Fragment에서의 ActionBar 조작은 OnCreateView에서 하자

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 출처: https://youngjaekim.wordpress.com/2015/02/26/fragment에서의-actionbar-조작은-oncreateview에서-하자/

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이 글은 Vardhan님의 포스트를 요약한 것입니다.

우리는 보통 ActionBar를 조작할 때 Fragment.OnCreate에 코드를 넣습니다.
아래와 같이 말이죠.

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@Override public void onCreate(Bundle savedInstanceState) {     super.onCreate(savedInstanceState);             ActionBar actionBar = getActivity().getSupportActionBar();             actionBar.setTitle("첫화면"); }

하지만 Fragment와 엮이면서 ActionBar를 Fragment에서 조작할 때는,
Fragment.OnCreate에서 조작할 때 Activity NullPointerException이 발생할 수 있습니다.

그 이유는 아주 복잡한 Activity-Fragment 라이프사이클과 관계 때문인데,
그 중 Activity가 destroy 되고 다시 살아날 때를 가정하면 다음의 흐름을 거칩니다.

생성

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MainActivity onCreate           Fragment onAttach           Fragment onCreate           Fragment onCreateView           Fragment onViewCreated           Fragment onActivityCreated MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume

파괴

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Fragment onStop MainActivity onStop           Fragment onDestroyView           Fragment onDestroy           Fragment onDetach MainActivity onDestroy

복원

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Fragment onAttach           Fragment onCreate // 문제 발생지점 MainActivity onCreate           Fragment onCreateView           Fragment onViewCreated           Fragment onActivityCreated MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume

What happens? If we navigates to another activity from fragment and come back to main activity?
Here we have two scenarios, where fragment life cycle works.

1.    Activity is not destroyed when user leaves it.
2.    Activity got destroyed when user leaves it.

Activity Normal flow	Activity got destroyed
MainActivity onCreate           Fragment onAttach           Fragment onCreate           Fragment onCreateView           Fragment onViewCreated           Fragment onActivityCreated MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume	MainActivity onCreate           Fragment onAttach           Fragment onCreate           Fragment onCreateView           Fragment onViewCreated           Fragment onActivityCreated MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume
Fragment onStop MainActivity onStop	Fragment onStop MainActivity onStop           Fragment onDestroyView           Fragment onDestroy           Fragment onDetach MainActivity onDestroy
MainActivity onRestart MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume	Fragment onAttach           Fragment onCreate MainActivity onCreate           Fragment onCreateView           Fragment onViewCreated           Fragment onActivityCreated MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume

문제는 마지막의 “복원” 과정에서 발생합니다. Fragment.onCreate를 호출했지만,
ActionBar는 Activity.onCreate를 지나야 활성화가 되기 때문에 Null인 상태인겁니다.

참고로, 복원 과정을 쉽게 테스트하려면 개발자 옵션에 “액티비티 유지 안함” 옵션을 켠 후
앱을 구동 → 홈버튼을 눌렀다가 → 다시 앱을 켜면 복원 과정을 재현할 수 있습니다.