API 29以下设备中Compose setShadowLayer替代实现方案咨询

由于setShadowLayer在硬件加速渲染模式下仅支持API 29及以上版本，我们可以通过版本分支处理，为API 29以下设备提供Canvas渐变模拟阴影的方案，同时保留高版本的原生实现。

修改后的完整代码 #

import android.os.Build
import androidx.compose.ui.Modifier
import androidx.compose.ui.draw.drawBehind
import androidx.compose.ui.geometry.CornerRadius
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.geometry.Size
import androidx.compose.ui.graphics.BlendMode
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.Paint
import androidx.compose.ui.graphics.drawscope.drawIntoCanvas
import androidx.compose.ui.graphics.toArgb
import androidx.compose.ui.unit.Dp
import androidx.compose.ui.unit.toPx

fun Modifier.advancedShadow(
    color: Color = Color.Black,
    alpha: Float = 1f,
    cornersRadius: Dp = 0.dp,
    shadowBlurRadius: Dp = 0.dp,
    offsetY: Dp = 0.dp,
    offsetX: Dp = 0.dp
) = drawBehind {
    val shadowColor = color.copy(alpha = alpha)
    val blurRadiusPx = shadowBlurRadius.toPx()
    val offsetXPx = offsetX.toPx()
    val offsetYPx = offsetY.toPx()
    val cornersRadiusPx = cornersRadius.toPx()

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
        // API 29+ 保留原生setShadowLayer实现
        val transparentColor = color.copy(alpha = 0f).toArgb()
        drawIntoCanvas {
            val paint = Paint()
            val frameworkPaint = paint.asFrameworkPaint()
            frameworkPaint.color = transparentColor
            frameworkPaint.setShadowLayer(
                blurRadiusPx,
                offsetXPx,
                offsetYPx,
                shadowColor.toArgb()
            )
            it.drawRoundRect(
                0f,
                0f,
                size.width,
                size.height,
                cornersRadiusPx,
                cornersRadiusPx,
                paint
            )
        }
    } else {
        // API 29以下 用径向渐变+遮罩模拟阴影
        val shadowAreaSize = Size(
            width = size.width + blurRadiusPx * 2,
            height = size.height + blurRadiusPx * 2
        )
        val shadowOffset = Offset(
            x = offsetXPx - blurRadiusPx,
            y = offsetYPx - blurRadiusPx
        )

        // 绘制径向渐变阴影
        drawCircle(
            color = shadowColor,
            radius = maxOf(shadowAreaSize.width, shadowAreaSize.height) / 2,
            center = Offset(shadowAreaSize.width / 2 + shadowOffset.x, shadowAreaSize.height / 2 + shadowOffset.y),
            alpha = alpha * 0.25f // 调整透明度适配视觉效果
        )

        // 用圆角矩形遮罩裁剪出符合控件形状的阴影
        drawRoundRect(
            color = Color.Transparent,
            topLeft = shadowOffset,
            size = shadowAreaSize,
            cornerRadius = CornerRadius(cornersRadiusPx + blurRadiusPx),
            blendMode = BlendMode.DstIn
        )
    }
}

关键说明 #

• 版本分支：通过Build.VERSION.SDK_INT判断系统版本，高版本保留原生高效实现，低版本用Canvas模拟
• 模拟原理：用径向渐变生成模糊效果，再通过圆角矩形遮罩裁剪出与控件匹配的阴影形状，最后通过偏移参数控制阴影位置
• 视觉调整：可以修改模拟部分的alpha值、渐变范围，让阴影效果更接近原生
• 性能注意：Canvas模拟的阴影性能略低于原生，但在大多数场景下足够流畅，避免在高频重绘的控件中过度使用复杂参数

内容的提问来源于stack exchange，提问作者galihif