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In Android, every tutorial teaching you the basics describe how to design screen through XML files. It’s also possible to achieve the same result with Java (or any JVM-based language). Android screen design is not the only domain where XML and Java are valid options. For example, Spring configuration and Vaadin screen design allow both. In all those domains, there’s however a trade-off involved: on one hand, XML has a quite rigid structure enforced by an XML-schema while Java gives power to do pretty well anything at the cost of readability. "With great power comes great responsibility". In the latter case, it’s up to the individual developer to exercise his/her judgement in order to keep the code as readable as possible.

Domain-Specific Languages sit between those two ends of the spectrum, as they offer a structured syntax close to the problem at hand but with the full support of the underlying language constructs when necessary. As an example, the AssertJ library provides a DSL for assertions, using a fluent API. The following snippet is taken from its documentation:

// entry point for all assertThat methods and utility methods (e.g. entry)
import static org.assertj.core.api.Assertions.*;

// collection specific assertions (there are plenty more)
// in the examples below fellowshipOfTheRing is a List<TolkienCharacter>
assertThat(fellowshipOfTheRing).hasSize(9)
                               .contains(frodo, sam)
                               .doesNotContain(sauron);

DSLs can be provided in any language, even if some feel more natural. Scala naturally comes to mind, but Kotlin is also quite a great match. Getting back to Android, Jetbrains provides the excellent Anko library to design Android screen using a Kotlin-based DSL. The following snippet highlights Anko (taken from the doc):

verticalLayout {
    val name = editText()
    button("Say Hello") {
        onClick { toast("Hello, ${name.text}!") }
    }
}

There are two Kotlin language constructs required for that.

Extension functions

I already wrote about extension functions, so I’ll be pretty quick about it. In essence, extension functions are a way to add behavior to an existing type.

For example, String has methods to set in lower/upper case but nothing to capitalize. With extension functions, it’s quite easy to fill the gap:

fun String.toCapitalCase() = when {
    length < 2 -> toUpperCase()
    else -> this[0].toUpperCase() + substring(1).toLowerCase()
}

At this point, usage is straightforward: "foo".toCapitalCase().

The important point is the usage of this in the above snippet: it refers to the actual string instance.

Function types

In Kotlin, function are types. Among all consequences, this means functions can be passed as function parameters.

fun doSomethingWithInt(value: Int, f: (Int) -> Unit) {
    value.apply(f);
}

The above function can be now passed any function that takes an Int as a parameter and returns Unit e.g. { print(it) }. It’s called like that:

doSomethingWithInt(5, { print(it) })

Now, Kotlin offers syntactic sugar when calling methods: if the lambda is the last parameter in a function call, it can be separated from the other arguments like this:

doSomethingWithInt(5) { print(it) }

Putting it all together

Getting back to the Anko snippet above, let’s check how the verticalLayout { …​ } method is defined:

inline fun ViewManager.verticalLayout(theme: Int = 0, init: _LinearLayout.() -> Unit): LinearLayout {
    return ankoView(`$$Anko$Factories$CustomViews`.VERTICAL_LAYOUT_FACTORY, theme, init)
}

As seen in the first paragraph, the init parameter is an extension function defined on the _LinearLayout type. this used in its context will refer to the instance of this latter type.

The second paragraph explained what represents the content of the braces: the init parameter function.

Conclusion

Hopefully, this post show how easy it is to create DSL with Kotlin thanks to its syntax.

I developed one such DSL to create GUI for the Vaadin framework: the name is Kaadin and the result is available online.