Kotlin - How to pass a Runnable as this in Handler

Question

I'm beginner in kotlin. I try to create a task that will repeat every 2 seconds. So I created something like this.

val handler = Handler()
    handler.postDelayed(Runnable {
        // TODO - Here is my logic

        // Repeat again after 2 seconds
        handler.postDelayed(this, 2000)
    }, 2000)

But in postDelayed(this) it gives error - required Runnable!, found MainActivity. I've tried even this@Runnable but it didn't work.

But when I write the same function like this, it works

val handler = Handler()
    handler.postDelayed(object : Runnable {
        override fun run() {
            // TODO - Here is my logic

            // Repeat again after 2 seconds
            handler.postDelayed(this, 2000)
        }
    }, 2000)

So why the this keyword doesn't work in first function, but in second function it works good?

Answer 1

You have several options to go about here:

1. make both the runnable and the handler be in the same scope

       //class scope
       val handler = Handler()
       val runnable = object : Runnable {
          override fun run () {
            handler.removeCallbacksAndMessages(null) 
            //make sure you cancel the 
             previous task in case you scheduled one that has not run yet
            //do your thing
   
            handler.postDelayed(runnable,time)
         }
      }
   

then in some function

handler.postDelayed(runnable,time)

2. You can run a timertask, which would be better in this case

    val task = TimerTask {
       override fun run() {
        //do your thing
       }
    }
   
    val timer = Timer()
   
    timer.scheduleAtFixedRate(task,0L, timeBetweenTasks)
   

Answer 2

The first one is a function that accepts a lambda and returns a Runnable. In this case this means nothing.

The second one you're defining an anonymous object that implements Runnable. In this case this refers to that object instance.