Shader-based fill painters

Baseline support for #4

theming/src/desktopMain/kotlin/org/pushingpixels/aurora/theming/painter/fill/ShaderWrapperFillPainter.kt

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2020-2021 Aurora, Kirill Grouchnikov
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pushingpixels.aurora.theming.painter.fill
+
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.geometry.Size
+import androidx.compose.ui.graphics.Outline
+import androidx.compose.ui.graphics.Path
+import androidx.compose.ui.graphics.ShaderBrush
+import androidx.compose.ui.graphics.addOutline
+import androidx.compose.ui.graphics.drawscope.DrawScope
+import androidx.compose.ui.graphics.drawscope.clipPath
+import org.jetbrains.skia.Shader
+import org.pushingpixels.aurora.theming.colorscheme.AuroraColorScheme
+
+/**
+ * Implementation of [AuroraFillPainter] that uses a Skia shader to paint on
+ * filled areas.
+ *
+ * @author Kirill Grouchnikov
+ */
+abstract class ShaderWrapperFillPainter(
+    val shaderGenerator: (AuroraColorScheme, Size) -> Shader,
+    val baseFillPainter: AuroraFillPainter
+) : AuroraFillPainter {
+    private val shaders = hashMapOf<Pair<AuroraColorScheme, Size>, Shader>()
+
+    override fun paintContourBackground(
+        drawScope: DrawScope,
+        size: Size,
+        outline: Outline,
+        fillScheme: AuroraColorScheme,
+        alpha: Float
+    ) {
+        with(drawScope) {
+            baseFillPainter.paintContourBackground(
+                drawScope = this,
+                size = size,
+                outline = outline,
+                fillScheme = fillScheme,
+                alpha = alpha
+            )
+
+            val key = Pair(fillScheme, size)
+            var shader = shaders[key]
+            if (shader == null) {
+                shader = shaderGenerator.invoke(fillScheme, size)
+                shaders[key] = shader
+            }
+
+            val clipPath = Path()
+            clipPath.addOutline(outline)
+            clipPath(path = clipPath) {
+                drawRect(
+                    brush = ShaderBrush(shader),
+                    topLeft = Offset(0.0f, 0.0f),
+                    size = size
+                )
+            }
+        }
+    }
+}