Introduce the FadingBackground modifier

/*

 * 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)"

}