Polymer: How to watch for change in <content> properties

Question

I'm just starting to learn Polymer. Here is a generic version of my polymer element:

<polymer-element name="my-element">
    <template>
        <style>
        :host {
            position:absolute;
            width: 200px;
            height: 100px;
            background-color: green;
        }
        </style>
        <p>my element</p>
        <content id="first-in"></content>
        <content id="second-in"></content>
    </template>
    <script>
    Polymer('my-element', {
        domReady: function() {
            alert(this.children[0].getAttribute('title')); 
            //this returns the value I want to observe
        }
    });
    </script>
<polymer-element>

The content tags are both being filled with another custom element (again slightly simplified):

<polymer-element name="in-element" attributes="title">
    <template>
        <style>
        ...
        </style>
        <p>{{title}}</p>
    </template>
    <script>
    Polymer('in-element', {
        title: 'me'
    });
    </script>
<polymer-element>

What I want to be able to do is call a function in my-element when the title attribute in (any) in-element (put in the content tag) is changed (by a click event or whatever). I'm not sure how to access this using observe or if I need to use mutationObserver, etc. How is this done/Is it possible?

Answer 1

Native properties like title are not observable by Polymer's data-binding system (e.g. Object.observe() [info]). It's generally a good idea to avoid them.

In your example, I've changed title to mytitle and published it with reflect: true so the property value reflects back to the attribute. This way you can completely avoid .getAttribute() and just check .mytitle on in-elements. You can also use {{mytitle}} in bindings.

You can do this through mutation observers [1]. Polymer provides onMutation to monitor children but you want to monitor attributes of children. For that, you need a pure MO:

ready: function() {
  var observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(m) {
      if (m.target.localName == 'in-element') { // only care about in-element
        console.log(m.attributeName, m.oldValue, m.target.mytitle);
      }
    });  
  });
  // Observe attribute changes to child elements
  observer.observe(this, {
    attributes: true,
    subtree: true,
    attributeOldValue: true
  }); 
}

Demo: http://jsbin.com/doxafewo/1/edit

In domReady(), I also changed your alert of this.children[0] to this.$.firstin.getDistributedNodes()[0].mytitle. Using getDistributedNodes() is better because you're guaranteed to have the nodes that are actually passing through the <content> insertion point [2].