활동 수명 주기에 관한 이해  |  Android 개발자  |  Android Developers

The Activity instances in your app transition through different states in their lifecycle. The Activity class provides a number of callbacks that let the activity know when a state changes or that the system is creating, stopping, or resuming an activity or destroying the process the activity resides in.

→ Doing the right work at the right time and handling transitions properly make your app more robust and performant.

(callback : 이벤트 발생 후 호출하는 함수)

Activity-lifecycle concepts

Lifecycle callbacks

onCreat()

It is must be implemented, which fires when the system first creates the activity. In the onCreate() method, perform basic application startup logic that happens only once for the entire life of the activity.

• bind data to lists
• associate the activity with a ViewModel
• instantiate some class-scope variables

→ layout inflate, data initailization

lateinit var textView: TextView

// some transient state for the activity instance
var gameState: String? = null

override fun onCreate(savedInstanceState: Bundle?) {
    // call the super class onCreate to complete the creation of activity like
    // the view hierarchy
    super.onCreate(savedInstanceState)

    // recovering the instance state
    gameState = savedInstanceState?.getString(GAME_STATE_KEY)

    // set the user interface layout for this activity
    // the layout file is defined in the project res/layout/main_activity.xml file
    setContentView(R.layout.main_activity)

    // initialize member TextView so we can manipulate it later
    textView = findViewById(R.id.text_view)
}

// This callback is called only when there is a saved instance that is previously saved by using
// onSaveInstanceState(). We restore some state in onCreate(), while we can optionally restore
// other state here, possibly usable after onStart() has completed.
// The savedInstanceState Bundle is same as the one used in onCreate().
override fun onRestoreInstanceState(savedInstanceState: Bundle?) {
    textView.text = savedInstanceState?.getString(TEXT_VIEW_KEY)
}

// invoked when the activity may be temporarily destroyed, save the instance state here
override fun onSaveInstanceState(outState: Bundle?) {
    outState?.run {
        putString(GAME_STATE_KEY, gameState)
        putString(TEXT_VIEW_KEY, textView.text.toString())
    }
    // call superclass to save any view hierarchy
    super.onSaveInstanceState(outState)
}

onCreate method receives the parameter savedInstanceState, which is a Bundle object containing the activity's previously saved state.

onStart()

This call makes the activity visible to the user as the app prepares for the activity to enter the foreground and become interactive.

→ running animation, updating data