What different this::functionA between functionA() as params in kotlin

Question

I have two extension method

Function A is:

fun Activity.postDelay1s(run: () -> Unit) {
Handler().postDelayed({ run() }, 1000)
}

Function B is:

fun CustomRefreshLayout.setUp(containerRecycleView: RecyclerView, onResfresh: () -> Unit) {
with(this) {
    recyclerView = containerRecycleView
    setColorSchemeColors(Color.BLUE,
            Color.GREEN,
            Color.YELLOW,
            Color.RED)
    setDistanceToTriggerSync(300)
    setProgressBackgroundColorSchemeColor(Color.WHITE)
    setSize(SwipeRefreshLayout.LARGE)
    setOnRefreshListener { onResfresh() }
}

}

When I call function a I use this way postDelay1s { initData() }, but call b I need use this way swipe.setUp(trade_list, this::initData).

I want to kown what different with initData() between this::initDatain this two extension function

s1m0nw1 · Answer 1 · 2018-02-01T09:45:16.240

You can call b with a function reference as you already did (::initData) and also with a lambda like in a:

swipe.setUp(trade_list){
    initData()
}

It’s also possible to pass the lambda inside the parentheses which isn’t recommended in most cases:

swipe.setUp(trade_list, { initData() })

Documentation says:

In Kotlin, there is a convention that if the last parameter to a function is a function, and you're passing a lambda expression as the corresponding argument, you can specify it outside of parentheses

Also read this thread.

score 2 · Answer 2 · answered Feb 01 '18 at 09:44

Your functions take the same type in their parameters (() -> Unit), so you can use call both of them in either way.

Let's see what the first syntax does:

postDelay1s { initData() }

Here you're creating a new lambda (anonymous function), whose body consists of only a single call to the initData function. You're basically introducing an additional level of redirection into your code.

With the second syntax:

postDelay1s(this::initData)

You're passing in a reference to your initData method. This way, the initData method takes over the role that the lambda played in the previous version - the postDelay1s function will call this method directly, without the need for the additional step.

Both of these syntaxes can be used with your setUp method as well.

What different this::functionA between functionA() as params in kotlin

2 Answers2