CODEWITHSUNDEEP
kotlin
user1639348
user1639348

Reputation: 119

How to use typealias of function type?

how can I use this typalias?

typealias MyHandler = (Int, String) -> Unit

I tried this but it doesn't work:

var myHandler = MyHandler(1, "2")

Upvotes: 1

Views: 3356

Answers (3)

vagdevi k
vagdevi k

Reputation: 1685

Adding to theThapa and Fred, function typealiases are a way to declare the type of a function. It can be later used.

For example, the following shows a good example of how to declare it and use it:

import kotlin.test.*
import java.util.*

//define alias
typealias MyHandler = (Int, Int) -> Int

fun main(args: Array<String>) {


//define the function
val myHandler: MyHandler = {intValue, bValue -> 
intValue + bValue
}

// class Foo takes MyHandler type as parameter to instantiate
class Foo(val action: MyHandler) {

  val stateOne: Boolean = false

  fun bar() {

    if (stateOne) {
      println(action.invoke(1, 45))
    } else {
      println(action.invoke(0, 65))
    }
  }
}

// instantiate Foo class along with constructor whose parameter is of type MyHandler
val f = Foo(myHandler)
f.bar()
}

Upvotes: 0

Fred
Fred

Reputation: 17095

typealias are just an alias for the type :) in other words, it's just another name for the type.

Imagine having to write all the time (Int, String) -> Unit. With typealias you can define something like you did to help out and write less,i.e. instead of:

fun Foo(handler: (Int, String) -> Unit)

You can write:

fun Foo(handler: MyHandler)

They also help giving hints, meaning they can give you a way to describe types in a more contextualized way. Imagine implementing an app where in it's entire domain time is represented as an Int. One approach we could follow is defining:

typealias Time = Int

From there on, every time you want to code something specifically with time, instead of using Int you can provide more context to others by using Time. This is not a new type, it's just another name for an Int, so therefore everything that works with integers works with it too.

There's more if you want to have a look

Upvotes: 2

theThapa
theThapa

Reputation: 631

You are trying to instantiate a typealias and are getting interface doesn't have a constructor error. To my understanding, typealias with function types work with three steps:

  1. Define the typealias itself

    typealias MyHandler = (Int, String) -> Unit

  2. declare an action of that type

    val myHandler: MyHandler = {intValue, stringValue -> 
 // do something
}

  3. use that action, e.g.

    class Foo(val action: MyHandler) {

  val stateOne: Boolean = false

  // ...

  fun bar() {

    if (stateOne) {
      action.invoke(1, "One")
    } else {
      action.invoke(0, "notOne")
    }
  }
}

Upvotes: 4

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