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machine.js

// DEMO: Call events anywhere (including parent machines/states) with callbacks

const echoCallbackHandler = (context, event) => 
(callback, onEvent) => {
  onEvent(e => {
    if(e.type === 'HEAR') {
      callback('ECHO')
    }
  })
}

const echoMachine = Machine({
  id: "echo",
  initial: "listening",
  states: {
    listening: {
      invoke: {
        id: 'echoCallback',
        src: echoCallbackHandler
      },
      on: {
        SPEAK: {
          actions: send('HEAR', {
            to: 'echoCallback'
          })
        },
        ECHO: {
          actions: () => {
            console.log("echo, echo");
          }
        }
      }
    }
  }
});