Cannot Capture Mutable List Mockk

January 29, 2022

Kotlin error when compiling such code mentioned in https://mockk.io/#capturing. What’s wrong actually?

class Foo{
   fun fx(parm: MutableList<Double>) {}
}

val foo = Foo()
val parm: MutableList<Double> = mutableListOf()

every { foo.fx(capture(parm)) }

Error written below

Kotlin: Type mismatch: inferred type is Double but MutableList<Double> was expected

>Solution :

The capture function expects either a Slot – when you want to capture the parameter of a single invocation – or MutableList – when attempting to capture the parameters of multiple invocations. Both of type T, where T is the type of the parameter. This has to be used in conjunction with verify, not with every.

So you have to use either of the two mechanisms to capture the call arguments.

val param = slot<MutableList<Double>>()
verify { foo.fx(capture(param)) }

// or
val params = mutableListOf<MutableList<Double>>()
verify { foo.fx(capture(params)) }

Also, this only works with a mocked object.
So instead of creating an actual object of type Foo, you would need to create a mock of it.

// set up the mock
val foo = mockk<foo>()
every { foo.func(any()) } just runs

// call the mocked function, most likely you want to do this indirectly instead
foo.fx(mutableListOf())

// assert that the function has been called
val slot = slot<MutableList<Double>>()
verify { foo.fx(capture(slot)) }
val param = slot.captuted

// assert what the function actually has been called with - this example uses "Hamkrest"
assertThat(param, hasSize(equalTo(0)))