Hey! New to Kotlin and this group and struggling t...

# announcements

[Tomaz Kragelj]

Hey! New to Kotlin and this group and struggling to get an answer for how to use callback in the form of optional property. For example:

var onDownloadCompleted: ((data: ByteArray) -> Unit)? = null

Coming from Swift, I tried calling it using ? mark but it yields compiler error “(Error:(108, 26) Unexpected tokens (use ‘;’ to separate expressions on the same line)“:

onDownloadCompleted?(bytes)

So currently I’m using longer form which works:

if (onDownloadCompleted != null) {
    onDownloadCompleted!!(bytes)
}

But it feels so un-DRY for such a simple task, is there a simpler way? Or am I thinking in wrong direction and should use different callback approach?

[everald]

You could try using the invoke eg.

onDownloadCompleted?.invoke(bytes)

[Tomaz Kragelj]

Allright, this works! Thanks!!!

[marstran]

You shouldn't need !! when you do the null-check btw. The variable gets smart casted to the non-nullable type.

[kirillrakhman]

not if it's a var property

[marstran]

Oh, true

[radityagumay]

Remove .invoke you dont need extra code.

[marstran]

@radityagumay invoke is the whole point with the answer. You can't do onDownloadCompleted?(bytes).

[radityagumay]

Ah, yes. Because this expect nullable.