Shadow Dom - Know When Dom Is Rendered/changed

I am writing a chrome extension that modifies elements properties as a page is loaded or changes. I do this using a Mutation Observer. However the observer's handler is not called

Solution 1:

You can simply observe() the shadowRoot property of the element with a Shadow DOM.

customElements.define('image-list', classextendsHTMLElement {
  connectedCallback() {
    var observer = newMutationObserver(function(mutations) {
      mutations.forEach(function(mutation) {
        //Detect <img> insertionif (mutation.addedNodes.length)
          console.info('Node added: ', mutation.addedNodes[0])
      })
    })

    this.attachShadow({mode: 'open'}).innerHTML = '<img alt="image 1">'

    observer.observe(this.shadowRoot, {childList: true})
  }

  addImage() {
    var img = document.createElement('img')
    img.alt = ' new image 'this.shadowRoot.appendChild(img)
  }
})

<image-listid=LI></image-list><buttononclick="LI.addImage()">Add image</button>

Solution 2:

if the contents are simply getting slotted using the slotchange event is easiest, it doesn't fire on deeper changes, just the slotting (unlike a mutation observer watching a subtree and childList); it's like an observer of childList on the slot and the best way to separate concerns between a component and slotted content; when observing changes in a component's own respective shadowRoot or deeper in a tree for some reason, use the mutation observer in @Supersharp's answer except adding subtree to the config (or possibly setup children with their respective shadowRoots to dispatch an event with {composed: true, cancelable: true, bubbles: true} notifying ancestors up to your component of the change;

there'd need to be a slot added to the shadowRoot and the event listener added to it like shadowRoot.innerHTML = <slot onslotchange=...>empty slot</slot> adapted to work in-context

https://developer.mozilla.org/docs/Web/API/HTMLSlotElement/slotchange_event