RE: [Custom Elements] attributeChanged not sufficient?

As a point of information, IE supports an "elementresize" event in order to support this scenario. It's not great because it puts a lot of implementation requirements on our layout engine, but it's one potential approach to addressing this scenario.

(Note, the event is prefixed in our implementation: "mselementresize")

-----Original Message-----
From: Ryosuke Niwa [] 
Sent: Monday, March 31, 2014 5:24 PM
To: Ondřej Žára
Subject: Re: [Custom Elements] attributeChanged not sufficient?

> On Mar 31, 2014, at 8:20 PM, Ondřej Žára <> wrote:
> Hi,
> let me introduce my Custom Element scenario: an interactive map element, powered by one of the well-known JS APIs (such as Google Maps API or so).
> Typically, the markup will be like
> <my-map lat="..." lon="..." zoom="..." controls>
> However, the underlying JS needs to know when this element's rendered size changes; the viewport needs to be filled with new map tiles and other geo data.
> Typically, when using a plain JS API (and not a custom declarative markup), users are used to call a size synchronization routine, shall the map viewport change. This is no longer the case when a Custom Element is introduced (and scripting is replaced by declarative HTML).

I don't think this problem is specific to custom elements. Users can always affect the size of the element with or without custom elements.

> A user may insert a map element anywhere in the page (see for reference), including a variable-width box in a sidebar or so. This means that the <my-map> element itself cannot determine when its own (rendered) size changes, as the attributeChanged callback only applies to own attributes.


> Is there some recommended way of dealing with this?

I think the only way to detect this would be constantly querying the size of your custom element, which will be a performance nightmare.

Perhaps we can add asynchronous event that fires whenever the dimension of an element changes? The hardest part of spec'ing such an event will be figuring how often the change will be checked. We can't do it after every DOM modification, every micro task, etc... due to performance constraints.

- R. Niwa

Received on Tuesday, 1 April 2014 17:11:35 UTC