> For the complete documentation index, see [llms.txt](https://fsm2.onepub.dev/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://fsm2.onepub.dev/transitions/sideeffects.md).

# SideEffects

FSM2 allows you to trigger individual side effects for each transition that occurs.

 The side effect is a lambda which will be called when the transition is triggered. 

Side effects make it easy to carry out a task when a transition occurs.  For example if the Event is 'OnApplyHeat' you could use a side effect to recalculate the temperature of a the water as the result of being heated.

A transition that fails to pass its [guard condition](/transitions/guard-conditions.md) will not be triggered and its side effect will not be called.

The sideEffect is called AFTER the original states `onExit` method is called but before the new states `onEnter` method is called.

When the sideEffect is called the State Machine is still considered to be in the original state.

It is safe to call `stateMachine.applyEvent` call whilst in sideEffect. The event will be queued and processed once the current transition has been completed.

```dart
void main() {
  final machine = StateMachine.create((g) => g
    ..initialState(Solid())
    ..state<Solid>((b) => b
      ..on<OnHeat, Liquid>(sideEffect: (e) => print('I melted')))
 ));
```

In this example the side effect is only called when the `onHeat` event is sent when the FSM is in the `Solid` state.

```dart
machine.applyEvent(OnHeat());
> I melted
```

The side effect will be called every time the transition occurs.

## Nested and Concurrent Regions

A single transition may result in multiple state changes when Nested or Concurrent Regions are used.

In these cases the sideEffect is still only called once&#x20;