要使用 Fragment 在 dependency import

dependencies {
implementation "androidx.navigation:navigation-fragment-ktx:2.3.0"
}

Fragment 二種方式使用

**定義在 xml **

*MainActivity *

class MainActivity : AppCompatActivity() {

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

}

activitymain.xml 改成 fragment

<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/nav_graph.xml"
    app:startDestination="@id/mainFragment">

    <fragment
        android:id="@+id/mainFragment"
        android:name="tw.andyang.kotlinandroidworkshop.TodoListFragment"
        tools:layout="@layout/fragment_todo_list" />

</navigation>

create TodoListFragment fragment

class TodoListFragment : Fragment() {

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_todo_list, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

  
    }
}

*create TodoListFragment xml *

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:clipToPadding="false"
    tools:context=".TodoListFragment">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:paddingBottom="60dp"
        tools:listitem="@layout/item_todo" />

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/buttonAdd"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/add_todo"
        app:layout_constraintBottom_toBottomOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

FragmentManager

FragmentManager:
管理Activity中的fragments，通過呼叫getFragmentManager() | getSupportFragmentManager()獲取FragmentManager

FragmentManager fragmentManager = getFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

Fragment Transactions:
使用Fragment時，通過使用者互動來執行如增加、移除、替換等。

// 建立 Fragment 
Fragment newFragment = new testFragment();
FragmentTransaction transaction = getFragmentManager().beginTransaction();

transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);
transaction.commit();

newFragment將取代在R.id.fragment_container fragment。如果本身沒有就直接新增新的fragment。
呼叫addToBackStack()，最後commit()被儲存在back stack中，按Back鍵可以返回上一個轉換前的狀態。

Fragment的replace、add、hide、show 說明

reference :https://ithelp.ithome.com.tw/articles/10221199