Kotlin高阶函数如何工作？

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

Frankly speaking, I am not entirely sure what your code was to achieve, but let me clarify what your snippet is doing at least:

private fun updateSearchResults(results : List<Game>?):
         (foo: List<Game>?) -> Unit = { parameter: List<Game>? ->

    if (null != results && results.size > 0) {
        // code
        Unit
    } else {
        // code
        Unit
    }
}

Here you have a function updateSearchResults which accepts a parameter results and returns a function of type (foo: List<Game>?) -> Unit . Note that I renamed some things to avoid name clashes and clarify what is what. The naming foo hast no effect what so ever, I am not sure why you are allowed to write it. The returning lambda has one parameter parameter of type List<Game>? , which you completely ignore in your code. Over all, the result of the if depends exclusively on the parameter of updateSearchResults .

Answer 2

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

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

Answer 3

There is an excellent article created by Juan Ignacio Saravia that talk about Higher-Order Functions

I'll try to summarize here:

A higher-order function is a function that takes functions as parameters, or returns a function.

Pass a function as parameter

fun logExecution(func: () -> Unit) {
    Log.d("tag", "before executing func")
    func()
    Log.d("tag", "after executing func")
}

This function “logExecution” allows you to pass a function as parameter and log before and after the execution of this function.

func: ()-> Unit

Here “func” is the name of the parameter and “() -> Unit” is the “type” of the parameter, in this case, we are saying that func will be a function that doesn't receive any parameter and doesn't return any value (remember that Unit works like void in Java).

You can call this function by passing a lambda expression that must not receive or return any value, like in this way:

logExecution( { Log.d("tag", "I'm a function") } )

but also Kotlin allows you to remove the parenthesis if there is only one function parameter or if the last parameter is a function:

logExecution { Log.d("tag", "I'm a function") }

Receive another parameter

We can change the logExecution signature to receive another parameter and then put the function parameter at the end like this:

// added tag parameter:
fun logExecution(tag: String, func: () -> Unit) { ... }
// call in this way:
logExecution("tag") { Log.d("tag", "I'm a function") }

or:

logExecution("tag") {
    Log.d("tag", "I'm a function")
}

Make the function receive and return values

fun logExecution(func: (String, String) -> Int) {
    val thisIsAnInt = func("Hello", "World")
}

Async function example

This is a function that receives a function and execute it in another thread:

fun runAsync(func: () -> Unit) {
    Thread(Runnable { func() }).start()
}

and we can execute a function outside of the Main UI Thread easily:

runAsync {
    // i.e.: save something in the Database
}

Maybe you want to run some specific code for Lollipop devices and instead of doing the regular if check, you can use this function:

fun isLollipopOrAbove(func: () -> Unit) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
        func()
    }
}

and use it in this way:

isLollipopOrAbove {
    // run lollipop specific code safely
}

I hope with this, it has become a little clearer about Higher-Order Functions

Answer 4

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

There is no passing at runtime or compile time. If you use the function only once like updateSearchResults(null) , the if is always false, and the whole thing is equivalent to { showNoResultsFoundMessage() }