Android - Lifecycle 사용 방법

class MyObserver(private val lifeCycle: Lifecycle) : LifecycleObserver {
    companion object {
        const val TAG = "MyObserver"
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun onCreated(source: LifecycleOwner) {
        Log.d(TAG, "onCreated")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun onStart() {
        Log.d(TAG, "onStart")
        if (lifeCycle.currentState.isAtLeast(Lifecycle.State.INITIALIZED)) {
            Log.d(TAG, "currentState is greater or equal to INITIALIZED")
        }
        if (lifeCycle.currentState.isAtLeast(Lifecycle.State.CREATED)) {
            Log.d(TAG, "currentState is greater or equal to CREATED")
        }
        if (lifeCycle.currentState.isAtLeast(Lifecycle.State.STARTED)) {
            Log.d(TAG, "currentState is greater or equal to STARTED")
        }
        if (lifeCycle.currentState.isAtLeast(Lifecycle.State.RESUMED)) {
            Log.d(TAG, "currentState is greater or equal to RESUMED")
        }
        if (lifeCycle.currentState.isAtLeast(Lifecycle.State.DESTROYED)) {
            Log.d(TAG, "currentState is greater or equal to DESTROYED")
        }
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun onResume() {
        Log.d(TAG, "onResume")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun onPause() {
        Log.d(TAG, "onPause")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun onStop() {
        Log.d(TAG, "onStop")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun onDestroy() {
        Log.d(TAG, "onDestroy")
    }
}

class LifeCycleActivity : AppCompatActivity() {

    private var observer = MyObserver(lifecycle)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_lifecycle)

        lifecycle.addObserver(observer)
    }
}

MyObserver: onCreated
MyObserver: onStart
MyObserver: currentState is greater or equal to INITIALIZED
MyObserver: currentState is greater or equal to CREATED
MyObserver: currentState is greater or equal to STARTED
MyObserver: currentState is greater or equal to DESTROYED
MyObserver: onResume

MyObserver: onPause
MyObserver: onStop

public enum Event {
    ON_CREATE,
    ON_START,
    ON_RESUME,
    ON_PAUSE,
    ON_STOP,
    ON_DESTROY,
    ON_ANY
}

/**
 * Lifecycle states. You can consider the states as the nodes in a graph and
 * {@link Event}s as the edges between these nodes.
 */
@SuppressWarnings("WeakerAccess")
public enum State {
    /**
     * Destroyed state for a LifecycleOwner. After this event, this Lifecycle will not dispatch
     * any more events. For instance, for an {@link android.app.Activity}, this state is reached
     * <b>right before</b> Activity's {@link android.app.Activity#onDestroy() onDestroy} call.
     */
    DESTROYED,

    /**
     * Initialized state for a LifecycleOwner. For an {@link android.app.Activity}, this is
     * the state when it is constructed but has not received
     * {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} yet.
     */
    INITIALIZED,

    /**
     * Created state for a LifecycleOwner. For an {@link android.app.Activity}, this state
     * is reached in two cases:
     * <ul>
     *     <li>after {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} call;
     *     <li><b>right before</b> {@link android.app.Activity#onStop() onStop} call.
     * </ul>
     */
    CREATED,

    /**
     * Started state for a LifecycleOwner. For an {@link android.app.Activity}, this state
     * is reached in two cases:
     * <ul>
     *     <li>after {@link android.app.Activity#onStart() onStart} call;
     *     <li><b>right before</b> {@link android.app.Activity#onPause() onPause} call.
     * </ul>
     */
    STARTED,

    /**
     * Resumed state for a LifecycleOwner. For an {@link android.app.Activity}, this state
     * is reached after {@link android.app.Activity#onResume() onResume} is called.
     */
    RESUMED;

    /**
     * Compares if this State is greater or equal to the given {@code state}.
     *
     * @param state State to compare with
     * @return true if this State is greater or equal to the given {@code state}
     */
    public boolean isAtLeast(@NonNull State state) {
        return compareTo(state) >= 0;
    }
}