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Function Literals with Receiver

September 16, 2024

Kotlin

Contents

1. Introduction of Function Literals

  • Function literals are frequently used as a constructor of objects.

  • Well known examples are build.gradle.kts and jetpack-compose for Andriod or Desktop UIs.

In the sequel let's consider the following builder function, a function literal takes the form

fun builder(dummyName: ClassA.() -> Unit) {
    ...
}

which means that:

  1. We define a lambda function that can be treated as a method of ClassA
  2. This lambda function takes the signature () -> Unit.
  3. To call this lambda function, we invoke classA.dummyName().
  4. builder orchestrates the execution of the above defined operations.

2. Example

class Properties {
    var time: String? = null
}

class BuildInfo {
    private var properties: Properties? = null

    fun withProperties(configure: Properties.() -> Unit) {
        val _properties = Properties()
        _properties.configure()
        properties = _properties
    }
}

class SpringBootExtension {
    private var buildInfo: BuildInfo? = null

    fun withBuildInfo(configure: BuildInfo.() -> Unit) {
        val _buildInfo = BuildInfo()
        _buildInfo.configure()
        this.buildInfo = _buildInfo
    }
}

Let's create a builder function which builds a SpringBootExtension object.

// builder function
fun springBootEextension(build: SpringBootExtension.() -> Unit): SpringBootExtension {
    val extension = SpringBootExtension()
    extension.build()
    return extension
}

Which means that

  1. springBootEextension accepts a lambda function which is defined as if we are inside of the class definition of SpringBootExtension,
  2. The name build is in fact dummy and can be anything we want, it is just a name to invoke the execution of the lambda.
fun createExtension() {
    val extension = springBootEextension { // trailing closure without input params
        withBuildInfo { // it is a method inside of SpringBootEextension
                        // it is accessible by the closure because our context is now SpringBootEextension
            withProperties { // accepts another closure for configuration
                time = null  // context is now Property class, can make changes to the created Property object
            }
        }
    }
}

  • In short, ClassA.() -> Unit can be read as configuration of ClassA in many cases (and bear in mind that it is a lambda function defined inside of ClassA).

  • In terms of this terminology:

    • springBootEextension(_: SpringBootExtension.() -> Unit) accepts a configuration of SpringBootExtension
    • withBuildInfo(_: BuildInfo.() -> Unit) accepts a configuration of BuildInfo
    • withProperties(_: Properties.() -> Unit) accepts a configuration of Properties