Java – unit test of kotlin lambda callback

Suppose we have the following functions to test

fun loadData(dataId: Long,completion: (JsonElement?,Exception?) -> Unit) {
    underlayingApi.post(url = "some/rest/url",completion = { rawResult,exception ->
                val processedResult = processjson(rawResult)
                completion(processedResult,exception)
            })
}

I know how to simulate, inject, stub and validate calls to underlaying API

How to verify the result (processed result, exception) returned through completion?

Solution

To test Lambdas behavior, you must simulate the underlaying API and call lambda. Com through an invoactiononmock object like this

`when`(underlayingApi.post(eq("some/rest/url"),any())).thenAnswer {
        val argument = it.arguments[1]
        val completion = argument as ((rawResult: String?,exception: Exception?) -> Unit)
        completion.invoke("result",null)
    }

This causes a callback to be called within the object under test Now check that the callback of the tested object works properly and verify that it works properly

objUnderTest.loadData(id,{ json,exception ->
                              assert....
                          })