Android | Handling Click Events Kotlin's Way

Exploring Kotlin

Less than 100 pages covering Kotlin syntax and features in straight and to the point explanation.

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Listening to a button clicks is a common task in application development. A listener is set to listen and act upon click events. OnClickListener is an interface used to listen to click events of a button or any other components of type View.

View & Click Listener

  • View is the basic building block for UI components.
  • Views can register or set listeners that will be notified when an event happens to the view.
  • Components of type View can set click listener that get notified on click event.
  • setOnClickListener(OnClickListener l) is used to set the click listener.
  • Click listener is an instance of type OnClickListener.

View.java

 public void setOnClickListener(OnClickListener l) { .... }
  • OnClickListener is an interface with a single abstract method “SAM”.
public interface OnClickListener {
        void onClick(View v);
}
  • So, we need to pass an instance of type OnClickListener to setOnClickListener() function.
  • We can achieve this in the following ways.

( 1 ) The simplest old way, implement OnClickListener interface

  • The simplest way to create an instance of type OnClickListener is by implementing it.
  • Our activity can implement the interface OnClickListener and override onClick().
// 1. extend View.OnClickListener
class MainActivity : AppCompatActivity(), View.OnClickListener {

    private lateinit var textView:TextView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val button = findViewById<Button>(R.id.button)
        textView = findViewById<TextView>(R.id.textView)

        // 2. pass this as instance of OnClickListener
        button.setOnClickListener(this)

    }

    // 3. override onClick()
    override fun onClick(v: View?) {
        textView.setText("Clicked!")
    }
}

( 2 ) Use object expression

  • Use object expression to create an instance from an anonymous class implementing OnClickListener.
class MainActivity : AppCompatActivity() {

    private lateinit var textView:TextView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val button = findViewById<Button>(R.id.button)
        textView = findViewById<TextView>(R.id.textView)

        // create an instance from anonymous class
        button.setOnClickListener{
            object: View.OnClickListener{
                override fun onClick(v: View?) {
                    textView.setText("Clicked!")
                }

            }
        }

    }
}

( 3 ) Lambda

  • Since OnClickListener is an interface with a single abstract function, we can use lambda.
  • The onClick() function takes a parameter of type View and returns void or Unit for Kotlin.
  • So, the function type of the lambda should be (View) -> Unit
class MainActivity : AppCompatActivity() {

    private lateinit var textView:TextView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val button = findViewById<Button>(R.id.button)
        textView = findViewById<TextView>(R.id.textView)

        // create an instance from anonymous class
       button.setOnClickListener{ v -> textView.setText("Clicked!") }

    }
}
  • Or, even simpler
    button.setOnClickListener{ textView.setText("Clicked!") }
    
Exploring Kotlin

Less than 100 pages covering Kotlin syntax and features in straight and to the point explanation.