Blocker.js

const findOBR = v => {
  while(v.length || !(v.tag.onbeforeremove || v.attrs && v.attrs.onbeforeremove))
    v = v.instance || v.children || v[0]
  
  return v
}

const persist = v => {
  const target = findOBR(v.instance || v.children)

  if(v.attrs.link)
    v.attrs.link.set(target.dom, target)
}

const Blocker = {
  oninit: v => {
    v.state.link = new Map
  },
  view: v => (
      v.attrs.view
    ?
      v.attrs.view(v.state.link)
    :
      v.children
  ),
  oncreate: persist,
  onupdate: persist,
  onremove: v => {
    if(v.attrs.link)
      v.attrs.link.delete(v.dom)
  },
  onbeforeremove: v =>
    Promise.all(
      [...v.state.link.values()]
        .map(v =>
          Promise.all(
            ['attrs', 'tag']
              .map(host =>
                v[host].onbeforeremove
              )
              .map(obr =>
                obr && obr.call(v.state, v)
              )
          )
        )
    ),
}