kotlin-styled-next

This is a reimplementation of kotlin-styled in pure Kotlin that doesn't use styled-components. It is designed to be a drop-in replacement for kotlin-styled. It's currently feature-complete and its performance characteristics are very similar to kotlin-styled.

In development mode it is superior to kotlin-styled because CSSOM API is used by default with an ability to fall back to <style> insertion during run time (see Debugging below).

React 18 (useInsertionEffect) is already supported by kotlin-styled-next.

To sum it up, if you're using kotlin-styled, we recommend switching to kotlin-styled-next.

However, please do note that kotlin-styled-next is only compatible with kotlin-react-legacy. If you are using kotlin-react, use kotlin-emotion instead.

Gradle

repositories {
    mavenCentral()
}

implementation("org.jetbrains.kotlin-wrappers:kotlin-styled-next:VERSION")

Getting Started

kotlin-styled-next is a great fit for applications built using kotlin-react. It gives you not only a type-safe way of authoring stylesheets, but it also takes care of adding vendor prefixes for your CSS rules, assembling stylesheets, and injecting them into the DOM.

If you are not familiar with styled-components or CSS-in-JS in general, now would be a good time to inform yourself about the concept , because kotlin-styled-next implements this exact idea... in Kotlin.

When using just kotlin-react you would create a regular CSS file and then you would reference CSS classes from Kotlin like this:

fun RBuilder.div() {
    div("some-class") {
        +"Hello world!"
    }
}

With kotlin-styled-next you never have to leave Kotlin:

fun RBuilder.div() {
    styledDiv {
        css {
            padding = Padding(vertical = 16.px)
            backgroundColor = Color.green
        }

        +"Hello world!"
    }
}

While you can mix markup and styles in one-off scenarios like the example above, most times you would probably want to have them separated to enable code reuse:

object ComponentStyles : StyleSheet("ComponentStyles") {
    val wrapper by css {
        padding = Padding(vertical = 16.px)
        backgroundColor = Color.green
    }
}

fun RBuilder.div() {
    styledDiv {
        css {
            +ComponentStyles.wrapper
        }

        +"Hello world!"
    }
}

The latter is much easier to debug in the browser as well: when inspecting the element you'll see readable class names, e.g. class="ComponentStyles-wrapper" rather than generated ones.

CSS Properties

The DSL supports most common CSS properties and values, including animations, transforms, shadows, flexbox, and grids. SVG properties are not supported yet, contributions are welcome. However, you can use put("property", "value") syntax for any unsupported property:

fun RBuilder.div() {
    styledDiv {
        css {
            put("will-change", "transform")
        }
    }
}

CSS Selectors

The DSL allows you to use most CSS selectors. See CssBuilder for more details. Contributions are welcome.

After creating a StyleSheet just go ahead and start using it in a component, it will be injected automatically.

object ComponentStyles : StyleSheet("ComponentStyles") {
    // Example of an ".element:hover" selector
    val element by css {
        backgroundColor = Color.green

        hover {
            backgroundColor = Color.red
        }
    }

    // Example of a ".wrapper *" selector
    val wrapper by css {
        children {
            // CSS properties
        }
    }

    // Example of a ".wrapper div" selector
    val wrapper by css {
        children("div") {
            // CSS properties
        }
    }

    // Example of a ".wrapper:hover .inner" selector
    val wrapper by css {
        // CSS properties
    }

    val inner by css {
        backgroundColor = Color.green

        // Use reflection to refer to other elements, it's longer but safer than using hard-coded class names
        ancestorHover("${ComponentStyles.name}-${ComponentStyles::wrapper.name}") {
            backgroundColor = Color.red
        }
    }
}

fun RBuilder.div() {
    styledDiv {
        css {
            +ComponentStyles.element
        }

        +"An element"
    }

    styledDiv {
        css {
            +ComponentStyles.wrapper
        }

        styledDiv {
            css {
                +ComponentStyles.inner
            }

