How do I test generators for delegated properties?

In my current project there is a class that will later on be implemented by many others. This class provides some generators for delegated properties.

abstract class BaseClass {
    protected val delegated1 get() = new Delegated1Impl()
    protected val delegated2 get() = new Delegated2Impl()
    ...
}

This base class can be used this way:

class Example : BaseClass() {
    var field1 by delegated1
    var field2 by delegated2
}

Now I want to test these delegated generators. Some of them contain logic which I want to test, but for now I only want to know that everytime they are called they return a new instance.

Now my question is: how can I test these generators?
The generators are not visible outside of extending classes so I cannot simply create an instance of it and call these methods.

@Test
fun `delegated1 should always return a new instance`() {
    val target = object: BaseClass()

    val first = target.delegated1    // This does not work since it is protected
    val second = target.delegated1

    assertTrue(first !== second)
}

Solution

You need a new object created whenever you "call" the get method. So how to test it? With a provider

A Provider<T> is just an object that provides you new instances of a concrete class. Its signature is something like this:

interface Provider<T> {
    fun get() : T
}

So you need to inject a new Provider<T> into your BaseClass:

abstract class BaseClass(
        private val implementation1Provider : Provider<YourInterface>,
        private val implementation2Provider : Provider<YourInterface>) {
    protected val delegated1 get() = implementation1Provider.get()
    protected val delegated2 get() = implementation2Provider.get()
    ...
}

Now you can inject your custom providers in the test and assert that they have been called:

@Test
fun `delegated1 should always return a new instance`() {
    val implementation1Provider = ...
    val target = Example(implementation1Provider, ...)

    val first = target.field1

    // assert that implementation1Provider.get() has been called
}