# Overview

## Finite State Machines

FSM2 is a partial implementation of the UML2 State Machine.

State Machines allow you to model the behaviour of a system by specifying the sequence of states that a system can be in, as well as the events that cause the system to move between states.

![](https://3303650144-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MMSgL-sFaUnTbQGLLbQ%2F-MODg6iVaZOIaipVelcM%2F-MODgOUJBL0diYEAyViA%2Ffsm_test.svg?alt=media\&token=7cd28859-49f6-43ba-844d-78ab6f862014)

In the above FSM for Water we can see water has three states and we have modelled four events that causes Water to transition from one state to another.

FSM2 uses a builder model to create a state machine with support for both static and dynamic transitions.

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To make your life easier FSM2 is able to generate a graphical visualisation of your FSM using the [Graphwiz](http://www.graphviz.org/) dot notation. This makes designing and debugging your state machine much easier.

Let's have a look at the code required to generate the above model:

```dart
var machine = StateMachine.create((g) => g
          ..initialState<Solid>()
          ..state<Solid>((b) => b
            ..on<OnMelted, Liquid>(sideEffect: (e) => print('its melting')
            ..onEnter((s, e) => print('we are now solid')
            ..onExit((s, e) => print('bye bye solid existance'))
          ..state<Liquid>((b) => b
            ..on<OnFroze, Solid>(sideEffect: (e) => print('its getting cold'))
            ..on<OnVaporized, Gas>(sideEffect: (e) => print('its getting very hot'))
          ..state<Gas>((b) => b
            ..on<OnCondensed, Liquid>(sideEffect: (e) => print('move a little closer will you'))
        );
```

Your state machine will have an initial state of `Solid` and to transition to an new state you send the state machine an event.

```bash
machine.applyEvent(OnMelted);
```

Once you have applied all events (which are processed asynchronously) you need to call:

````
```dart
await machine.complete;
```
````

The call will only return once all events have been processed. Until `complete` returns the StateMachine is considered to be in an indeterminate state.

To create a visualization of the state machine:

```bash
machine.export('test.gv');
```

To view the FSM

```bash
apt install xdot
dotx test.gv
```

##

## Credits

Credit goes where credit is due.

FSM2 started out life as a fork of FSM by Kirill Bubochkin <https://github.com/ookami-kb/fsm> which in turn was inspired by [Tinder StateMachine library](https://github.com/Tinder/StateMachine).

All of the good bits are his, all of the mistakes are mine :)