在创建项目的时候，studio会默认生成一个以项目名+Theme名称的Theme函数，例如MyAPPTheme

MaterialTheme 使用

@Composable
fun MyAPPTheme(
    darkTheme: Boolean = isSystemInDarkTheme(),
    content: @Composable() () -> Unit
) {
    val colors = if (darkTheme) {
        DarkColorPalette
    } else {
        LightColorPalette
    }

    MaterialTheme(
        colors = colors, //颜色集合
        typography = Typography, //字体
        shapes = Shapes, //形状
        content = content //视图
    )
}

private val DarkColorPalette = darkColors(
    primary = Color_00c389,
    primaryVariant = Purple700,
    secondary = Teal200
)

private val LightColorPalette = lightColors(
    primary = Color_00c389,
    primaryVariant = Purple700,
    secondary = Teal200

    /* Other default colors to override
    background = Color.White,
    surface = Color.White,
    onPrimary = Color.White,
    onSecondary = Color.Black,
    onBackground = Color.Black,
    onSurface = Color.Black,
    */
)

看代码，其主要控制Colors颜色，接下来我们看下Color有哪些参数

MaterialTheme colors 配置

class Colors(
primary: Color, // 主颜色，屏幕和元素都用这个颜色
primaryVariant: Color, // 主颜色变种色，用于与主颜色做区分的场景，比如app bar和system bar
secondary: Color, // 次选色之强调色，悬浮按钮，单选/复选按钮，高亮选中的文本，链接和标题
secondaryVariant: Color, // 次选色变种色，用于区分强调色
background: Color, // 背景色，在可滚动项下面展示
surface: Color, // 平面色之表层色，展示在平面组件背景色，比如Surface组件，sheet组件和Menu菜单(CardView,SheetLayout,Menu)等
error: Color, // 错误色，组件中展示错误信息，比如TextField的提示信息
onPrimary: Color, // 在主颜色primary之上的文本和图标的颜色
onSecondary: Color, // 在强调色secondary之上的文本和图标的颜色
onBackground: Color, // 在背景色background之上的文本和图标的颜色
onSurface: Color, // 在表层色surface之上的文本和图标的颜色
onError: Color, // 在错误色error之上的文本和图标的颜色
isLight: Boolean // 是否是亮色模式
)

MaterialTheme Typography 配置

在Type.kt中定义app字体样式内容：

// Set of Material typography styles to start with
val Typography = Typography(
    //一级内容文字样式
    body1 = TextStyle(
        fontFamily = FontFamily.Default,
        fontWeight = FontWeight.Normal,
        fontSize = 16.sp
    )

    /* Other default text styles to override
    //按钮文字
    button = TextStyle(
        fontFamily = FontFamily.Default,
        fontWeight = FontWeight.W500,
        fontSize = 14.sp
    ),
    //说明文字
    caption = TextStyle(
        fontFamily = FontFamily.Default,
        fontWeight = FontWeight.Normal,
        fontSize = 12.sp
    )
    ,
    //页脚页眉文字样式
    overline = TextStyle(
        fontFamily = FontFamily.Default,
        fontWeight = FontWeight.Normal,
        fontSize = 12.sp
    )
    //一级标题
    h1 = TextStyle(
        fontFamily = FontFamily.Default,
        fontWeight = FontWeight.Normal,
        fontSize = 12.sp
    )
    */
)

MaterialTheme Shapes配置

在Shape.kt中定义app背景形状样式内容：

val Shapes = Shapes(
    // 小组件使用的形状，比如: Button，SnackBar，悬浮按钮等
    small = RoundedCornerShape(4.dp),
    // 中组件使用的形状，比如Card(就是CardView)，AlertDialog等
    medium = RoundedCornerShape(4.dp),
    // 大组件使用的形状，比如ModalDrawer或者ModalBottomSheetLayout(就是抽屉布局和清单布局)
    large = RoundedCornerShape(0.dp)
)

MaterialTheme 原理

MaterialTheme它本身是一个Comsable组件，相关配置最终传入ProvideTextStyle，在其内部进行调用。

以Colors为例，其中使用CompositionLocalProvider函数通过provides提供给LocalColorScheme进行使用。

@Composable
fun MaterialTheme(
    colorScheme: ColorScheme = MaterialTheme.colorScheme,
    shapes: Shapes = MaterialTheme.shapes,
    typography: Typography = MaterialTheme.typography,
    content: @Composable () -> Unit
) {
    val rippleIndication = androidx.compose.material.ripple.rememberRipple()
    val selectionColors = rememberTextSelectionColors(colorScheme)
    CompositionLocalProvider(
        LocalColorScheme provides colorScheme,
        LocalIndication provides rippleIndication,
        androidx.compose.material.ripple.LocalRippleTheme provides MaterialRippleTheme,
        LocalShapes provides shapes,
        LocalTextSelectionColors provides selectionColors,
        LocalTypography provides typography,
    ) {
        ProvideTextStyle(value = typography.bodyLarge, content = content)
    }
}

那我们是如何引用到MaterialTheme中的相关配置呢，如MaterialTheme.colorScheme.surface，接下来看单例MaterialTheme

object MaterialTheme {
    /**
     * Retrieves the current [ColorScheme] at the call site's position in the hierarchy.
     */
    val colorScheme: ColorScheme
        @Composable
        @ReadOnlyComposable
        get() = LocalColorScheme.current

    /**
     * Retrieves the current [Typography] at the call site's position in the hierarchy.
     */
    val typography: Typography
        @Composable
        @ReadOnlyComposable
        get() = LocalTypography.current

    /**
     * Retrieves the current [Shapes] at the call site's position in the hierarchy.
     */
    val shapes: Shapes
        @Composable
        @ReadOnlyComposable
        get() = LocalShapes.current
}

最终发现获取配色时，是通过MaterialTheme单例对象的colorScheme属性，间接使用了LocalColorScheme，而LocalColorScheme又是什么呢

/**
 * CompositionLocal used to pass [ColorScheme] down the tree.
 *
 * Setting the value here is typically done as part of [MaterialTheme].
 * To retrieve the current value of this CompositionLocal, use
 * [MaterialTheme.colorScheme].
 */
internal val LocalColorScheme = staticCompositionLocalOf { lightColorScheme() }

由代码可知CompositionLocal，初始化值是lightColorScheme返回的Colors配置