Is there a way to write an extension function that changes the value of the object?

Question

In my case I want to change a primitive - Boolean

I never liked the following type of code:

private var firstTime: Boolean = true
...

    if (firstTime) {
        // do something for the first time here
        firstTime = false
    }

would be nice if I could have an extension function like:

if (firstTime.checkAndUnset()) {
    // do something for the first time here
}

Is this possible?

Answer 1

A solution that would work for properties is to write an extension function for mutable properties and then use it with the property reference. This code work with Kotlin 1.1:

fun KMutableProperty0<Boolean>.checkAndUnset(): Boolean {
    val result = get()
    set(false)
    return result
}

Usage:

class MyClass {
    private var firstTime = true

    fun f() {
        if (this::firstTime.checkAndUnset()) {
            println("First time")
        }
    }
}

fun main(args: Array<String>) {
    val c = MyClass()
    c.f() // First time
    c.f() //
}

_{(check the runnable demo)}

This, however, will not work for local variables (at least not in Kotlin 1.1).

---

In Kotlin 1.0.x, bound callable references are not yet available, and the solution above can be rewritten to this somewhat clumsier code:

fun <T> KMutableProperty1<T, Boolean>.checkAndUnset(receiver: T): Boolean {
    val result = get(receiver)
    set(receiver, false)
    return result
}

MyClass::firstTime.checkAndUnset(this)

Answer 2

Just create following property

private var firstTime: Boolean = true
    get() {
        return if (field) { //Note: field - is keyword in current context
            field = false 
            true
        } else false 
    }
    set(v) { field = v }

And usage as simple as

if (firstTime) {
    //It's first time
    //firstTime is false now
}