如何在活动启动时在recyclerview中显示已选中的复选框（多对多关系）

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【中文标题】如何在活动启动时在recyclerview中显示已选中的复选框（多对多关系）【英文标题】：How to display already checked checkboxes in recyclerview, on activity start-up(many to many relationship) 【发布时间】：2021-12-20 04:55:27 【问题描述】：

每个“Note”都以不同的方式“添加”到“Folders”中，或者我应该说，每个“Note”都有不同的选中“Folders”。

当我检查“文件夹”时，使用返回按钮返回上一个笔记活动，然后打开相同的“笔记”转到“添加到文件夹活动”并将“笔记”添加到文件夹，它应该记住之前检查过“文件夹”并重新检查。

我已经在 onBindViewHolder 和 FolderHolder(RecyclerView Holder) 中实现了该功能。

问题是当我取消选中复选框时，recyclerview 中存在多个“文件夹”，然后向下滚动，然后备份，未选中的文件夹会按原样重新检查自身。

我知道每次滚动时都会调用 onBindViewHolder 并回收视图，因此选中的“文件夹”会在滚动时重新检查。

我想在活动启动时检查之前已经检查过的文件夹，就像现在一样，但不会在滚动时重新检查它们。

请您给我推荐一个代码吗？我真的很感激！

我的 recyclerview 适配器：

public class FoldersAddToFoldersAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> 

    private final Context context;

    private final ArrayList<Folder> folders;

    private final List<Folder> checkedFolders;

    private final Note note;

    private final FoldersDAO foldersDao;

    private final NotesFoldersJoinDAO notesFoldersJoinDao;

    public FoldersAddToFoldersAdapter(ArrayList<Folder> folders, List<Folder> checkedFolders, Note note, Context context,
                                      FoldersDAO foldersDao, NotesFoldersJoinDAO notesFoldersJoinDao) 
        this.folders = folders;
        this.checkedFolders = checkedFolders;
        this.note = note;
        this.context = context;
        this.foldersDao = foldersDao;
        this.notesFoldersJoinDao = notesFoldersJoinDao;
    

    @NonNull
    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) 
        View v = LayoutInflater.from(context).inflate(R.layout.folder_add_to_folders_layout, parent, false);
        return new FolderHolder(v, this, notesFoldersJoinDao);
    

    @Override
    public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, int position) 
        final FolderHolder folderHolder = (FolderHolder) holder;
        final Folder folder = getFolder(position);
        if (folder != null) 
            //[..]

            folderHolder.setData(note, folder);


            // **THIS IS THE CODE WHICH CHECKS THE FOLDERS, WHICH I WAS SAYING ABOUT.** 
            for (Folder checkedFolder : checkedFolders) 
                if (checkedFolder.getId() == folder.getId()) 
                    folder.setChecked(true);
                
            
            folderHolder.checkBoxAddToFolders.setChecked(folder.isChecked());
        
    

    public void addCheckedFolder(Folder folder) 
        checkedFolders.add(folder);
    

    public void removeCheckedFolder(Folder folder) 
        checkedFolders.remove(folder);
    


    public static class FolderHolder extends RecyclerView.ViewHolder 

        CheckBox checkBoxAddToFolders;

        FoldersAddToFoldersAdapter adapter;

        Note note;

        Folder folder;

        NotesFoldersJoinDAO notesFoldersJoinDao;

        public FolderHolder(@NonNull View itemView, FoldersAddToFoldersAdapter adapter, NotesFoldersJoinDAO notesFoldersJoinDao) 
            super(itemView);
            checkBoxAddToFolders = itemView.findViewById(R.id.checkbox_add_to_folders);
            this.notesFoldersJoinDao = notesFoldersJoinDao;
            this.adapter = adapter;


            // THIS IS ALSO IMPORTANT FOR CHECKBOX.
            checkBoxAddToFolders.setOnClickListener(v -> 
                boolean isChecked = checkBoxAddToFolders.isChecked();
                NoteFolderJoin noteFolderJoin = new NoteFolderJoin(note.getId(), folder.getId());
                if (isChecked) 
                    folder.setChecked(true);
                    adapter.addCheckedFolder(folder);
                    notesFoldersJoinDao.insertNoteFolderJoin(noteFolderJoin);
                 else 
                    folder.setChecked(false);
                    adapter.removeCheckedFolder(folder);
                    notesFoldersJoinDao.deleteNoteNoteFolderJoin(noteFolderJoin);
                
            );

        public void setData(Note note, Folder folder) 
            this.note = note;
            this.folder = folder;

我的 Activity 有 recyclerview：

public class AddToFoldersActivity extends AppCompatActivity 

    public static final String NOTE_EXTRA_KEY = "note_id";

    private RecyclerView foldersListRv;

    private FoldersDAO folderDao;

    private FoldersAddToFoldersAdapter foldersAdapter;

    private Note note;

