When you run the below code you will see x-bar's valueChanged() is being called twice. Debugging shows that in instance/properties.js, this._observeNames has ["value", "value"], and that causing it to register 2 observers on the same property. I believe the root cause is in declaration/properties.js, optimizePropertyMaps() simply concats base's observeNames and this observeNames, and doesn't remove duplicates.
https://github.com/Polymer/polymer/blob/master/src/declaration/properties.js#L25
<polymer-element name="x-foo" attributes="value">
<script>
Polymer('x-foo', {
value: null,
valueChanged: function() {
}
});
</script>
</polymer-element>
<polymer-element name="x-bar" extends="x-foo">
<script>
Polymer('x-bar', {
valueChanged: function() {
console.log('x-bar valueChanged:', this.value);
}
});
</script>
</polymer-element>
<x-bar></x-bar>
<script>
document.addEventListener('WebComponentsReady', function() {
document.querySelector('x-bar').value = {};
});
</script>output:
x-bar valueChanged: Object {}
x-bar valueChanged: Object {}
p.s. This issue is only happening in master branch.
When you run the below code you will see x-bar's valueChanged() is being called twice. Debugging shows that in instance/properties.js, this._observeNames has ["value", "value"], and that causing it to register 2 observers on the same property. I believe the root cause is in declaration/properties.js, optimizePropertyMaps() simply concats base's observeNames and this observeNames, and doesn't remove duplicates.
https://github.com/Polymer/polymer/blob/master/src/declaration/properties.js#L25
output:
p.s. This issue is only happening in master branch.