Local transition

Also known as Internal transition, Opposite of External transition

A transition from a compound state to its direct children normally causes the compound state to exit, then enter again. In a local transition, the compound state does not enter and exit again.

Consider the following state:

The state starts out in state A, but the “SIGNAL” signal moves it between its substates, A, B and C. The transitions between A, B, C don’t cause the top level state S to exit, so the SIGNAL state does not, for example cause the entry or exit actions to happen.

But the “RESET” signal however, goes from state S to state A. If this is a normal transition, it would cause state S to exit and then enter again, before finally enter A. However, sometimes the exit and entry of state S is unwanted. IF RESET is declared to be a local transition, it does not exit the source state.

Local transitions are only possible to define from a composite state and to its children, or from a child to its containing ancestor.

Notation

XState

In XState, an internal transition is described by prefixing the target state with a dot.

{
  initial: 'A',
  onEntry: 'something',
  onExit: 'something',
  on: {
    RESET: '.A'
  },
  states: {
    A: {
      on: { SIGNAL: 'B' }
    },
    B: {
      on: { SIGNAL: 'C' }
    },
    C: {
      on: { SIGNAL: 'A' }
    }
  }
}

You can also specify an internal transition more explicitly:

on {
  RESET: [
    { target: 'A', internal: true }
  ]
}

Important to note that internal: false is the default, taking the lead from SCXML.

See xstate.js.org/docs/guides/internal for more information.

SCXML

SCXML’s <transition> has a type attribute which can be set to internal to declare a transition as local:

<state id="ALPHABET" initial="A">
  <onentry>something</onentry>
  <onexit>something</onexit>
  <transition event="RESET" target="A" type="internal" />
  <state id="A">
    <transition event="SIGNAL" target="B"/>
  </state>
  <state id="B">
    <transition event="SIGNAL" target="C"/>
  </state>
  <state id="C">
    <transition event="SIGNAL" target="A"/>
  </state>
</state>

Sources

Wikipedia

UML defines local and external transitions in TransitionKind as:

• local — Implies that the Transition, if triggered, will not exit the composite (source) State, but it will exit and re-enter any state within the composite State that is in the current state configuration.
• external — Implies that the Transition, if triggered, will exit the composite (source) State.