Instantiating an Interface Listener in Kotlin

Question

I cannot, for the life of me, instantiate an interface outside of a fragment in Kotlin or Kotlin for Android. It was standard procedure in Java to say something like:

MyInterface mInterfaceListener = new MyInterface(this);
mInterfaceListener.invokeSomeGenericMethod();

Note that mInterfaceListener is referring to an Interface, not an onCLickListener or anything like that

How are interfaces instantiated in Kotlin? How do I make a "listener" and trigger an interface's functions?

Below are some attempts in a very simple app I am doing for learning purposes. Notice the variable mPresenterListener which is an Interface

    class QuoteActivity : QuoteContract.ViewOps, AppCompatActivity() {

    private lateinit var vText: TextView
    private lateinit var vFab: FloatingActionButton
    private lateinit var mPresenterListener: QuoteContract.PresenterOperations

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        mPresenterListener = this.mPresenterListener
        vText=findViewById(R.id.main_quote)
        vFab=findViewById(R.id.main_fab)
        vFab.setOnClickListener{
            mPresenterListener.onQuoteRequested()
        }
    }

    override fun displayQuote(quote: String) {
        vText.text = quote
    }

}

And my presenter:

    class QuotePresenter(private val viewListener: QuoteContract.ViewOps): QuoteContract.PresenterOperations {

    private lateinit var modelListener: QuoteContract.ModelOperations

    init {
        modelListener = this.modelListener
    }


    override fun onQuoteRequested() {
        modelListener.generateQuote()
    }

    override fun onQuoteGenerated(quote: String) {
        viewListener.displayQuote(quote)
    }


}

The interface:

interface QuoteContract {



//Methods available to Presenter (Presenter -> View)
interface ViewOps{
    fun displayQuote(quote: String)
}


//Ops offered from presenter to view (Presenter->View)

interface PresenterOperations {
    //Presenter->View
    fun onQuoteRequested()
    //Presenter->Model
    fun onQuoteGenerated(quote: String)
}

//Ops offered from Model to Presenter (Model -> Presenter)
interface ModelOperations {
    fun generateQuote()
}

}

Solution 1

You can do watchers/listeners like this:

val textView: TextView = this.findViewById(R.id.amountEdit)
val watcher = object : TextWatcher {
    override fun afterTextChanged(p0: Editable?) {
        val inputAmount = textView.text.toString
        val amount = if (!inputAmount.isEmpty()) inputAmount.toDouble() else 0.0
        conversionViewModel?.convert(amount)
        }

    override fun beforeTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) {
        println("before text changed called..")
    }

    override fun onTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) {
        println("text changed..")
    }
}

and then:

textView.addTextChangedListener(watcher)

Notice the object in declaring the watcher. Can do the same with a listener.

Solution 2

Also we can use listener without some interface and with default value like:

val someButtonListener: (isChecked: Boolean) -> Unit = {_ -> }
val someButtonListener: (v: View) -> Unit = {_ -> }

Comments

[Josh Ribeiro]

Ok, so just to wrap my head around your method: Textwatcher is an interface and you are assigning it to a singleton? How would this work in a typical MVP scenario? Thank you for your answer

[Rob]

Why is it a singleton? it's an instance. This is one of the many great things about Kotlin: you can make instances out of interfaces, as objects.

[Josh Ribeiro]

The mini-course I had taken defined the Singleton in Kotlin as an Object which can only have one instance of itself. I see Singleton sometimes in dependency injection, so I am not sure if there is some vocabulary overlap. Anyway, thank you for following up with my "answer". Yours is the answer, I just got lost with all the TextView shinnanigans and I wasn't sure if TextWatcher was an interface or some Android library class. Regardless, you helped me out and answered the question: assign it to an object. Much much easier than my solution. Thank you

[Rob]

Sure, glad it helped @JoshRibeiro, good on you for wanting to learn the right way to do things. This question caught my eye because when I was learning the new Android Blueprints I had the exact question of how best to adapt the Java approach.