How to use kotlin functional programming to access a property - unit-testing

Question

I currently have the following function:
fun createMask(mask : String){
val ssnField : mywidgets.SSNField = findViewById (R.id.editTextText)
ssnField.hint = mask
}
To unit test this I want to wrap the untestable code within createMask into a closure. (The untestable code is the view layer logic that's difficult to instantiate and execute in a unit test.) Here is what I want to do in pseudo code:
createMask(closure, mask : String){
closure = mask // closure function returns pointer to property (depending on closure return type, might need to use setter: closure.set(mask))
}
With the above, the caller then does:
fun caller(){
createMask((){
val ssnField : mywidgets.SSNField = findViewById (R.id.editTextText)
return ssnField.hint
}, "xxx-xx-xxx")
}
How do do what is expressed in pseudo code work in kotlin?

Answer 1

You can return a reference of the property if you make createMask accept a parameter of type () -> KMutableProperty0<String>. Then you can call the set method:
fun createMask(mask : String, block: () -> KMutableProperty0<String>) {
block().set(mask)
}
// caller
createMask("xxx-xx-xxx") {
val ssnField = ...
ssnField::hint
}
Alternatively, use (String) -> Unit to represent "any function that takes a string", if you want to allow callers to pass any function that has the "form" of a setter.
fun createMask(mask : String, block: () -> (String) -> Unit) {
block()(mask)
}
// caller
createMask("xxx-xx-xxx") {
val ssnField = ...
ssnField::hint.setter
}
Note that this method involves reflection, which may not be desirable. Alternatively, you can accept a closure that takes the string to be set, and let the caller set it in the closure:
fun createMask(mask: String, block: (String) -> Unit) {
block(mask)
}
// caller
createMask("xxx-xx-xxx") {
val ssnField = ...
// note that rather than responsible for returning a property, the caller
// is responsible for setting "it" to the property
ssnField.hint = it
}
(I'm assuming createMask does more than just setting a property. Otherwise it is quite pointless...)