Observers called multiple times in inheritance chain

[LarsDenBakker]

Description

I have two classes, base-element defines an observer and my-element extends base-element with no own properties. Both are placed in the dom with the property set. The observer of just base-element is called once. The observer of my-element (which was inherited from base-element) is called twice.

http://jsbin.com/wimozeravo/1/edit?html,console,output

Adding an empty properties set to my-element makes the observer fire only once:

http://jsbin.com/giniqeweco/1/edit?html,console,output

It seems that when a class in an inheritance chain defines a property observer, for each class in the inheritance chain that does not define a properties getter, the property observer is called once extra.

This can be illustrated with mixins (which are just classes).

Here there are two mixins and a class. The first mixin defines an observer and the others don't define any properties. The observer is fired thrice:

http://jsbin.com/hilularuce/1/edit?html,console,output

With empty properties it's fired once:

http://jsbin.com/domakayufu/1/edit?html,console,output

[LarsDenBakker]

I use this pattern a lot in my codebase, and I was thinking how this could possibly not have caused any issues for me before.

Then I realized that 2.4 just got released yesterday. After downgrading this issue no longer there, so it seems that this is a regression in the new PropertiesMixin. :(

I couldn't get the jsbin pointing to the 2.3.1 working.

[web-padawan]

I can confirm the issue, this is affecting us as well, especially by breaking observers used to set and then check some flags. See this build for vaadin-combo-box and the observer code here.

I quickly investigated and was able to reproduce it locally: adding console.log inside of the single property observer indicates that it does fire twice for the same old and new values.

[TimvdLippe]

Thanks for reporting! Did quite some digging, but I was able to figure it out. Fix up at #5068

[LarsDenBakker]

Thanks!

[LarsDenBakker]

Any updates on this? 2.4 is announced now, and this will affect most peoples codebases.

[TimvdLippe]

@LarsDenBakker #5068 will be reviewed by @kevinpschaaf and @sorvell and hopefully merged today 🎉

[LarsDenBakker]

When can we expect a 2.4.1? Checking so that I can plan accordingly for the release of our app.