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

Question

In my case, I want to change the 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 } 

it would be nice if I had an extension function, for example:

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

Is it possible?

Answer 1

The solution that will work for properties is to write an extension function for mutable properties, and then use it with a reference to the property. This code works with Kotlin 1.1:

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

Using:

 class MyClass { private var firstTime = true fun f() { if (this::firstTime.checkAndUnset()) { println("First time") } } } fun main(args: Array<String>) { val c = MyClass() cf() // First time cf() // } 

_{(check out the runnable demo)}

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

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In Kotlin 1.0.x, related called links are not yet available, and the above solution can be rewritten to this somewhat more awkward 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 the 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 use is as simple as

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