Kotlin - How to pass a Runnable as this in Handler

user_11204066 0
Viewed 10165

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?

kotlin runnable handler android
4 Answers

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)

  1. 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)
user_21368120

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.

user_8984780

The below example will work.

val runnable = object : Runnable { 
        override fun run() {
         
            handler.postDelayed(this,1000)
        }
    }
user_57207450

In your case , when use this it means "local final class <no name provided> : Runnable" , refer to a header runnable.

runnable=object : Runnable {
            override fun run() {
                // i is a counter
                println("No. "+i++)
                // Repeat again after 2 seconds
                handler.postDelayed(this, 2000)
            }
        }
        handler.postDelayed(runnable,0)

Where runnable is used inside of a method. As Handler() is deprecated, we must use like this:

var handler: Handler = Handler(Looper.getMainLooper())
var runnable: Runnable = Runnable { }

Moreover, anywhere you can stop this method by:

handler.removeCallbacks(runnable)
user_122726870
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