儲存和恢復碎片狀態

片段還有一個 onSaveInstanceState() 方法，當需要儲存狀態時呼叫它：

public class MySimpleFragment extends Fragment {
    private int someStateValue;
    private final String SOME_VALUE_KEY = "someValueToSave";
   
    // Fires when a configuration change occurs and fragment needs to save state
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        outState.putInt(SOME_VALUE_KEY, someStateValue);
        super.onSaveInstanceState(outState);
    }
}

然後我們可以在 onCreateView 中從這個儲存狀態中提取資料：

public class MySimpleFragment extends Fragment {
   // ...

   // Inflate the view for the fragment based on layout XML
   @Override
   public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.my_simple_fragment, container, false);
        if (savedInstanceState != null) {
            someStateValue = savedInstanceState.getInt(SOME_VALUE_KEY);
            // Do something with value if needed
        }
        return view;
   }
}

為了正確儲存片段狀態，我們需要確保在配置更改時不會不必要地重新建立片段。這意味著當它們已經存在時，要小心不要重新初始化它們。在配置更改後，需要通過標記查詢在 Activity 中初始化的任何片段：

public class ParentActivity extends AppCompatActivity {
    private MySimpleFragment fragmentSimple;
    private final String SIMPLE_FRAGMENT_TAG = "myfragmenttag";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        if (savedInstanceState != null) { // saved instance state, fragment may exist
           // look up the instance that already exists by tag
           fragmentSimple = (MySimpleFragment)  
              getSupportFragmentManager().findFragmentByTag(SIMPLE_FRAGMENT_TAG);
        } else if (fragmentSimple == null) { 
           // only create fragment if they haven't been instantiated already
           fragmentSimple = new MySimpleFragment();
        }
    }
}

這要求我們在將片段放入事務中的活動時要小心包含用於查詢的標記：

public class ParentActivity extends AppCompatActivity {
    private MySimpleFragment fragmentSimple;
    private final String SIMPLE_FRAGMENT_TAG = "myfragmenttag";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // ... fragment lookup or instantation from above...
        // Always add a tag to a fragment being inserted into container
        if (!fragmentSimple.isInLayout()) {
            getSupportFragmentManager()
                .beginTransaction()
                .replace(R.id.container, fragmentSimple, SIMPLE_FRAGMENT_TAG)
                .commit();
        }
    }
}

使用這種簡單的模式，我們可以正確地重用碎片並在配置更改中恢復其狀態。