Introduce the FadingBackground modifier
This CL introduces an overload of Modifier.background which takes a
`alpha: () -> Float` parameter to set the alpha of the background. This
should be used when the alpha of the background is animated, to avoid
recompositions during an animation.
The content of FadingBackground.kt was mostly copy pasted from
Background.kt [1].
[1] http://cs/androidx-platform-dev/compose/foundation/foundation/src/commonMain/kotlin/androidx/compose/foundation/Background.kt
Bug: 242040009
Test: Manual
Change-Id: I3296f7bc7382e2afe77a088cfdbefbfae134051d
diff --git a/packages/SystemUI/compose/core/src/com/android/systemui/compose/modifiers/FadingBackground.kt b/packages/SystemUI/compose/core/src/com/android/systemui/compose/modifiers/FadingBackground.kt
new file mode 100644
index 0000000..121bf2c
--- /dev/null
+++ b/packages/SystemUI/compose/core/src/com/android/systemui/compose/modifiers/FadingBackground.kt
@@ -0,0 +1,117 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * 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 com.android.systemui.compose.modifiers
+
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.DrawModifier
+import androidx.compose.ui.geometry.Size
+import androidx.compose.ui.graphics.Brush
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.Outline
+import androidx.compose.ui.graphics.RectangleShape
+import androidx.compose.ui.graphics.Shape
+import androidx.compose.ui.graphics.SolidColor
+import androidx.compose.ui.graphics.drawOutline
+import androidx.compose.ui.graphics.drawscope.ContentDrawScope
+import androidx.compose.ui.platform.InspectorInfo
+import androidx.compose.ui.platform.InspectorValueInfo
+import androidx.compose.ui.platform.debugInspectorInfo
+import androidx.compose.ui.unit.LayoutDirection
+
+/**
+ * Draws a fading [shape] with a solid [color] and [alpha] behind the content.
+ *
+ * @param color color to paint background with
+ * @param alpha alpha of the background
+ * @param shape desired shape of the background
+ */
+fun Modifier.background(
+ color: Color,
+ alpha: () -> Float,
+ shape: Shape = RectangleShape,
+) =
+ this.then(
+ FadingBackground(
+ brush = SolidColor(color),
+ alpha = alpha,
+ shape = shape,
+ inspectorInfo =
+ debugInspectorInfo {
+ name = "background"
+ value = color
+ properties["color"] = color
+ properties["alpha"] = alpha
+ properties["shape"] = shape
+ }
+ )
+ )
+
+private class FadingBackground
+constructor(
+ private val brush: Brush,
+ private val shape: Shape,
+ private val alpha: () -> Float,
+ inspectorInfo: InspectorInfo.() -> Unit
+) : DrawModifier, InspectorValueInfo(inspectorInfo) {
+ // naive cache outline calculation if size is the same
+ private var lastSize: Size? = null
+ private var lastLayoutDirection: LayoutDirection? = null
+ private var lastOutline: Outline? = null
+
+ override fun ContentDrawScope.draw() {
+ if (shape === RectangleShape) {
+ // shortcut to avoid Outline calculation and allocation
+ drawRect()
+ } else {
+ drawOutline()
+ }
+ drawContent()
+ }
+
+ private fun ContentDrawScope.drawRect() {
+ drawRect(brush, alpha = alpha())
+ }
+
+ private fun ContentDrawScope.drawOutline() {
+ val outline =
+ if (size == lastSize && layoutDirection == lastLayoutDirection) {
+ lastOutline!!
+ } else {
+ shape.createOutline(size, layoutDirection, this)
+ }
+ drawOutline(outline, brush = brush, alpha = alpha())
+ lastOutline = outline
+ lastSize = size
+ lastLayoutDirection = layoutDirection
+ }
+
+ override fun hashCode(): Int {
+ var result = brush.hashCode()
+ result = 31 * result + alpha.hashCode()
+ result = 31 * result + shape.hashCode()
+ return result
+ }
+
+ override fun equals(other: Any?): Boolean {
+ val otherModifier = other as? FadingBackground ?: return false
+ return brush == otherModifier.brush &&
+ alpha == otherModifier.alpha &&
+ shape == otherModifier.shape
+ }
+
+ override fun toString(): String = "FadingBackground(brush=$brush, alpha = $alpha, shape=$shape)"
+}