            +"Inner element"
        }
    }
}

Debugging

By default, kotlin-styled-next uses the CSSOM API for stylesheet creation both in development in production modes, because it's much more performant than setting the text content of a <style> tag. This, however, makes the generated styles harder to debug. To ease debugging, a special API is available in development mode that you can use from the browser console:

StyledNext.downloadCss() // download all CSS generated by the application
StyledNext.getCss() // get list of all CSS rules
StyledNext.downloadCss("some-classname") // download a .css file with rules that contain some-classname
StyledNext.getCss("some-classname") // get list of CSS rules that contain some-classname

Alternatively, you can disable the CSSOM API in development mode altogether by calling GlobalStyles.useDevSheet() in your application code or StyledNext.useDevSheet(isDev: Boolean = true) in the browser console.

Global Styles

To create a global stylesheet use the CssBuilder class and the StyledComponents.injectGlobal() function:

val styles = CssBuilder(allowClasses = false).apply {
    body {
        margin = Margin(0.px)
        padding = Padding(0.px)
    }

    "[draggable=\"true\"]" {
        put("user-select", "none")
    }
}

StyledComponents.injectGlobal(styles)

Defining Static CSS Class Names Based on Enumerations

Typically, styling of a component consists of two parts: a fixed one, and a dynamic one. Imagine styling a button: a CSS rule such as "text-align: center" will be defined for all kinds of buttons, while the text color will depend on the type of the button: colors used for the "primary" and "secondary" buttons will differ. One could write the following code (example was simplified intentionally):

object ButtonStyles : StyleSheet() {
    val button by css {
        textAlign = TextAlign.center
    }
}

private val button = fc<ButtonProps> {
    styledButton {
        css {
            +ButtonStyles.button

            if (props.kind == "primary") {
                color = Color.white
            } else if (props.kind == "secondary") {
                color = Color.blue
            }
        }
    }
}

In this case, three CSS classes will be generated. but only the first one will have a semantic name (ButtonStyles-button), the other two will be auto-generated (ksc-1, ksc-2, etc.)

While this example is purposely naive — it is of course possible to define val primary by css and val secondary by css in the stylesheet – in case of longer enumerations that could prove cumbersome. The dynamicCss delegate comes to the rescue:

private val demoComponent = fc<Props> {
    val (screenSize, setScreenSize) = useState(ScreenSize.fromRawWidth(window.innerWidth))

    useEffectOnce {
        val eventListener: (Event) -> Unit = {
            setScreenSize(ScreenSize.fromRawWidth(window.innerWidth))
        }
        window.addEventListener("resize", eventListener)
        cleanup {
            window.removeEventListener("resize", eventListener)
        }
    }

    styledP {
        css(DemoComponentStyles.demoDynamicStyle(screenSize))

        +"Hi, it seems like I'm running on the ${screenSize.name.lowercase()} screen!"
    }
}

private object DemoComponentStyles : StyleSheet() {
    val demoDynamicStyle by dynamicCss<ScreenSize> {
        fontSize = when (it) {
            ScreenSize.DESKTOP -> 2.rem
            ScreenSize.TABLET  -> 1.5.rem
            ScreenSize.PHONE   -> 1.rem
        }
    }
}

enum class ScreenSize(private val startsFromWidth: Int) : HasCssSuffix {
    DESKTOP(992),
    TABLET(768),
    PHONE(0);

    companion object {
        fun fromRawWidth(rawWidth: Int): ScreenSize {
            for (value in values()) {
                if (rawWidth >= value.startsFromWidth) return value
            }
            return DESKTOP
        }
    }

    override val cssSuffix: String get() = name.lowercase()
}

Note that one doesn't have to use the HasCssSuffix interface, it's only required in complex cases when the name of the CSS property can't be derived easily.

You can use any CssValue, Enum, KProperty, as well as primitive types (Boolean, Number, String) to derive CSS classes.

Packages

styled

styled.dynamicCss