[css-paint-api] Allow paint function arguments, properties, or both?

[bfgeek]

Example with just properties:

.foo {
  --bar: 5;
  background-image: paint(foo), paint(foo);
}

registerPaint(class {
  static inputProperties = ['--bar', 'color'];
  void paint(ctx, geom, styleMap) {
    // --bar & color both available here.
  }
});

Pros:

• Can use color.

Cons:

• Can't use same function to get different result (like linear-gradient).

Example with just input arguments

.foo {
  --bar: 5;
  background-image: paint(foo, var(--bar)), paint(foo, 10);
}

registerPaint(class {
  static inputArguments = ['<number>'];
  void paint(ctx, geom, styleMap) {
    // --bar & color both available here.
  }
});

Pros:

• Can use same function with different arguments.

Cons:

• Can't access native properties like color.

[bfgeek]

paint(foo 0deg, red);
paint(foo , red);

inputArgumentType = ['<magical-thingy>']

[bfgeek]

The HoudiniTF F2F resolved to add both for paint lvl1.

[bfgeek]

It turns out this is hard to spec. This requires defining a grammar for what the paint image function can accept (to be fully flexible).

For example:

registerPaint('foo', class {
  static inputArgumentSyntax = '<number>? <length>';
})

This is going to be difficult to co-erce to a typed representation. It is ambiguous, and we might get it wrong. I.e. for the above example:

.foo { background-image: paint(foo 0) }

A better way of doing this, is with a future parser API thingy. For example:

paint() = paint( <ident> <declaration-value>? )

registerPaint(class {
  void paint(ctx, geom, styleMap, inputTokens) {
    const lengthTypedOM = inputTokens.parseLength(); // Pops tokens which is a length off.
    if (!lengthTypedOM) {
      // Fail.
    }
    const numberTypedOM = inputTokens.parseNumber(); // Etc.
  }
});

^ This would be very powerful for authors.

I think we should leave this out for FPWD, but leave it as an open issue, to wait until Parse API could support this.

[bfgeek]

Updated issue in: 7677ff9 .

[tabatkins]

This would have the same restrictions on grammar that we have for typed custom properties, with the same ambiguity-resolving behavior.

[shans]

New approach: will provide an argument syntax to paint registration. This will match the things that are allowed in properties & values, including '*', which is a token-stream. NOTE: This should be exposed as a string rather than a stream of tokens. By default will be nothing (no arguments).