StateMachine
StateMachine apis are used for some complex components with some state management.
Each time the state changes, a new version of it is available for reading in state.
Each state change may also emmit a temporarily existing effects (for instance, to show Toast in case of error).
The widget decided itself when the state needs to be changed. It may also provide some public methods to have limited control over the state changes (e.g. settings the params).