Kotlin 액티비티(Activity) : 생명주기(LifeCycle)
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    MainActivity.kt
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class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        Toast.makeText(this,"onCreate", Toast.LENGTH_SHORT).show()
    }

    override fun onStart() {
        super.onStart()
        Toast.makeText(this,"onStart", Toast.LENGTH_SHORT).show()
    }
    override fun onResume() {
        super.onResume()
        Toast.makeText(this,"onResume", Toast.LENGTH_SHORT).show()
    }
    override fun onPause() {
        super.onPause()
        Toast.makeText(this,"onPause", Toast.LENGTH_SHORT).show()
    }
    override fun onStop() {
        super.onStop()
        Toast.makeText(this,"onStop", Toast.LENGTH_SHORT).show()
    }
    override fun onRestart() {
        super.onRestart()
        Toast.makeText(this,"onRestart", Toast.LENGTH_SHORT).show()
    }
    override fun onDestroy() {
        super.onDestroy()
        Toast.makeText(this,"onDestroy", Toast.LENGTH_SHORT).show()
    }
}