events-stop-propagating-once-handled.md

Events Stop Propagating Once Handled

In a hierarchical (or nested) XState machine, events are sent to the inner most sub-state of the current state. If that state handles the event, then it won't propagate up to parent states. If a state receives an event that it doesn't handle, that event will propagate up to parent states until it is handled or there is nothing to handle it.

Let's look at an example.

{
  initial: "active",
  context: { count: 0 },
  states: {
    active: {
      on: {
        COUNT: {
          actions: "changeCount",
        },
      },
      initial: "increment",
      states: {
        increment: {
          on: {
            SWITCH_COUNT_DIRECTION: "decrement",
          },
        },
        decrement: {
          on: {
            SWITCH_COUNT_DIRECTION: "increment",
            COUNT: {
              actions: 'logEvent'
            },
          },
        },
      },
    },
  },
};

This machine starts in the active.increment state.

If I send the COUNT event, it will first go to the active.increment state. It isn't handled there, so it will propagate up to active where it is handled.

Now let's say I transition to active.decrement. If I send the COUNT event, it will first go to active.decrement where it is handled. The logEvent action happens and then the event stops. The active state does not receive nor handle the COUNT event.

source