如何在jetpack compose中将contentColor应用于表面的孩子

Question

contentColor如何在Jetpack内部影响 Surface 的子元素，例如在此示例中，文本颜色将为青色。

Answer 1

让我们看一下Surface代码：

@Composable
private fun Surface(
    modifier: Modifier,
    shape: Shape,
    color: Color,
    contentColor: Color,
    border: BorderStroke?,
    elevation: Dp,
    clickAndSemanticsModifier: Modifier,
    content: @Composable () -> Unit
) {
    val elevationOverlay = LocalElevationOverlay.current
    val absoluteElevation = LocalAbsoluteElevation.current + elevation
    val backgroundColor = if (color == MaterialTheme.colors.surface && elevationOverlay != null) {
        elevationOverlay.apply(color, absoluteElevation)
    } else {
        color
    }
    CompositionLocalProvider(
        LocalContentColor provides contentColor,
        LocalAbsoluteElevation provides absoluteElevation
    ) {
        Box(
            modifier
                .shadow(elevation, shape, clip = false)
                .then(if (border != null) Modifier.border(border, shape) else Modifier)
                .background(
                    color = backgroundColor,
                    shape = shape
                )
                .clip(shape)
                .then(clickAndSemanticsModifier),
            propagateMinConstraints = true
        ) {
            content()
        }
    }
}

您可以在此处看到Surface正在为值LocalContentColor设置CompositionLocalProvider 。 许多 Material 可组合物使用LocalContentColor作为其颜色的默认值，在您的示例Text中：

@Composable
fun Text(
    text: AnnotatedString,
    modifier: Modifier = Modifier,
    color: Color = Color.Unspecified,
    fontSize: TextUnit = TextUnit.Unspecified,
    fontStyle: FontStyle? = null,
    fontWeight: FontWeight? = null,
    fontFamily: FontFamily? = null,
    letterSpacing: TextUnit = TextUnit.Unspecified,
    textDecoration: TextDecoration? = null,
    textAlign: TextAlign? = null,
    lineHeight: TextUnit = TextUnit.Unspecified,
    overflow: TextOverflow = TextOverflow.Clip,
    softWrap: Boolean = true,
    maxLines: Int = Int.MAX_VALUE,
    inlineContent: Map<String, InlineTextContent> = mapOf(),
    onTextLayout: (TextLayoutResult) -> Unit = {},
    style: TextStyle = LocalTextStyle.current
) {
    val textColor = color.takeOrElse {
        style.color.takeOrElse {
            LocalContentColor.current.copy(alpha = LocalContentAlpha.current)
        }
    }
    val mergedStyle = style.merge(
        TextStyle(
            color = textColor,
            fontSize = fontSize,
            fontWeight = fontWeight,
            textAlign = textAlign,
            lineHeight = lineHeight,
            fontFamily = fontFamily,
            textDecoration = textDecoration,
            fontStyle = fontStyle,
            letterSpacing = letterSpacing
        )
    )
    BasicText(
        text,
        modifier,
        mergedStyle,
        onTextLayout,
        overflow,
        softWrap,
        maxLines,
        inlineContent
    )
}

尤其是这部分：

val textColor = color.takeOrElse {
    style.color.takeOrElse {
        LocalContentColor.current.copy(alpha = LocalContentAlpha.current)
    }
}

在这里，您可以看到Text正在尝试将颜色定义为参数。 如果它是Unspecified的，它会尝试从TextStyle中获取一个。 如果那里没有颜色 - Text使用来自LocalContentColor.current的值（与LocalContentAlpha混合，但这是另一个问题的主题）。