why does onmutation disconnect after first mutation?

[Seth Ladd]

Hi all,

I see onMutation (a nice helper!) here: https://github.com/Polymer/polymer/commit/528129c  Why does it disconnect after the first mutation?

If we expect that there will be many mutations to a node, should we just use the raw MutationObserver ?

I like onMutation... it's easier to use, but I'd like it to not disconnect (unless I'm missing a subtle part of the API)

Thanks for the context,

Seth

[Scott Miles]

It's tempting for users to use `async` or `timeout` to try to wait for DOM updates after modifying some data. As we know, it's easy to get into a race-condition this way, and so it's an anti-pattern.

Going through our examples, I found some usage like this and wanted to replace it with a mutation observer. In all those use cases, these were one-off situations. I change this, then when the DOM updates, change that. For this reason, I coded `onMutation` as a one-time use API.

There are clearly use-cases that want continuous monitoring. It's possible to do this using onMutation, but it's not ideal (see example). We are working on a modification to make this easier, or to allow declarative monitoring of shadow-roots, light-dom, or possibly individual nodes.

Example of using onMutation for monitoring:

      watchMutation: function() {

        var change = function() {
          this.presentationChanged();
          this.onMutation(this, change);
        }.bind(this);
        this.onMutation(this, change);
      }


Scott

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[Seth Ladd]

Ah ha, makes sense. And helpful.

Warning! Bikeshedding!  Perhaps a name that better conveys this is a one-time callback?

* onFirstMutation

* whenMutates

Thanks for the explanation,

Seth

[Scott Miles]

I think there's universal agreement the name (`onMutation`) is not good.

So, thanks for the suggestions!

Scott

[sepand...@gmail.com]

I came across the same problem. In my case I want to run some code when a new element is dropped into a container element in the designer tool. Just wanted to check if there is a better way of doing this rather than using the 'not ideal' watchMutation function.

Thanks

Sepand