Consider not resetting scroll position on items change

[MaKleSoft]

Current any change to the items array causes the scroll position to be reset. In the _itemsChanged handler:

if (this._scrollTop > this._scrollOffset) {
  this._resetScrollPosition(0);
}

There may be some cases where this is appropriate but I would argue in the majority of cases (e.g. editing an item, deleting a single item etc.) , this is not the desired behavior. What is the reasoning behind doing this? Personally, I would like to simply remove those three lines and have users take care of resetting the scroll position manually when appropriate. Thoughts?

[MaKleSoft]

Bump. Can I at least get some feedback on whether this is has any chance to get fixed? Otherwise I'll have to fork, which I'd like to avoid.

[keanulee]

Looks like that code was added in #350, and I believe it's necessary to ensure the scroll region is filled with items. @blasten may have some more insight.

[MaKleSoft]

It's only necessary if the size of the items array has changed significantly which for the use cases I've pointed out is rarely the case. In any case, resetting all the way to 0 seems excessive. It would be nice if we could figure out what the new boundary is and then reset the scroller to that limit if and only if the current scroll position is beyond that boundary. Maybe that's what #350 was trying to achieve but apparently it doesn't work. I.e. editing only one item in the array will still reset the scroller to 0 every time.

[ernsheong]

Whoops I opened a new issue on the same matter: https://github.com/PolymerElements/iron-list/issues/476 Favoring this one.

You can use Polymer's splice method so that the "items.splices" portion of the code in _itemsChanged gets triggered instead.

However I find that I needed to call Polymer.flush myself to make the render happen. See https://github.com/PolymerElements/iron-list/issues/475

My use case is that I am using the Redux pattern, and replacing the items array instance with a new one.

[MaKleSoft]

@ernsheong The splice method is no longer supported in Polymer 2.0 afaik and we're not using hybrid mode.

[ernsheong]

@MaKleSoft According to 2.0 docs splice is still here https://www.polymer-project.org/2.0/docs/devguide/model-data#array-mutation, and I am calling it in 2.0 code, it works... just that you might have to call Polymer.flush on your own to force render

[MaKleSoft]

@ernsheong huh, no shit. I was sure I had read something about it being removed somewhere. Maybe it was in the preview and they added it back later. Thanks for pointing this out!

Anyway, I think my original point is still valid, especially if you are using the ImmutableData behavior.

[ernsheong]

Yea, my use case was using Redux pattern, replacing the entire array. Cheers!

[daniele-orlando]

I'm waiting for this feature too.

[derhuebiii]

Me, too.

I work around by temporary increasing $.iron-list.scrollOffset to a number higher than the $.iron-list.scrollTop and reset it after the data has been loaded, but not optimal. :)

[ernsheong]

Polymer 2/3 is in maintenance mode. If you are starting a new project, the recommended step is to start with https://github.com/Polymer/lit-element

It is unlikely that this component or any of its sister components will be getting any features anytime soon, if ever.