[Android 앱 개발 숙련] 5. 프래그먼트의 데이터 전달

[Android 앱 개발 숙련] 5. 프래그먼트의 데이터 전달

📌참고자료: Why to use Fragment Arguments in Android | Medium

Fragment Recreation

• Fragment recreation:
  when fragment instance has to be destroyed and be initiated again
  -> fragment constructer will be called again
  -> fragment will live a whole new lifecycle
• Configuration change triggers recreation
  -> recreation of currently visible fragment (and activity)

class MainActivity : AppCompatActivity(){ 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        if (savedInstanceState == null) {
            supportFragmentManager.beginTransaction()
                .replace(R.id.container, RecreationExampleFragment())
                .commit()
        }
    }
}

• Activity will create fragment...
  • in onCreate()
  • when restoring activity's state (savedInstanceState)

Don'ts when using fragments

• 우리가 필요로하는 데이터를 파라미터로 갖는 cunstom constructor를 정의하는 경우
  -> not guaranteed to be always safe
  -> The android system requires each fragment to have a default constructor with no parameter
  (default constructor is called on fragment recreation)

class BadFragment1(
    // will crash, on recreation because no default constructor is defined
    private val textToShow: String
): Fragment()

• 우리가 필요로하는 데이터를 프로퍼티로 갖도록 정의하는 경우
  -> value will be lost in fragment recreation

class BadFragment2: Fragment() {
    // Will be reset to empty string after recreation
    var textToShow: String = ""
}

Fragment arguments

• It is strongly recommended to supply arguments with setArguments and later retrieved by the Fragment with getArguments
  -> These arguments are automatically saved and restored alongside the Fragment
• Fragment argument로 사용할 수 있는 data type에 제한 존재
  • Only simple data types are supported (ex. String, Int, ...)
  • Parcelables, Serializables
• newInstance() 패턴

class GoodFragment: Fragment() {
    // retrieve value from arguments in custom getter 
    private val textToShow: String
        get() = requireArguments().getString(ARG_TEXT_TO_SHOW)
            ?: throw IllegalArgumentException("Argument $ARG_TEXT_TO_SHOW required")
    companion object {
        // the name for the argument
        private const val ARG_TEXT_TO_SHOW = "argTextToShow"
        // Use this function to create instances of the fragment
        // and set the passed data as arguments
        fun newInstance(textToShow: String) = GoodFragment().apply {
            arguments = bundleOf(
                ARG_TEXT_TO_SHOW to textToShow
            )
        }
    }
}

(1) Activity -> Fragment 데이터 전달

• newInstance 패턴 사용

(2) Fragment -> Fragment 데이터 전달

• newInstance 패턴 사용

(3) Fragment -> Activity 데이터 전달

• 콜백 인터페이스(FragmentDataListner) 정의

interface FragmentDataListener {
    fun onDataReceived(data: String)
}

• 데이터를 보내는 Fragment
  • 프래그먼트가 액티비티에 attach될 때, 액티비티가 콜백 인터페이스(FragmentDataListner)를 구현하는지 확인

    class SecondFragment : Fragment() {
    	private var listener: FragmentDataListener? = null
    	//...
      	override fun onAttach(context: Context) {
           super.onAttach(context)
           if (context is FragmentDataListener) {
               listener = context
           } else {
               throw RuntimeException("$context must implement FragmentDataListener")
           }
        }
        //...
        override fun onDestroyView() {
           super.onDestroyView()
           _binding = null
           listener = null
       }
    }

  • 콜백 인터페이스의 onDataReceived 메서드를 호출하여 데이터 전달

      val dataToSend = "Hello from SecondFragment!"
       listener?.onDataReceived(dataToSend)

• 데이터를 받는 Activity
  • 콜백 인터페이스(FragmentDataListner) 구현

    class MainActivity : AppCompatActivity(), FragmentDataListener {
        //...
       override fun onDataReceived(data: String) {
           // Fragment에서 받은 데이터를 처리
           Toast.makeText(this, data, Toast.LENGTH_SHORT).show()
       }
    }