How Kotlin Higher-Order Functions work?

Question

I'm struggling a bit to understand Higher-Order Functions and how to pass functions as parameters to other functions using Kotlin. I have a basic example that I want to fufill:

fun addOnSearchGameResultListener(
            activity: AppCompatActivity,
            releaseThread: () -> Unit,
            showNoResultsFoundMessage: () -> Unit,
            updateSearchResults: (result: List<Game>) -> Unit) {
        var event0017Handler: TaskExecutor = object : TaskExecutor {
            override fun executeOnSuccessTask(response: JSONObject) {
                async() {
                    uiThread {
                        try {
                            releaseThread()
                            mLoaderManager.hideIndeterminateProgressBar(activity)
                            val result = mJSONParser.getGamesByGameKey(response)
                            Log.i(GameController::class.simpleName, "response: ${result.toString()}")
                            updateSearchResults(result)
                        } catch (e: JSONException) {
                            showNoResultsFoundMessage()
                        }
                    }
                }
            }

            override fun executeOnErrorTask(payload: JSONObject) {
                releaseThread()
                mNotificationManager.showErrorPopUp(activity, payload.getString("data"))
            }
        }
        NotificationCenter.RegistrationCenter.registerForEvent(EventCatalog.e0017, event0017Handler)
    }

I'm calling the method above this way:

mGameService.addOnSearchGameResultListener(
            this,
            releaseThread(),
            showNoResultsFoundMessage(),
            updateSearchResults(null)
    )

And updateSearchResults(null) is declared as:

private fun updateSearchResults (results : List<Game>?) : (results : List<Game>?) -> Unit = {
        if (null != results && results.size > 0) {
            mLastMatchingQuery = query_container.text.toString()
            hideNoResultsFoundMessage()
            mGames = results
            mAdapter!!.dataSet = results.toMutableList()
        } else {
            showNoResultsFoundMessage()
        }
    }

I know I passed null to the func when I declared it ('cause I need to pass something at compilation time), however, the call made from inside addOnSearchGameResultListener() is not made passing the parameter from runtime, I mean, in addOnSearchGameResultListener() I always get null for results. How exactly this works and what am I doing wrong?

Answer 1

I think the confusion comes from parameter names, results in particular. To resolve that you can change the updateSearchResults to i.e.:

private fun updateSearchResults() : (List<Game>?) -> Unit = { results ->
    if (null != results && results.size > 0) {
        mLastMatchingQuery = query_container.text.toString()
        hideNoResultsFoundMessage()
        mGames = results
        mAdapter!!.dataSet = results.toMutableList()
    } else {
        showNoResultsFoundMessage()
    }
}

However I do feel that it would easier to follow the code if you'd apply following changes:

• make updateSearchResults regular method:

  private fun updateSearchResults (results : List<Game>?) {
      if (null != results && results.size > 0) {
          mLastMatchingQuery = query_container.text.toString()
          hideNoResultsFoundMessage()
          mGames = results
          mAdapter!!.dataSet = results.toMutableList()
      } else {
          showNoResultsFoundMessage()
      }
  }
  

• change the addOnSearchGameResultListener invocation and pass a lambda

  mGameService.addOnSearchGameResultListener(
          this,
          releaseThread(),
          showNoResultsFoundMessage(),
          { updateSearchResults(it) }
  )
  

• apply similar changes to releaseThread, showNoResultsFoundMessage