CSS Color Mix (#1109)

* Add color/Alpha transformer

* use cssColorMix in shadows

* add changeset

* dont apply new transformer

.changeset/poor-kids-trade.md

@@ -0,0 +1,5 @@
+---
+'@primer/primitives': minor
+---
+
+use cssColorMix instead of converting variables to hex when we use transparency

scripts/buildTokens.ts

@@ -30,7 +30,7 @@ const getStyleDictionaryConfig: StyleDictionaryConfigGenerator = (
   include,
   log: {
     warnings: 'disabled', // 'warn' | 'error' | 'disabled'
-    verbosity: 'verbose', // 'default' | 'silent' | 'verbose'
+    verbosity: 'silent', // 'default' | 'silent' | 'verbose'
     errors: {
       brokenReferences: 'throw', // 'throw' | 'console'
     },

src/primerStyleDictionary.ts

@@ -37,6 +37,7 @@ import {
   jsonFigma,
 } from './formats/index.js'
 import {themeOverrides} from './preprocessors/themeOverrides.js'
+import {colorAlphaToCss} from './transformers/colorAlphaToCss.js'
 
 /**
  * @name {@link PrimerStyleDictionary}
@@ -104,6 +105,8 @@ PrimerStyleDictionary.registerFormat({
  * Transformers
  *
  */
+PrimerStyleDictionary.registerTransform(colorAlphaToCss)
+
 PrimerStyleDictionary.registerTransform(colorToRgbAlpha)
 
 PrimerStyleDictionary.registerTransform(colorToRgbaFloat)

src/transformers/colorAlphaToCss.test.ts

@@ -0,0 +1,49 @@
+import {getMockToken} from '../test-utilities/index.js'
+import {colorAlphaToCss} from './colorAlphaToCss.js'
+
+describe('Transformer: colorAlphaToCss', () => {
+  it('transforms hex3, hex6, hex8 `color` tokens without alpha value', () => {
+    const input = [
+      getMockToken({$value: '#123'}),
+      getMockToken({$value: '#343434'}),
+      getMockToken({$value: '#34343455'}),
+    ]
+    const expectedOutput = ['#123', '#343434', '#34343455']
+    expect(input.map(item => colorAlphaToCss.transform(item, {}, {}))).toStrictEqual(expectedOutput)
+  })
+
+  it('transforms hex3, hex6, hex8 `color` tokens with alpha value', () => {
+    const input = [
+      getMockToken({$value: '#123', alpha: 0.25}),
+      getMockToken({$value: '#343434', alpha: 0.6}),
+      getMockToken({$value: '#34343466', alpha: 0.1}),
+    ]
+    const expectedOutput = [
+      'color-mix(in srgb, #123, transparent 75%)',
+      'color-mix(in srgb, #343434, transparent 40%)',
+      'color-mix(in srgb, #34343466, transparent 90%)',
+    ]
+    expect(input.map(item => colorAlphaToCss.transform(item, {}, {}))).toStrictEqual(expectedOutput)
+  })
+
+  it('transforms references with and without alpha value', () => {
+    const input = [
+      getMockToken({$value: '{base.color.green.5}'}),
+      getMockToken({$value: '{base.color.red.5}', alpha: 0.25}),
+    ]
+    const expectedOutput = ['{base.color.green.5}', 'color-mix(in srgb, {base.color.red.5}, transparent 75%)']
+    expect(input.map(item => colorAlphaToCss.transform(item as TransformedToken, {}, {}))).toStrictEqual(expectedOutput)
+  })
+
+  it('transforms color-mix with and without alpha value', () => {
+    const input = [
+      getMockToken({$value: 'color-mix(in srgb, {base.color.red.5}, transparent 25%)'}),
+      getMockToken({$value: 'color-mix(in srgb, {base.color.red.5}, transparent 25%)', alpha: 0.35}),
+    ]
+    const expectedOutput = [
+      'color-mix(in srgb, {base.color.red.5}, transparent 25%)',
+      'color-mix(in srgb, color-mix(in srgb, {base.color.red.5}, transparent 25%), transparent 65%)',
+    ]
+    expect(input.map(item => colorAlphaToCss.transform(item as TransformedToken, {}, {}))).toStrictEqual(expectedOutput)
+  })
+})

src/transformers/colorAlphaToCss.ts

@@ -0,0 +1,26 @@
+import {Transform, TransformedToken} from 'style-dictionary/types'
+import {isColorWithAlpha} from '../filters/isColorWithAlpha.js'
+import {getTokenValue} from './utilities/getTokenValue.js'
+
+export const cssColorMix = (colorA: string, colorB: string, colorBPercent: number) => {
+  if (colorBPercent < 0 || colorBPercent > 1) {
+    throw new Error(
+      `Invalid argument for "cssColorMix", colorBPercent must be between 0 and 1, ${colorBPercent} provided.`,
+    )
+  }
+  if (colorBPercent === 0) return colorA
+  if (colorBPercent === 1) return colorB
+
+  return `color-mix(in srgb, ${colorA}, ${colorB} ${colorBPercent * 100}%)`
+}
+
+export const colorAlphaToCss: Transform = {
+  name: 'colorAlpha/css',
+  type: 'value',
+  transitive: true,
+  filter: isColorWithAlpha,
+  transform: (token: TransformedToken) => {
+    if (!token.alpha || token.alpha === null) return getTokenValue(token)
+    return cssColorMix(getTokenValue(token), 'transparent', 1 - token.alpha)
+  },
+}

src/transformers/colorToHex.ts

@@ -15,7 +15,7 @@ export const colorToHex: Transform = {
   transitive: true,
   filter: isColor,
   transform: (token: TransformedToken) => {
-    const alphaValue = token.alpha ?? token.$extensions?.alpha
+    const alphaValue = token.alpha
     if (alphaValue === null || alphaValue === undefined) {
       return toHex(getTokenValue(token))
     }