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dom - In Dart why the code below (about MutationObserver) dose not work?

I modified one the Dart polymer example to test MutationObserver. It does not work! Any suggestion?

This is the HTML code:

<body>   
<ul>      
  <template id="tmpl" repeat>
    <li>{{}}</li>
  </template>
</ul>
</body>

This is Dart code:

MutationObserver observer = new MutationObserver(_onMutation);
observer.observe(query('#tmpl'), childList: true, subtree: true); 
List timestamps = toObservable([]); 
query('#tmpl').model = timestamps;

new Timer.periodic(const Duration(seconds: 1), (_) {
    timestamps.add(new DateTime.now());
});

_onMutation(List<MutationRecord> mutations, MutationObserver observer) {
 print('hello test MutationObserver');  **//there is not any print!!!!!!!!!!!**
}

Any idea about why it would not work?

[Note: The webpage display is fine, problem is just about the MutationObserver]

Thanks!

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I think you don't want to listen on #tmpl, but on its parentNode. HTML Template element expands its contents as siblings when a model is set. Try this change:

observer.observe(query('#tmpl').parent, childList: true, subtree: true); 

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