java - 在加载了 LiveDate 对象的 fragment 中旋转屏幕后如何保持 ScrollView 中的滚动位置

Question

我有一个 fragment ，用于显示歌曲的歌词，该歌词是根据给定的 id 从房间数据库加载的。

我想在旋转屏幕后保留滚动位置。现在，旋转后，歌曲会再次从数据库加载，并且无论旋转之前的滚动位置如何， View 都位于最顶部。

我认为我可以在onSaveInstanceState中保存滚动位置，在onCreateView()中使用命令mSongDisplayScrollView.scrollTo(x, y) >

fragment 代码:

public class SongDisplayFragment extends Fragment {

    private Song mSongToDisplay;

    private ScrollView mSongDisplayScrollView;
    private TextView mSongTitleTextView;
    private RecyclerView mSongLyricsRecyclerView;

    private GuitarSongbookViewModel mGuitarSongbookViewModel;

    public static final String SONG_ID_KEY = "SONG_ID_KEY";


    public SongDisplayFragment() {

    }

    public static SongDisplayFragment newInstance(Long songId) {
        SongDisplayFragment songDisplayFragment = new SongDisplayFragment();
        Bundle arguments = new Bundle();
        arguments.putLong(SONG_ID_KEY, 
        songDisplayFragment.setArguments(arguments);
        return songDisplayFragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(false);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_song_display, container, false);

        mSongDisplayScrollView = view.findViewById(R.id.song_display_scroll_view);
        mSongLyricsRecyclerView = view.findViewById(R.id.lyrics_rv_);
        mSongTitleTextView = view.findViewById(R.id.

        mGuitarSongbookViewModel = ViewModelProviders.of(this).get(GuitarSongbookViewModel.class);

        final SongDisplayAdapter songDisplayAdapter = new SongDisplayAdapter(getContext());

        Long songId = null;
        if (getArguments().containsKey(SONG_ID_KEY)) {
            songId = getArguments().getLong(SONG_ID_KEY);
        }

        if (songId != null) {
            final Long finalSongId = songId;


            mGuitarSongbookViewModel.getSongById(songId).observe(this, new Observer<Song>() {
                @Override
                public void onChanged(@Nullable final Song song) {
                    mSongToDisplay = song;
                    mSongTitleTextView.setText(mSongToDisplay.getMTitle());
                    songDisplayAdapter.setSong(song);

                }
            });


        }


        mSongLyricsRecyclerView.setAdapter(songDisplayAdapter);
        mSongLyricsRecyclerView.setLayoutManager(new LinearLayoutManager(getContext()));

        return view;
    }

}

fragment XML:

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

    <ScrollView
        android:id="@+id/song_display_scroll_view"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintBottom_toTopOf="@+id/autoscroll_bar"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="1.0"
        tools:layout_editor_absoluteX="138dp">

        <LinearLayout

            android:id="@+id/son_display_linear_layout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">

            <TextView
                android:id="@+id/displayed_song_title_txt_"
                style="@style/TitleOfDisplayedSong"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="16dp"
                android:layout_marginTop="8dp"
                android:text="@string/title_placeholder" />

            <androidx.recyclerview.widget.RecyclerView
                android:id="@+id/lyrics_rv_"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_marginTop="8dp"
                android:layout_marginBottom="12dp"
                android:nestedScrollingEnabled="false" />
        </LinearLayout>
    </ScrollView>
</androidx.constraintlayout.widget.ConstraintLayout>

该 fragment 在 textView 和 RecyclerView 中显示歌曲的标题和歌词，适配器类代码为:

package com.example.guitarsongbook.adapters;

import android.content.Context;
import android.graphics.Rect;
import android.text.Html;
import android.util.TypedValue;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.GridLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.example.guitarsongbook.R;
import com.example.guitarsongbook.daos.SongChordJoinDao;
import com.example.guitarsongbook.model.Chord;
import com.example.guitarsongbook.model.Song;

import java.util.ArrayList;
import java.util.List;


public class SongDisplayAdapter extends RecyclerView.Adapter<SongDisplayAdapter.SongViewHolder> {

    private Context context;
    private final LayoutInflater mInflater;

    private Song mSong;
    private ArrayList<String> mLyrics;

    public SongDisplayAdapter(Context context) {
        this.context = context;
        mInflater = LayoutInflater.from(context);
    }

