Delegation with by in Kotlin

Forward a whole interface to a wrapped instance in one clause.

interface Logger {
    fun log(line: String)
}

class TrailLog : Logger {
    override fun log(line: String) = println("[log] $line")
}

// by hands every Logger call to the wrapped instance -- no boilerplate.
class GuidedTrip(logbook: Logger) : Logger by logbook {
    fun depart(trail: String) = log("Departing $trail")
}

fun main() {
    val trip = GuidedTrip(TrailLog())
    trip.depart("Ridgeline")
    trip.log("All hikers accounted for")
}

How it works

1. TrailLog implements Logger the ordinary way.
2. class GuidedTrip(logbook: Logger) : Logger by logbook writes the forwarding for you.
3. The class adds depart on top and still answers every Logger call.

Keywords and builtins used here

• GuidedTrip
• Logger
• String
• TrailLog
• by
• class
• depart
• fun
• interface
• log
• main
• override
• val

The run, in numbers

Lines

18

Characters to type

423

Tokens

95

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 88 seconds.

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Step 2 of 3 in Generics & delegation, step 23 of 27 in Language basics.

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