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Type-Safe Builders in Kotlin

Type-safe tree structure builder​

Builders can be defined as a set of extension functions taking lambda expressions with receivers as arguments. In this example, a menu of a JFrame is being built:

import javax.swing.*

fun JFrame.menuBar(init: JMenuBar.() -> Unit) {
    val menuBar = JMenuBar()
    menuBar.init()
    setJMenuBar(menuBar)
}

fun JMenuBar.menu(caption: String, init: JMenu.() -> Unit) {
    val menu = JMenu(caption)
    menu.init()
    add(menu)
}

fun JMenu.menuItem(caption: String, init: JMenuItem.() -> Unit) {
    val menuItem = JMenuItem(caption)
    menuItem.init()
    add(menuItem)
}

These functions can then be used to build a tree structure of objects in an easy way:

class MyFrame : JFrame() {
    init {
        menuBar {
            menu("Menu1") {
                menuItem("Item1") {
                    // Initialize MenuItem with some Action
                }
                menuItem("Item2") {}
            }
            menu("Menu2") {
                menuItem("Item3") {}
                menuItem("Item4") {}
            }
        }
    }
}

Remarks​

A type-safe builder is a concept, rather than a language feature, so it is not strictly formalized.

A typical structure of a type-safe builder​

A single builder function usually consists of 3 steps:

  1. Create an object.
  2. Execute lambda to initialize the object.
  3. Add the object to structure or return it.

Type-safe builders in Kotlin libraries​

The concept of type-safe builders is widely used in some Kotlin libraries and frameworks, eg.:

  • Anko
  • Wasabi
  • Ktor
  • Spec