android color hint

Author: intergalacticspacehighway

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/uimanager/style/Gradient.kt

@@ -8,11 +8,21 @@
 package com.facebook.react.uimanager.style
 
 import android.content.Context
+import android.graphics.Color
 import android.graphics.Rect
 import android.graphics.Shader
+import androidx.core.graphics.ColorUtils
 import com.facebook.react.bridge.ColorPropConverter
+import com.facebook.react.bridge.ReadableArray
 import com.facebook.react.bridge.ReadableMap
 import com.facebook.react.bridge.ReadableType
+import com.facebook.react.uimanager.FloatUtil
+import kotlin.math.ln
+
+private data class ColorStop(
+  var color: Int? = null,
+  val position: Float
+)
 
 internal class Gradient(gradient: ReadableMap?, context: Context) {
   private enum class GradientType {
@@ -36,23 +46,19 @@ internal class Gradient(gradient: ReadableMap?, context: Context) {
         gradient.getMap("direction")
             ?: throw IllegalArgumentException("Gradient must have direction")
 
-    val colorStops =
+    val colorStopsRaw =
         gradient.getArray("colorStops")
             ?: throw IllegalArgumentException("Invalid colorStops array")
 
-    val size = colorStops.size()
-    val colors = IntArray(size)
-    val positions = FloatArray(size)
-
-    for (i in 0 until size) {
-      val colorStop = colorStops.getMap(i) ?: continue
-      colors[i] =
-          if (colorStop.getType("color") == ReadableType.Map) {
-            ColorPropConverter.getColor(colorStop.getMap("color"), context)
-          } else {
-            colorStop.getInt("color")
-          }
-      positions[i] = colorStop.getDouble("position").toFloat()
+    val colorStops = processColorTransitionHints(colorStopsRaw, context);
+    val colors = IntArray(colorStops.size)
+    val positions = FloatArray(colorStops.size)
+
+    colorStops.forEachIndexed { i, colorStop ->
+      colorStop.color?.let { color ->
+        colors[i] = color
+        positions[i] = colorStop.position
+      }
     }
 
     linearGradient = LinearGradient(directionMap, colors, positions)
@@ -64,4 +70,117 @@ internal class Gradient(gradient: ReadableMap?, context: Context) {
           linearGradient.getShader(bounds.width().toFloat(), bounds.height().toFloat())
     }
   }
+
+  // Spec: https://drafts.csswg.org/css-images-4/#coloring-gradient-line (Refer transition hint section)
+  // Browsers add 9 intermediate color stops when a transition hint is present
+  // Algorithm is referred from Blink engine [source](https://github.com/chromium/chromium/blob/a296b1bad6dc1ed9d751b7528f7ca2134227b828/third_party/blink/renderer/core/css/css_gradient_value.cc#L240).
+  private fun processColorTransitionHints(originalStopsArray: ReadableArray, context: Context): List<ColorStop> {
+    val colorStops = ArrayList<ColorStop>(originalStopsArray.size())
+    for (i in 0 until originalStopsArray.size()) {
+      val colorStop = originalStopsArray.getMap(i) ?: continue
+      val position = colorStop.getDouble("position").toFloat()
+      val color = if (colorStop.hasKey("color") && !colorStop.isNull("color")) {
+        if (colorStop.getType("color") == ReadableType.Map) {
+          ColorPropConverter.getColor(colorStop.getMap("color"), context)
+        } else {
+          colorStop.getInt("color")
+        }
+      } else null
+
+      colorStops.add(ColorStop(color, position))
+    }
+
+    var indexOffset = 0
+    for (i in 1 until colorStops.size - 1) {
+      val colorStop = colorStops[i]
+      // Skip if not a color hint
+      if (colorStop.color != null) {
+        continue
+      }
+
+      val x = i + indexOffset
+      if (x < 1) {
+        continue
+      }
+
+      val offsetLeft = colorStops[x - 1].position
+      val offsetRight = colorStops[x + 1].position
+      val offset = colorStop.position
+      val leftDist = offset - offsetLeft
+      val rightDist = offsetRight - offset
+      val totalDist = offsetRight - offsetLeft
+
+      val leftColor = colorStops[x - 1].color ?: Color.TRANSPARENT
+      val rightColor = colorStops[x + 1].color ?: Color.TRANSPARENT
+
+      if (FloatUtil.floatsEqual(leftDist, rightDist)) {
+        colorStops.removeAt(x)
+        --indexOffset
+        continue
+      }
+
+      if (FloatUtil.floatsEqual(leftDist, .0f)) {
+        colorStop.color = rightColor
+        continue
+      }
+
+      if (FloatUtil.floatsEqual(rightDist, .0f)) {
+        colorStop.color = leftColor
+        continue
+      }
+
+      val newStops = ArrayList<ColorStop>(9)
+      // Position the new color stops
+      if (leftDist > rightDist) {
+        for (y in 0..6) {
+          newStops.add(ColorStop(
+            position = offsetLeft + leftDist * ((7f + y) / 13f)
+          ))
+        }
+        newStops.add(ColorStop(
+          position = offset + rightDist * (1f / 3f)
+        ))
+        newStops.add(ColorStop(
+          position = offset + rightDist * (2f / 3f)
+        ))
+      } else {
+        newStops.add(ColorStop(
+          position = offsetLeft + leftDist * (1f / 3f)
+        ))
+        newStops.add(ColorStop(
+          position = offsetLeft + leftDist * (2f / 3f)
+        ))
+        for (y in 0..6) {
+          newStops.add(ColorStop(
+            position = offset + rightDist * (y / 13f)
+          ))
+        }
+      }
+
+      // calculate colors for the new color hints.
+      // The color weighting for the new color stops will be
+      // pointRelativeOffset^(ln(0.5)/ln(hintRelativeOffset)).
+      val hintRelativeOffset = leftDist / totalDist
+      for (newStop in newStops) {
+        val pointRelativeOffset = (newStop.position - offsetLeft) / totalDist
+        val weighting = Math.pow(
+          pointRelativeOffset.toDouble(),
+          ln(0.5) / ln(hintRelativeOffset.toDouble())
+        ).toFloat()
+
+        if (weighting.isInfinite() || weighting.isNaN()) {
+          continue
+        }
+
+        newStop.color = ColorUtils.blendARGB(leftColor, rightColor, weighting)
+      }
+
+      // Replace the color hint with new color stops.
+      colorStops.removeAt(x)
+      colorStops.addAll(x, newStops)
+      indexOffset += 8
+    }
+
+    return colorStops
+  }
 }

packages/rn-tester/js/examples/LinearGradient/LinearGradientExample.js

@@ -226,7 +226,6 @@ exports.examples = [
   },
   {
     title: 'Transition hint',
-    description: 'Linear gradient with transition hint',
     render(): React.Node {
       return (
         <GradientBox