Add `keyframes` support for scoped style ?

[jihchi]

Consider following example:
https://svelte.technology/repl/?version=1.6.3&gist=5390c2517520e606bb02ce8168f3b8b0

Currently, Svelte will output results as example:

.foo[svelte-3750005346], [svelte-3750005346] .foo {
    color: red;
    font-size: 2em;
    font-family: 'Comic Sans MS';
    animation: spin-360 infinite 20s linear;
}

@keyframes spin-360 {
    from[svelte-3750005346], [svelte-3750005346] from { transform: rotate(0deg); }
    to[svelte-3750005346], [svelte-3750005346] to { transform: rotate(360deg); }
}

Which animation won't works.

Expect that css should be:

@@ -6,6 +6,6 @@
 }
 
 @keyframes spin-360 {
-    from[svelte-3750005346], [svelte-3750005346] from { transform: rotate(0deg); }
-    to[svelte-3750005346], [svelte-3750005346] to { transform: rotate(360deg); }
+    from { transform: rotate(0deg); }
+    to { transform: rotate(360deg); }
 }

[Conduitry]

I'm looking at the code responsible for this and it's hurting my head https://github.com/sveltejs/svelte/blob/master/src/generators/shared/css/transform.js :( There's no real parsing of the CSS, it's just a bunch of regular expressions.

This is kind of icky because things like @media declarations should have what's inside them transformed, but @keyframes declarations like the one in the example should not - and what's inside the @keyframes declaration looks exactly like CSS selectors that we'd want to transform in other contexts.

I don't know how robust we ought to make this. I believe one workaround now (without any changes to Svelte) would be to just use 0% and 100% instead of from and to - Svelte won't mistake these for selectors. A somewhat hacky solution within Svelte would be to disregard anything that looks like a from or to selector in all cases, and hope no one is creating and attempting to style elements with those names.

Other than that, I don't know what to suggest, because I suspect we don't want to bring in a whole CSS parsing and processing library. The least-impact solution might be to just put a warning in the docs that you need to use 0% and 100% in @keyframes so the CSS processor doesn't get confused.

[Conduitry]

@jihchi Using 0% and 100% instead of from and to works around this for now - https://svelte.technology/repl/?version=1.6.3&gist=cf921c3813f02e5c2b78d39e1ae448d5

[Rich-Harris]

My initial thought was that we probably should use a dedicated CSS parser (most likely PostCSS) because that would make it a lot easier to do things like minification/optimisation in a sourcemap-aware way: #8 (comment). Because Svelte only runs at build time, there isn't the same pressure to be lean.

The code in that file is copied straight from Ractive, where there was pressure to keep things compact as in many cases the code runs in the browser. It was only really intended as a stopgap measure.

That said, @keyframes is a tricky one... other CSS is scoped, but keyframes are global. I wonder if we need to manipulate the animation name (and all its references) to achieve encapsulation.

FWIW, in the past when I've encountered this problem (with Ractive projects) I've reluctantly included a separate .css file with my keyframe declarations. Is a cop-out, but it works around the issue until we fix it properly.

[jihchi]

Thanks for the help / workaround. @Conduitry @Rich-Harris

[thysultan]

@Rich-Harris In case this is useful, i built a css compiler stylis.js that does handle name-spacing keyframes and animations.

[Conduitry]

I've been playing with this a little bit here. Not sure whether this is going to be a good direction to go in. In my branch, this particular issue is fixed, but there are a few not very nice things still. One is that PostCSS is introduced as an external (non-dev) dependency. It or one of its dependencies was depending on built-in Node modules, which the current build process doesn't seem to be able to handle. It's possible that this could be resolved with rollup-plugin-node-builtins but I didn't want to get bogged down in that for now.

Using PostCSS to process the CSS did remove some of the ugly code from transform.js, but sadly the actual processing of a single selector remains the same, and ugly. I'm not very familiar with PostCSS at all, but I didn't immediately see a way of getting at something more processed than a single selector.

[Conduitry]

Hm. Looks like this is not fixed yet with the new csstree-based parsing and transformation (REPL), but hopefully now this should be actually possible. I'll try to have a look at the new AST walker.

edit: In fact, the workaround I suggested before (which was based on limitations of the old regexes) doesn't work anymore in 1.7+ either. The 0% and 100% are now see as selectors.

[Conduitry]

Using this walker in processCSS.js seems to work. It adds a special case for @keyframes and doesn't process those at all.

	function walk ( node ) {
		if ( node.type === 'Rule' ) {
			transform( node );
		} else if ( node.type === 'Atrule' && node.name === 'keyframes' ) {
			// do not process keyframes declarations
		} else if ( node.children ) {
			node.children.forEach( walk );
		} else if ( node.block ) {
			walk( node.block );
		}

This doesn't take care of scoping the keyframes rules. Prefixing the @keyframes name (and the animation-name references to it) shouldn't be too hard, I'm looking into that now.

[Rich-Harris]

Nice! Re scoping – one thing that occurs to me is that while it should be possible to transform keyframe declarations and animation-name in the same component, it would behave slightly unexpectedly here:

<!-- Main.html -->
<Widget/>

<style>
  div {
    color: red;
  }

  @keyframes foo {...}
</style>

<!-- Widget.html -->
<div>this text is red, because child components inherit parent styles</div>

<style>
  div {
    /* foo would not be transformed correctly, so the animation would not
       be inherited */
    animation: foo 1s;
  }
</style>

So some CSS (color: red) would be inherited by child components, but some (i.e. keyframes) wouldn't. How do we feel about that? Maybe it's not a big deal. Or maybe there's some way that keyframes could be inherited that I haven't thought of? Or maybe no styles should ever be inherited by child components (though I often find it quite useful to be able to declare styles at a certain point in the tree, rather than repeating them for child components).

[Conduitry]

Something else I realized is that the keyframes names won't always show up in animation-name of course, they'll also show up in animation shorthand properties. We might have to do two passes, one where we collect keyframes names, and the next to replace instances of them in animation-name and animation declarations.

Doing two passes is probably necessary anyway - we don't want to be transforming keyframes names that aren't defined in that component, because that would break using keyframes that were defined globally.

As for inheriting keyframes ... I don't see, currently, how to do that in a nice way. It seems like it would necessarily involve some runtime manipulation of the css, because at compile time a component has no idea what context it will be used in, or what the keyframes it's using have been renamed to.

[Rich-Harris]

Yeah, I think you're right. Perhaps a workaround would be to have a way of marking keyframes as global, like this...

@keyframes !foo {...}

...though it feels a bit hacky, and I suppose it can be worked around by just adding a regular stylesheet or <style> tag somewhere on the page. Maybe it's just a case of documenting that particular (fairly minor) limitation

[Conduitry]

Just pushed what I have now to https://github.com/Conduitry/svelte/tree/gh-245-keyframes-scoping

It uses two passes - one where it collects the names of the keyframes definitions and transforms their definitions - and then a second where it does the normal [svelte-xxx] transformations and then also changes any identifiers in animation or animation-name properties that were previously found to be the names of keyframes.

This does allow you to use globally-defined keyframes within components, but will not allow you to globally define keyframes within components. I think that's reasonable behavior.

[thysultan]

@Conduitry You could also change the keyframe and animation names to ${name}+xxx where xxx is from [svelte-xxx] that way you can do it in one pass and allow the children of a component to inherit keyframes.