@Override
    protected void onCreate(Bundle savedInstanceState) 
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add_to_folders);
        MaterialToolbar topAppBar = findViewById(R.id.top_app_bar_add_to_folders);
        
        foldersListRv = findViewById(R.id.folder_list_add_to_folders);
        folderDao = NotesDB.getInstance(this).foldersDAO();
        
        int smallestScreenWidth = getResources().getConfiguration().smallestScreenWidthDp;
        NotesDAO notesDAO = NotesDB.getInstance(this).notesDAO();

        int id = getIntent().getExtras().getInt(NOTE_EXTRA_KEY, 0);
        note = notesDAO.getNoteById(id);

        LinearLayoutManager layoutManager = new LinearLayoutManager(this);

        foldersListRv.setLayoutManager(layoutManager);

        loadFolders();

        topAppBar.setNavigationOnClickListener(view -> finish());
    

    private void loadFolders () 
        NotesFoldersJoinDAO notesFoldersJoinDao = NotesDB.getInstance(this).notesFoldersJoinDAO();
        List<Folder> folderList = folderDao.getAllFolders();
        List<Folder> checkedFolderList = notesFoldersJoinDao.getFoldersFromNote(note.getId());
        ArrayList<Folder> folders = new ArrayList<>(folderList);
        this.foldersAdapter = new FoldersAddToFoldersAdapter(folders, checkedFolderList, note, this, folderDao, notesFoldersJoinDao);
        this.foldersListRv.setAdapter(foldersAdapter);

我的“文件夹”房间数据库实体：

@Entity(tableName = "folders")
public class Folder 
    @ColumnInfo(name = "folderId")
    @PrimaryKey
    private int id;

    @ColumnInfo(name = "folderName")
    private String folderName;

    //The checked state gets saved as a room db column, in each folder.
    private boolean checked;

    public Folder() 
    

    public String getFolderName() 
        return folderName;
    

    public void setFolderName(String folderName) 
        this.folderName = folderName;
    
    
    public int getId() 
        return id;
    

    public void setId(int id) 
        this.id = id;
    

    //From this method, it gets and returns the checked state of the checkbox.
    public boolean isChecked() 
        return checked;
    

    //And from this method, it sets the checked state of the checkbox.
    public void setChecked(boolean checked) 
        this.checked = checked;

我当然删除了一些代码，因为这会使这个问题变得太冗长。

【问题讨论】：

我认为您需要存储此复选框，例如共享首选项。因为 RecyclerView 仅显示您在展示中看到的项目。 RecyclerView 不知道底部的其他元素，而它们未显示在您的显示器上 @Spectator 感谢您的建议和评论！抱歉耽搁了……问题是，正如我所说的“每个注释都有不同的选中文件夹”。当我打开要添加到某些文件夹的便笺时，如果我有 2 个便笺和 2 个文件夹，则第一个便笺检查了第一个文件夹，第二个便笺检查了第二个文件夹。当然，在应用程序和活动启动时，应该记住选中的文件夹。我已经有 Room db 和它的 Dao 即。 checkFolderList 传递给 Adapter 中的 checkedFolders。我将尝试使用共享首选项，但如果放置 OnBindViewHolder，它的工作方式与现在相同。 谢谢@Spectator！我解决了！无论如何，谢谢你帮助我！ 【参考方案1】：

我终于解决了！我移动了：

for (Folder checkedFolder : checkedFolders) 
    if (checkedFolder.getId() == folder.getId()) 
        folder.setChecked(true);

从onBindViewHolder 到具有 recyclerview(AddToFoldersActivity) 的 Activity 并对其进行了一些修改以适应：

for (Folder checkedFolder : checkedFolderList) 
    for (Folder folder : folderList) 
        if (checkedFolder.getId() == folder.getId()) 
            folder.setChecked(true);

适配器（FoldersAddToFoldersAdapter）：

public class FoldersAddToFoldersAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> 

    private final Context context;

    private final ArrayList<Folder> folders;

    private final List<Folder> checkedFolders;

    private final Note note;

    private final FoldersDAO foldersDao;

    private final NotesFoldersJoinDAO notesFoldersJoinDao;

    public FoldersAddToFoldersAdapter(ArrayList<Folder> folders, List<Folder> checkedFolders, Note note, Context context,
                                      FoldersDAO foldersDao, NotesFoldersJoinDAO notesFoldersJoinDao) 
        this.folders = folders;
        this.checkedFolders = checkedFolders;
        this.note = note;
        this.context = context;
        this.foldersDao = foldersDao;
        this.notesFoldersJoinDao = notesFoldersJoinDao;
    

    @NonNull
    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) 
        View v = LayoutInflater.from(context).inflate(R.layout.folder_add_to_folders_layout, parent, false);
        return new FolderHolder(v, this, notesFoldersJoinDao);
    

    @Override
    public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, int position) 
        final FolderHolder folderHolder = (FolderHolder) holder;
        final Folder folder = getFolder(position);
        if (folder != null) 
            //[..]

            folderHolder.setData(note, folder);

            /* Moved code from here...
            for (Folder checkedFolder : checkedFolders) 
                if (checkedFolder.getId() == folder.getId()) 
                    folder.setChecked(true);
                
