Subtree retain flyd streams

example.js

import m from 'mithril';
import f from 'flyd';
import retain from './flyd-retain';

function myComponent(){
  let username = f.stream('');
  let username_header = username.map(name => m('h1', name));  
  let username_input = username.map(name => m('input', {value: name, oninput: m.withAttr('value', username)}));
  
  return retain(f.combine(function(h, i){
    return m('.user', [h(), i()]);
  }, [username_header, username_input]));
}

function component(c) {
  return {controller: c, view: v => v()};
}

m.mount(document.body, component(myComponent));

flyd-retain.js

export default function retain(src) {
  let _changed = true;
  src.map(()=> _changed = true);
  
  return function(){
    if (!_changed) {
      return {subtree: 'retain'};
    };
    _changed = false;
    return src();
  }
}