    @NonNull
    @Override
    public SongViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View itemView = mInflater.inflate(R.layout.song_lyrics_rv_item, parent, false);
        return new SongDisplayAdapter.SongViewHolder(itemView);
    }

    public void setSong(Song song){
        mSong = song;
        mLyrics = mSong.getMLyrics();
        notifyDataSetChanged();
    }



    @Override
    public void onBindViewHolder(@NonNull SongViewHolder holder, int position) {
        holder.bindTo(position);
    }

    @Override
    public int getItemCount() {
        if (mLyrics != null)
            return mLyrics.size();
        else return 0;
    }

    public class SongViewHolder extends RecyclerView.ViewHolder {

        private final TextView mLyricsLineTextView;


        public SongViewHolder(@NonNull View itemView) {
            super(itemView);
            mLyricsLineTextView = itemView.findViewById(R.id.song_lyric_line_txt_);
        }

        public void bindTo(int position) {
            if (mSong != null) {
                mLyricsLineTextView.setText(Html.fromHtml(mLyrics.get(position)));



            } else {
                mLyricsLineTextView.setText(context.getString(R.string.no_song_label));
            }
        }
    }


}

RecyclerView 项目 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="wrap_content">

    <TextView
        android:id="@+id/song_lyric_line_txt_"
        style="@style/LyricOfDisplayedSong"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="4dp"
        android:layout_marginEnd="24dp"
        app:layout_constraintEnd_toStartOf="@+id/song_chord_line_txt_"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:text="placeholder text" />


</androidx.constraintlayout.widget.ConstraintLayout>

我认为问题在于，旋转屏幕后，必须通过观察者的方法 onChange() 再次加载歌曲，因此有一小会儿，scrollView 没有任何内容可显示。我想找到一些解决方案，如何在再次加载歌曲后返回到旧位置。

Answer 1

第一步是通过 Fragment 的 onSaveInstanceState 方法保存对象的值。

我的更多解释将来自每个代码块中的注释，因此您可以更好地理解它是如何工作的。

Values to be persisted: mSongToDisplay and mScrollViewPosition

//new variable introduced
int mScrollViewPosition = 0;

@Override
protected void onSaveInstanceState(Bundle outState) {
    //If screen orientation changes, android redraws all views 
    //and non persistent data would be lost. 
    //Nevertheless, before this happens android informs the app via this method
    //So to prevent state loss we save the data to a temporary storage
     outState.putParcelable("song_data", mSongToDisplay);
    //save ScrollView current position
     outState.putInt("position_data", mSongDisplayScrollView.getScrollY());
    //call super to commit your changes 
    super.onSaveInstanceState(outState);
}

接下来是方向改变完成后恢复数据

在 fragments onCreateView 中添加以下 block ，并确保 if 语句 block 在初始化适配器之后出现

 ...
 //...add adapter initialization block here before checking for saved data 

 //Check for saved data
 if (savedInstanceState != null) {
  // Retrieve the data you saved
  mSongToDisplay = savedInstanceState.getParcelable("song_data");
  mScrollViewPosition = savedInstanceState.getInt("position_data");

  //Re-initialize and reload adapter record
  mSongToDisplay = song;
  mSongTitleTextView.setText(mSongToDisplay.getMTitle());
  songDisplayAdapter.setSong(song); 
 } 
 else {
 //No data to retrieve
 //initialize data model here. 
 }

   //Assign the adapter to recyclerView
   mSongLyricsRecyclerView.setAdapter(songDisplayAdapter);
   mSongLyricsRecyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
  try{
   //Finally we set the scrollview position at this point just to make sure
   //recyclerview has its data loaded and full length of scrollview is restored
   mSongDisplayScrollView.setScrollY(mScrollViewPosition);}
  catch(Exception ex){
     //position isn't right
   }
  return view;

此时所需的数据已被保留，并且在每个 screenOrientationChange 上，应用程序会将之前保存的歌曲对象加载到适配器中，以防止错误调用db 或 api >，并且还会使用保存的整数值mScrollViewPosition滚动到预期位置。