code: function startIfNeeded (target) {
    if (!lazyloadTimer) {
      lazyloadTimer = setTimeout(function () {
        fire()
        clearTimeout(lazyloadTimer)
        lazyloadTimer = null
      }, 16)
    }
  }
docs: we don't know when all images are appended just use setTimeout to do delay lazyload  -- actually everytime we add a element or update styles, the component manager will call startIfNeed to fire lazyload once again in the handleAppend function. so there is no way that any image element can miss it. See source code in componentMangager.js. just for the image component to start lazyload.