更改配置时无法在 Android 中保存片段状态

Question

这种类型的问题在 SO 中被问了很多次，我已经尝试了所有这些，但无法达到我真正想要的。在我的应用程序中我有Fragment，在那个片段中我有一个recycler 视图。 recyclerview 将被我从 API 获得的数据所操纵。

我想要的是，当旋转发生变化时，应用不会再次调用 API。为了实现这一点，我知道我必须将数据放入onSaveInstanceState，如果我想保存一个复杂的对象，那么该类必须实现Parcelable。我已经完成了推荐的所有操作，但无法实现我想要的。

我在这里分享我的代码，我创建了一个简化的示例。

SimpleFragment.java 的代码

public class SimpleFragment extends Fragment {

    RecyclerView recyclerView;
    ArrayList<TestModel> testModelList = new ArrayList<TestModel>() ;
    SimpleAdapter simpleAdapter;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.simple_fragment, container, false);
        recyclerView = (RecyclerView) rootView.findViewById(R.id.recycler);

        RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getActivity());
        recyclerView.setLayoutManager(mLayoutManager);

        simpleAdapter = new SimpleAdapter(getActivity());

        recyclerView.setAdapter(simpleAdapter);


        if(savedInstanceState != null){
            //if this fragment starts after a rotation or configuration change, load the existing model from a parcelable
            testModelList = savedInstanceState.getParcelableArrayList("key");
            // I have done necessary debug , after rotation , it comes here ,
            // but suddenly savedInstanceState becomes null and getTestModelList() get called


        }else {
            //if this fragment starts for the first time, load the list of model
           getTestModelList();
        }

        simpleAdapter.setModel(testModelList);

        return rootView;
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putParcelableArrayList("key",testModelList);
    }

    public void getTestModelList(){
        for (int i=0;i<10;i++){
            testModelList.add(new TestModel(i,"test"+i));
        }
        Toast.makeText(getActivity(),"Called",Toast.LENGTH_SHORT).show();
    }
}

SimpleFragment 的布局文件：simple_fragment.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler"
        android:background="@android:color/black"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>
</LinearLayout>

这是我的数据模型的代码TestModel.java

public class TestModel implements Parcelable {

    String text;

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public TestModel(int number, String text) {
        this.text = text;
    }


    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(this.text);
    }

    protected TestModel(Parcel in) {
        this.text = in.readString();
    }

    public static final Parcelable.Creator<TestModel> CREATOR
            = new Parcelable.Creator<TestModel>() {
        public TestModel createFromParcel(Parcel in) {
            return new TestModel(in);
        }

        public TestModel[] newArray(int size) {
            return new TestModel[size];
        }
    };
}

我的适配器类：SimpleAdapter.java

public class SimpleAdapter extends RecyclerView.Adapter<SimpleAdapter.ViewHolder> {

    Context context;
    ArrayList<TestModel> testModels;

    public SimpleAdapter(Context context) {
        this.context = context;
    }

    public void setModel(ArrayList<TestModel> model){
        this.testModels = model;
        notifyDataSetChanged();
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int viewType) {
        View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.single_row, viewGroup, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        holder.list_item.setText(testModels.get(position).getText());
    }

    @Override
    public int getItemCount() {
        return testModels.size();
    }

    public class ViewHolder extends RecyclerView.ViewHolder{
        TextView list_item;
        public ViewHolder(View itemView) {
            super(itemView);
            list_item = (TextView) itemView.findViewById(R.id.list_item);
        }
    }
}

适配器的单行布局代码single_row.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:padding="20dp"
        android:id="@+id/list_item"
        android:textColor="@android:color/white"/>

</LinearLayout>

这些是我所做的，如果再次调用 SimpleFragment 中的方向 getTestModelList() 方法，我想避免这种情况；在实际应用场景中，此方法将执行一些 API 调用。我想保存ArrayList<TestModel> testModelList 的状态。

Answer 1

您需要保存活动状态并在配置前检查它

遵循此代码

public class MainActivity extends AppCompatActivity{


    private HomeFragment homeFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        FragmentManager fm = getSupportFragmentManager();


           if (savedInstanceState == null) {
            homeFragment = new HomeFragment();


            ((fm.beginTransaction()).replace(R.id.mainframe, homeFragment)).commit();
        }
}}}

Answer 2

您可以像这样在 Fragment onCreate 方法中保存 Fragment 实例以更改方向以保存实例：

@Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
         // Set Instance true
        this.setRetainInstance(true);
    }

Answer 3

只要方向改变，Android 就会重新创建您的 Activity。这背后的原因是您可能需要更改布局，但您仍然可以使用 onConfigurationChanged() 的 onConfigurationChanged() 或 Fragment 我建议您添加

android:configChanges="orientation|keyboardHidden|screenSize"

在您的清单文件中（请参阅 this 和 this）。这将停止 Activity on Confifuration 更改的重新创建。