What does "activity?." mean in Kotlin?

Issue

I am trying to use a code snippet from the official android documentation (https://developer.android.com/training/printing/photos#kotlin) which is the doPhotoPrint() function in the code that I have attached, in order to learn how to use the PrintHelper class in Kotlin and Android Studio. See the attached image of the the code snippet:

The problem is that when I put the code in Main Activity in my test app, it keeps showing "activity?." as red. Why is this the case and how can I get the code to work so that is provides the user with the option to print? Thanks

Solution

The code you linked is just a general how to use a library function snippet - it's not put in any context, but we can assume it's probably written with a Fragment in mind. Fragments have a getActivity() method that returns the Activity it's currently attached to (or null if it's not)

Kotlin allows you to access getter and setter functions from Java code as if they were properties - so basically, instead of having to do value = getThing() and setThing(newValue) you can treat it like a variable: value = thing, thing = newValue etc. So when you access the activity property in this code, it's really calling getActivity(). And because the result can be null, you use the ? to null-check it before trying to do anything with it.

Really what that snippet is saying is, this code needs access to a Context, and it's using an Activity as one in the example. If you have a Fragment, you can just drop this code right in. If you're in an Activity though, then you obviously don't need to get one - this is your Activity! And you don't need to null-check it either of course. (And there's no activity property or getActivity() method to access, which is why it's showing up red and unrecognised)

So you can just replace that stuff and the checking code around it with:

private fun doPhotoPrint() {
    // using 'this' to pass the current activity as the context
    val printHelper = PrintHelper(this).apply {
        scaleMode = PrintHelper.SCALE_MODE_FIT
    }
    val bitmap = BitmapFactory.decodeResource(resources, R.drawable.droids)
    printHelper.printBitmap("droids.jpg - test print", bitmap)
}

Answered By - cactustictacs