class for executors

AppExecutors.kt

import android.os.Handler
import android.os.Looper
import java.util.concurrent.Executor
import java.util.concurrent.Executors


// Global executor pools.
class AppExecutors // singleton constructor
private constructor(private val diskIO: Executor, private val mainThread: Executor,
                    private val networkIO: Executor) {

    fun diskIO(): Executor {
        return diskIO
    }

    fun mainThread(): Executor {
        return mainThread
    }

    fun networkIO(): Executor {
        return networkIO
    }


    private class MainThreadExecutor : Executor {
        private val mainThreadHandler = Handler(Looper.getMainLooper())

        override fun execute(command: Runnable) {
            mainThreadHandler.post(command)
        }
    }

    companion object {
        // singleton instantiation
        @Volatile private var singleInstance: AppExecutors? = null

        val instance: AppExecutors
            get(){
                return singleInstance ?: synchronized(this) {
                    AppExecutors(
                        Executors.newSingleThreadExecutor(),
                        Executors.newFixedThreadPool(3),
                        MainThreadExecutor()
                    )
                }
            }
    }

}