Behaviour trees using generators

bt.js

// Some example behaviours

// Helper to wait X frames
function *wait(framecount) {
  while(--framecount) yield // Each call to yield will wait 1 frame
}

// Dance function will
function *dance() {
  console.log('\\_o_\\')
  yield wait(10) // wait 10 frames (1 second)
  console.log('/_o_/')
  yield wait(5)
  console.log('\\_o_/')
}



// *** Plumbing.
  
// The state is a stack of iterators. Each tick we'll take the top of the stack
// (the stack's last element) and iterate it.
let state = [dance()]


// Grab a reference to GeneratorFunction so we can detect nested iterators.
const GeneratorFunction = (function *(){})().constructor

const next = (iter) => {
  // Used to get values out of nested generators
  let lastVal = null

  while (state.length) {
    const iter = state[state.length - 1]

    const {value, done} = iter.next(lastVal)
    //console.log('iter next', value, done)

    if (done) {
      // value is the return value of the function. We'll pass that back to the
      // yield statement of the caller. We could instead force the caller to
      // return or something to save on error handling.
      state.pop()
      lastVal = value
    } else if (typeof value === 'object' && value.constructor === GeneratorFunction) {
      state.push(value)
      lastVal = null
    } else return null
  }

  return true // success.
}


let timer = setInterval(tick, 100)

function tick() {
  const val = next()
  //console.log('tick', val)

  // We're done
  if (val === true) clearInterval(timer)
}

output.txt

$ node bt.js 
\_o_\ # ... then 1 second pause
/_o_/ # then 500ms pause
\_o_/