tiny dog process, with its two finite states and two transitions is a Finite State Machine. A state machine is used to describe the behavior of something. The machine describes the thing’s states and the transitions between those states. It’s a Finite State Machine because it has a finite number of states. (Sometimes abbreviated to FSM by folks who love jargon).

Initial state​

Any process that has states will have an initial state, the default state the process exists in until an event happens to change the process’s state.

The initial state is represented by a filled circle with an arrow pointing from the circle to the initial state.

Using a statechart to describe the process of walking the dog, the initial state would be waiting to walk.

Final state​

Most processes with states will have a final state, the last state when the process is finished. The final state is represented by a double border on the state’s rounded rectangle box.

In the dog walking statechart, the final state would be walk complete.

Dog walking statechart showing waiting state transitioning through the leave home event to the on a walk state, then transitioning through the arrive home event to the final state of walk complete.

Compound states​

A compound state is a state that can contain more states, also known as child states. These child states can only happen when the parent compound state is happening. Inside the on a walk state, there could be the child states of walking, running and stopping to sniff good smells.

A compound state is symbolised by a labelled rounded rectangle box that acts as a container for its child states.

A compound state should also specify which child state is the initial state. In the on a walk compound state, the initial state is walking.

Compound states are what makes statecharts capable of handling more complexity than an everyday state machine.

Atomic states​

An atomic state is a state that doesn’t have any child states. Waiting, walk complete, walking, running and stopping to sniff good smells are all atomic states.

Parallel states​

A parallel state is a compound state where all of its child states, also known as regions, are active simultaneously. The regions are separated inside the compound state container by a dashed line.

Inside the on a walk compound state, there could be two regions. One region contains the dog’s activity child states of walking, running and stopping to sniff good smells, and the other region containing the dog’s tail states of wagging and not wagging. The dog can walk and wag its tail, run and wag its tail or stop and sniff while wagging its tail, it can also do any of these activities without wagging its tail.

Both regions should also specify which child state is the initial state. In our tail region, the initial state is not wagging.

Self-transition​

A self-transition is when an event happens, but the transition returns to the same state. The transition arrow exits and re-enters the same state.

A helpful way to describe a self-transition is “doing something, not going somewhere” in the process.

In a dog begging process, there would be a begging state with a gets treat event. And for the dogs who love their food, no matter how many times you go through the gets treat event, the dog returns to its begging state.

Planning statecharts​

One of the benefits of statecharts is that, in the process of putting a statechart together, you explore all the possible states in your process. This exploration will help you avoid bugs and errors in your code as you’re more likely to cover all the eventualities.

And because statecharts are executable, they can behave as both the diagram and the code, making it less likely that you’ll introduce differences or bugs interpreting between the diagramming and coding environments.

Planning a statechart for a login machine​

To draw a statechart for a login machine, start by listing the basic events in the process. Think about what your login process will do:

  • log in
  • log out

Then list the states that exist as a result of those events:

  • logged in
  • logged out

Once there’s some events and states, there’s the beginnings of a statechart.

Login statechart showing an initial logged out state transitioning through a log in event to a logged in state, then transitioning through a log out event back to the logged out state.

Don’t forget the initial state. In this case, the logged out state is the initial state, as any new user would come to the process logged out.

Delayed transitions​

Some login and logout processes will log out an inactive user after a fixed length of time as a security measure.

The active and idle states only happen when the user is logged in, so these become child states inside the logged in compound state.

The initial state inside the logged in compound state is active, as it happens as a direct result of the log in event, and logging in is a sign of user activity.

A delayed transition is a type of transition which happens after being in a state for a specified length of time. The delayed transition is labelled with “after” and a fixed duration to indicate how much time should pass before transitioning to the next indicated state.

In the login statechart, a delayed transition of 60000 milliseconds, or 1 minute, follows the active state to determine whether the user is idle. If there is an activity event before the transition reaches one minute, the process returns to the active state.

A delayed transition of 180000 milliseconds, or 3 minutes, follows the idle state to transition to the auto logged out state if the user remains idle.

Actions​

A statechart is used to set off actions in the system outside of the statechart. Actions are also commonly known as effects or side-effects. “Side effects” sounds like a negative or unimportant term, but setting off actions is the primary purpose in using statecharts.

Actions are events that have no impact or consequences for the rest of the sequence, the event is just triggered and the sequence moves on to the next step in the process. For example, the login statechart might execute actions that change the user interface.

An action can be fired upon entering or exiting a state, or on a transition. An action on a state is included inside the state’s container with an “entry /” or “exit /” label depending on whether the action should be fired on entry or exit from the state.

In the login statechart, there’s an entry action on the idle state to warn the user that they may be logged out.

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