# Simple Transitions

Simple transitions are declared using the 'on' builder.

```dart
import 'package:fsm2/fsm2.dart';

void main() {
  final machine = StateMachine.create((g) => g
    ..initialState(Solid())
    ..state<Solid>((b) => b
      ..on<OnMelted, Liquid>())
 );
```

The above examples creates a Finite State Machine (machine) which declares its initial state as being `Solid` and then declares a single transition Solid -> Liquid when the event `OnMelted` is applied to the FSM.

We can now write a unit test that validates our state machine when \`machine.applyEvent(OnMelted())\` is called.

```dart
import 'package:fsm2/fsm2.dart';
import 'package:test/test.dart';

test('test transition', () async {
  
  /// build the state machine.
  final machine = StateMachine.create((g) => g
    ..initialState(Solid())
    ..state<Solid>((b) => b
      ..on<OnMelted, Liquid>())
      
  /// check its initial state
  expect(machine.isInState<Solid>(), equals(true));

  /// trigger the state change
  await machine.applyEvent(OnMelted());

  /// check we arrived in the expected state.
  expect(machine.isInState<Liquid>(), equals(true));
});
```

### Many transitions

A State can have any number of `on` transitions including multiple transitions for the same Event providing a guard conditions is used.

See the section on [Guard Conditions](/transitions/guard-conditions.md) for details on defining conditional transitions.


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