            
            */
            folderHolder.checkBoxAddToFolders.setChecked(folder.isChecked());
        
    

    public void addCheckedFolder(Folder folder) 
        checkedFolders.add(folder);
    

    public void removeCheckedFolder(Folder folder) 
        checkedFolders.remove(folder);
    


    public static class FolderHolder extends RecyclerView.ViewHolder 

        CheckBox checkBoxAddToFolders;

        FoldersAddToFoldersAdapter adapter;

        Note note;

        Folder folder;

        NotesFoldersJoinDAO notesFoldersJoinDao;

        public FolderHolder(@NonNull View itemView, FoldersAddToFoldersAdapter adapter, NotesFoldersJoinDAO notesFoldersJoinDao) 
            super(itemView);
            checkBoxAddToFolders = itemView.findViewById(R.id.checkbox_add_to_folders);
            this.notesFoldersJoinDao = notesFoldersJoinDao;
            this.adapter = adapter;

            checkBoxAddToFolders.setOnClickListener(v -> 
                boolean isChecked = checkBoxAddToFolders.isChecked();
                NoteFolderJoin noteFolderJoin = new NoteFolderJoin(note.getId(), folder.getId());
                if (isChecked) 
                    folder.setChecked(true);
                    adapter.addCheckedFolder(folder);
                    notesFoldersJoinDao.insertNoteFolderJoin(noteFolderJoin);
                 else 
                    folder.setChecked(false);
                    adapter.removeCheckedFolder(folder);
                    notesFoldersJoinDao.deleteNoteNoteFolderJoin(noteFolderJoin);
                
            );

        public void setData(Note note, Folder folder) 
            this.note = note;
            this.folder = folder;

并将代码移至 Activity(AddToFoldersActivity)：

public class AddToFoldersActivity extends AppCompatActivity 

    public static final String NOTE_EXTRA_KEY = "note_id";

    private RecyclerView foldersListRv;

    private FoldersDAO folderDao;

    private FoldersAddToFoldersAdapter foldersAdapter;

    private Note note;

    @Override
    protected void onCreate(Bundle savedInstanceState) 
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add_to_folders);
        MaterialToolbar topAppBar = findViewById(R.id.top_app_bar_add_to_folders);
        
        foldersListRv = findViewById(R.id.folder_list_add_to_folders);
        folderDao = NotesDB.getInstance(this).foldersDAO();
        
        NotesDAO notesDAO = NotesDB.getInstance(this).notesDAO();

        int id = getIntent().getExtras().getInt(NOTE_EXTRA_KEY, 0);
        note = notesDAO.getNoteById(id);

        LinearLayoutManager layoutManager = new LinearLayoutManager(this);

        foldersListRv.setLayoutManager(layoutManager);

        //This is where the code really ends up.
        loadFolders();

        topAppBar.setNavigationOnClickListener(view -> finish());
    

    //The loadFolders() gets called in onCreate().
    private void loadFolders () 
        NotesFoldersJoinDAO notesFoldersJoinDao = NotesDB.getInstance(this).notesFoldersJoinDAO();
        List<Folder> folderList = folderDao.getAllFolders();
        List<Folder> checkedFolderList = notesFoldersJoinDao.getFoldersFromNote(note.getId());
        ArrayList<Folder> folders = new ArrayList<>(folderList);
        
        //The code gets called here.
        checkAlreadyCheckedFolders(checkedFolderList, folderList);
        
        this.foldersAdapter = new FoldersAddToFoldersAdapter(folders, checkedFolderList, note, this, folderDao, notesFoldersJoinDao);
        this.foldersListRv.setAdapter(foldersAdapter);
    

    //The code moved to here, with a bit of modification and gets called in loadFolders().
    private void checkAlreadyCheckedFolders(List<Folder> checkedFolderList, List<Folder> folderList) 
        for (Folder checkedFolder : checkedFolderList) 
            for (Folder folder : folderList) 
                if (checkedFolder.getId() == folder.getId()) 
                    folder.setChecked(true);

最后是“文件夹”房间数据库实体（表）：

@Entity(tableName = "folders")
public class Folder 
    @ColumnInfo(name = "folderId")
    @PrimaryKey
    private int id;

    @ColumnInfo(name = "folderName")
    private String folderName;

    //The checked state gets saved as a room db column, in each folder.
    private boolean checked;

    public Folder() 
    

    public String getFolderName() 
        return folderName;
    

    public void setFolderName(String folderName) 
        this.folderName = folderName;
    
    
    public int getId() 
        return id;
    

    public void setId(int id) 
        this.id = id;
    

    //From this method, it gets and returns the checked state of the checkbox.
    public boolean isChecked() 
        return checked;
    

    //And from this method, it sets the checked state of the checkbox.
    public void setChecked(boolean checked) 
        this.checked = checked;

这样，复选框检查仅​​在 Activity(AddToFoldersActivity) 启动/创建时运行一次，而不是在每次滚动中的 onBindViewHolder() 中一次又一次地运行。 onBindViewHolder() 在每个滚动条上运行。

【讨论】：

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