NowThen.js

export default () => {
  const queue = []
  let then

  return {
    onbeforeupdate : () => !blocker,
    oncreate       : resolve,
    onupdate       : resolve,
    view           : ({
      instance,
      children: [now],
      attrs   : {onchange = () => {}},
    }) => {
      let output = [[now]]

      onchange({
        now,
        then,
        go: guard(async directive => {
          output = [[then], [now]]
          
          await queue(directive)
        }),
      })

      return output
    },
  }
  
  function resolve(now){
    if(resolution)
      resolution(now.dom)
  }
  
  function guard(directive){
    return async () => {
      if(resolution)
        return console.error(
          `Resolution directives must be executed synchronously.`
        )

      if(blocker)
        return console.error(
          `Resolution directives are mutually exclusive.`
        )

      blocker = directive

      await directive()

      blocker = false
    }
  }
}