Observer will not work in a behavior? - polymer

Question

I found that only when I break out a observer into a behavior, no change will be detected. Are observers not able to be used in behaviors?
<iron-ajax
auto="[[activated]]"
url="[[HOST]][[LISTINGS]]/[[listingNumber]]"
handle-as="json"
verbose="true"
with-credentials="true"
on-error="_error"
loading="{{loading}}"
last-error="{{apiError}}"
last-response="{{listing}}"></iron-ajax>
</template>
<script>
Polymer({
is: 'single-listing',
behaviors: [ApiConstants, IronAjaxHelpers],
<script>
IronAjaxHelpers = {
listingNumber: {
type: Number,
value: 0,
notify: true
},
activated: {
type: Boolean,
value: false,
observer: 'setListingNumber'
},
setListingNumber: function(newValue, oldValue) {
console.log(newValue);
//this.listingNumber = id;
if (newValue === true) {
this.listingNumber = app.listingNumber;
}
}
};
</script>

Answer 1

Your behavior's properties should be defined inside the properties field, but it's currently at the top-level of the behavior object.
You should declare the properties inside the behavior like this:
IronAjaxHelpers = {
properties: {
/** PROPERTIES GO HERE **/
listingNumber: {
type: Number,
value: 0,
notify: true
},
activated: {
type: Boolean,
value: false,
observer: "setListingNumber"
}
},
setListingNumber: function(newValue, oldValue) {
console.log(newValue);
}
};
codepen