[英]How to keep scrolling position in ScrollView after rotating the screen in fragment with LiveDate object loaded
我有一個片段,用於顯示根據給定ID從房間數據庫加載的歌曲的歌詞。
旋轉屏幕后,我想保留滾動位置。 現在,旋轉之后,歌曲再次從db加載,並且視圖位於最頂部,無論旋轉之前的滾動位置如何。
我以為我可以在onSaveInstanceState
中將滾動位置保存在onCreateView()
某些包中,使用mSongDisplayScrollView.scrollTo(x, y)
上的命令
片段代碼:
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;
}
}
片段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>
該片段在textView和RecyclerView中顯示適配器的代碼的歌曲的歌詞和歌詞。
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項目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>
我認為問題在於,旋轉屏幕歌曲之后,必須由觀察者的methon onChange()
重新加載,因此有一小段時間,scrollView無需顯示任何內容。 我想找到一些解決方案,在再次加載歌曲后如何返回到舊位置。
第一步是通過Fragment的onSaveInstanceState
方法持久化對象的值。
我的更多解釋將來自每個代碼塊內的注釋,因此您可以更好地了解其全部工作原理。
要保留的值: mSongToDisplay和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);
}
接下來是在方向更改完成后恢復數據
在您的fragments onCreateView
添加以下塊,並確保在初始化適配器后添加if statement block
...
//...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;
此時,所需的數據已保留,並且在每個screenOrientationChange
該應用程序會將先前保存的歌曲對象加載到適配器中,從而防止對db or api
的錯誤調用,並且還將使用保存的整數值mScrollViewPosition
滾動到期望的位置。
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