How to close/shut down application using android studio kotlin when pressing the back button?

Question

at this part lines 49-51

override fun onBackPressed() {

    }

I tried to look in google but there are so many answers and so different.

now when I'm running the application from the phone I need then to make a long press on the application icon on the phone then info>force stop the back button does nothing the application is keep running. and I want when pressing once the back button that it will close and exit the application.

full code

package com.example.flashlight

import android.content.Context
import android.content.Intent
import android.hardware.camera2.CameraAccessException
import android.hardware.camera2.CameraManager
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.os.Handler
import android.os.Looper
import android.widget.SeekBar
import android.widget.TextView
import kotlin.math.log

class MainActivity : AppCompatActivity() {

    var flashLightStatus: Boolean = false
    var flashLightOn = false
    var counter: Long = 0

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val textView = findViewById<TextView>(R.id.textView)
        val seek = findViewById<SeekBar>(R.id.seekBar)
        seek.setOnSeekBarChangeListener(
            object : SeekBar.OnSeekBarChangeListener {
                override fun onProgressChanged(
                    seekBar: SeekBar?,
                    progress: Int,
                    fromUser: Boolean
                ) {
                    // Log.d("seekbar", "Your Progress: ${seekBar?.progress}"

                    counter = progress.toLong()
                    textView.text = counter.toString()
                }

                override fun onStartTrackingTouch(seekBar: SeekBar?) = Unit
                override fun onStopTrackingTouch(seekBar: SeekBar?) = Unit
            },
        )

        startFlashLight()

    }


    override fun onBackPressed() {
        
    }

    private fun openFlashLight() {
        val cameraManager = getSystemService(Context.CAMERA_SERVICE) as CameraManager
        val cameraId = cameraManager.cameraIdList[0]
        if (!flashLightStatus) {
            try {
                cameraManager.setTorchMode(cameraId, true)
                flashLightStatus = true

            } catch (e: CameraAccessException) {
            }
        } else {
            try {
                cameraManager.setTorchMode(cameraId, false)
                flashLightStatus = false
            } catch (e: CameraAccessException) {
            }
        }
    }

    private fun closeFlashLight()
    {
        val cameraManager = getSystemService(Context.CAMERA_SERVICE) as CameraManager
        val cameraId = cameraManager.cameraIdList[0]
        if (flashLightStatus) {
            try {
                cameraManager.setTorchMode(cameraId, false)
                flashLightStatus = false

            } catch (e: CameraAccessException) {
            }
        } else {
            try {
                cameraManager.setTorchMode(cameraId, true)
                flashLightStatus = true
            } catch (e: CameraAccessException) {
            }
        }
    }

    fun startFlashLight() {
        openFlashLight()
        flashLightOn = true

        val handler = Handler(Looper.getMainLooper())
        handler.postDelayed(object: Runnable {
            override fun run() {
                if (flashLightOn) {
                    closeFlashLight()
                } else {
                    openFlashLight()
                }

                // Changing the toggle and calling the same Runnable again after 5 seconds
                flashLightOn = !flashLightOn
                handler.postDelayed(this, counter)
            }
        }, counter)
    }
}

Answer 1

Since this is an XY-problem, I will answer the root problem.

You're trying to fix your bug by dumping the whole app from memory. Really, you need to fix your bug. (Aside from that, terminating the app is generally not a good idea. Read more here.)

Your bug is that you are creating an endless loop that keeps going even after the Activity is destroyed. It will be especially bad if you rotate your screen a few times and have multiple tasks going. It will be leaking many copies of your Activity and they'll all be running the same loop forever, with offset timings.

As with any resource/listener/task you create in onCreate() and need to be stopped when an Activity is destroyed, you must release or end it in onDestroy(). So in this case, move your handler to a property and release its work in onDestroy().

class MainActivity : AppCompatActivity() {

    var flashLightStatus: Boolean = false
    var flashLightOn = false
    var counter: Long = 0
    private val handler = Handler(Looper.getMainLooper())

    // ...

    fun startFlashLight() {
        openFlashLight()
        flashLightOn = true

        // DELETED HANDLER DECLARATION LINE HERE

        handler.removeCallbacksAndMessages(null) // Added this line to ensure no duplciate jobs

        handler.postDelayed(
        //... your original code
    }

    override fun onDestroy() { 
        super.onDestroy()
        handler.removeCallbacksAndMessages(null) 
    }
}

If you want to stop this flashlight flashing if someone switches to another app without closing your activity, then you should turn it off and remove the callbacks in onStop() and move the call to start them from onCreate() to onStart(). Keep the onDestroy() code to ensure cleanup.