Demonstration that the Android can destroy background activities on memory pressure without restarting the application

MainActivity.kt

/**
* Demonstration that the Android can destroy background activities on memory pressure without restarting
* the application
https://cs.android.com/android/platform/superproject/+/android-12.0.0_r4:frameworks/base/core/java/android/app/ActivityThread.java;l=7534
*/
const val ARG_ACTIVITY_NO = "ARG_ACTIVITY_NO"
const val TAG = "MainActivity"

val gcThresholds = GcActivityMemoryThresholds()

class MainActivity : AppCompatActivity() {
    val handler = Handler(Looper.getMainLooper())
    var activityNo: Int = -1
    val garbage: MutableList<ByteArray> = LinkedList()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        activityNo = intent.getIntExtra(ARG_ACTIVITY_NO, 1);
        Log.i(TAG, "onCreate: $activityNo")

        setContentView(R.layout.activity_main)
        findViewById<TextView>(R.id.textView).text = "activity $activityNo"

        if (activityNo == 1) {
            while (gcThresholds.usedMemory <= gcThresholds.threshold) {
                garbage += ByteArray(1024 * 1024)
            }
        } else {
            repeat(5 * 1024) {
                garbage += ByteArray(1024)
            }
        }

        if (activityNo < 10) {
            handler.postDelayed({
                startActivity(Intent(this@MainActivity, MainActivity::class.java).apply {
                    putExtra(ARG_ACTIVITY_NO, activityNo + 1);
                })
            }, 1000)
        }
    }

    override fun onStart() {
        super.onStart()
        Log.i(TAG, "onStart: $activityNo; Memory; $gcThresholds")
    }

    override fun onStop() {
        super.onStop()
        Log.i(TAG, "onStop: $activityNo")
    }

    override fun onDestroy() {
        super.onDestroy()
        Log.i(TAG, "ON_DESTROY: $activityNo")
        handler.removeCallbacksAndMessages(null)
    }
}

class GcActivityMemoryThresholds() {
    val threshold: Long = run {
        val runtime = Runtime.getRuntime()
        val dalvikMax = runtime.maxMemory()
        (3 * dalvikMax) / 4
    }

    val usedMemory: Long
        get() {
            val runtime = Runtime.getRuntime()
            return runtime.totalMemory() - runtime.freeMemory()
        }

    override fun toString(): String {
        return "GcActivityMemoryThresholds(threshold=$threshold, usedMemory=$usedMemory)"
    }
}