PolymerLabs/part-theme

Name: part-theme

Owner: PolymerLabs

Description: Experimental proof of concept shim for ::part/::theme css.

Created: 2017-10-17 00:53:08.0

Updated: 2018-02-27 10:12:23.0

Pushed: 2018-02-06 18:51:59.0

Homepage: null

Size: 9

Language: JavaScript

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README

part-theme

This is an experimental proof of concept shim for ::part or ::theme as defined here https://tabatkins.github.io/specs/css-shadow-parts/.

PartThemeMixin should be applied to any elements that define or consume css. Elements may instead just call applyPartTheme(element) to avoid use of the mixin.

Known Issues

• Works only for custom-elements that implement the PartThemeMixin.
• ::part/::theme selectors in main document styles are not applied.
• styling for dom that modifies part attributes after connectedCallback requires use of the custom api _applyPartTheme().
• Usable only with native Shadow DOM. Does not work with ShadyCSS polyfill.

Performance

Performance will likely be sub-optimal due to instance level work done to support parts.

1. Element's styles are parsed for ::part/::theme selectors. These are re-written using css custom properties.

2. Element's shadowRoot dom is scanned for elements with part attributes. A new style element is added with rules that match these attributes corresponding to part properties provided as css custom properties from the element ancestor.

NOTE: Any time dom is modified in the element containing part attributes, _applyPartTheme must be called. This is done automatically only when the element is connected.

Future optimization

1. Support for ::theme requires traversing the entire ancestor tree. Dropping support for ::theme would remove this requirement and then ancestor traversal would only be required for forwarding (foo => forwarded-foo).
2. Currently css is unique for every element instance but unique css is only required when part css is different. This should only be when (a) host type changes, (b) element part attribute is different, (c) ::theme changes in ancestor tree.