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?

Lena Bru · Accepted Answer · 2022-01-10T18:22:23.983

5

You have several options to go about here:

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)

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)

edited Jan 10 '22 at 18:22

answered Mar 19 '20 at 17:09

Lena Bru

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Thank you very much :) – Hayk Mkrtchyan Mar 19 '20 at 17:34
But the first code is not working for me. It doesn't override run method. – Hayk Mkrtchyan Mar 19 '20 at 17:37
In the first case, you still need `val runnable = object : Runnable { ... }` – Alexey Romanov Mar 19 '20 at 18:26

score 3 · Answer 2 · answered Mar 19 '20 at 16:58

3

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.

answered Mar 19 '20 at 16:58

m0skit0

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Thank you very much :) – Hayk Mkrtchyan Mar 19 '20 at 17:34
@m0skit0 When you said 'The first one', did you mean `Runnable{...}` in the OP's first code example? If that's the case, could I ask you if there's any reference for this usage? Well, yeah in Kotlin we can omit a function's parenthesis if the argument is lambda, but I thought that's not a function, rather it's just a lambda, and the notation `Runnable` is just nothing but for avoiding confusion(I read it from Kotlin docs). Please let me know if I understood incorrectly. Thanks in advance. – starriet Dec 30 '20 at 01:14
@starriet `Runnable` is a [SAM conversion to lambda from a functional interface](https://kotlinlang.org/docs/reference/fun-interfaces.html). It is equivalent to the second example, it's just syntactic sugar to avoid verbosity. – m0skit0 Dec 30 '20 at 09:47

score 2 · Answer 3 · answered Oct 05 '21 at 17:19

2

The below example will work.

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

answered Oct 05 '21 at 17:19

Jatinder Kumar

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Yeap, when we create runnable in this way, we can get a reference to it by this keyword. But using lambda, it's not possible)) – Hayk Mkrtchyan Oct 07 '21 at 07:45

score 1 · Answer 4 · answered Apr 01 '22 at 08:45

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)

Kotlin - How to pass a Runnable as this in Handler

4 Answers4