Store lambda in a variable in kotlin

Question

I'm starting developing in Android with kotlin and I have a problem with lambdas. I have a function to set a listener in my view, this looks like this:

fun setListener(listener: () -> Unit) {
}

The problem is that the code passed as lambda won't be executed in setListener function, it will be executed in another part of my code (specifically when an item of a spinner is selected) so I have to "save" or "store" this lambda into a variable/property so that I'm able to execute it when needed. Any idea about how to do it?

Edit: I've achieved it by doing:

private var listener: (() -> Unit)? = null

fun setListener(listener: () -> Unit) {
    this.listener = listener
}

Is there a better way to do it? Thanks

Bruch

Answer 1

Here's how you can do it:

class Foo {
    private var listener: () -> Unit = {}
    fun setListener(listener: () -> Unit) {
        this.listener = listener
    }
}

However, manually writing setters is discouraged in Kotlin. Instead, you can just make your property public:

class Foo {
    var listener: () -> Unit = {}
}

For reference, here are the docs about properties with lots of examples.

Amphitropous

"Getter visibility must be the same as property visibility" (i.e. you cannot have public set with private get). I suggest var listener: () -> Unit = {}. – Wade

Answer 2

You can store a function in a property easily. The simplest way:

var listener: (() -> Unit)? = null

Usage:

foo.listener = { println("called") }

If you want your property to be set-only, you can create one public property with unusable getter and one private property for internal use. Full example:

class Example {

    // for internal use
    private var _listener: (() -> Unit)? = null

    // public set-only
    var listener: (() -> Unit)?
        @Deprecated(message = "set-only", level = DeprecationLevel.ERROR)
        get() = throw AssertionError() // unusable getter
        set(value) { _listener = value } // write-through setter

    fun somethingHappend() {
        _listener?.invoke()
    }
}

Naughty

With this code I get an error also when trying to use _listener inside my class – Bruch

@Bruch I've augumented & clarified my answer. If you're still getting error, post error message. – Naughty

You could just make the getter private with private get() = .... Then you don't even need the _listener property. – Randers

@Randers you can't set setter vilibility lower than getter visibility. – Naughty

Answer 3

Here is a simple example.

fun main(){
    val result=::sum //function assign to variable (first method)
    val result1: (first: Int, second: Int) -> Int=::sum //function assign to variable (second method)
    print(result(5,6))

}
fun  sum(first:Int,second:Int):Int{
    return first+second;
}

Photothermic

This answer should be marked as correct solution : the =:: is the key – Duet