使用 SQLite 数据库将适配器 onBindViewHolder itemView 上下文转换为类上下文

Question

我有一个有效的回收视图：

public class MtbListAdapter extends RecyclerView.Adapter<MtbListAdapter.RecyclerViewHolder> implements Filterable {
private ArrayList<ApiObject> arrayList;
private ArrayList<ApiObject> arrayListFiltered;
private DatabaseHelper databaseHelper;

public MtbListAdapter(ArrayList<ApiObject> arrayList) {
    this.arrayList =arrayList;
    this.arrayListFiltered =arrayList;
}

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

@Override
public void onBindViewHolder(RecyclerViewHolder recyclerViewHolder, int position){

    //load texts to card
    recyclerViewHolder.tv_name.setText(arrayListFiltered.get(position).getName());
    recyclerViewHolder.tv_length.setText(arrayListFiltered.get(position).getLength());

    //is card already saved?
    String s_identifier = arrayListFiltered.get(position).getId();
    databaseHelper = new DatabaseHelper(recyclerViewHolder.tv_name.getContext());
    boolean isSaved = databaseHelper.isSaved(s_identifier);

    if (isSaved) recyclerViewHolder.iv_isSaved.setVisibility(View.VISIBLE);
    else recyclerViewHolder.iv_isSaved.setVisibility(View.INVISIBLE);
}

在onBindViewHolder 中，我检查该项目是否已保存，如果已保存，则在卡上放置一个“已保存”图像。我通过检查我的 SQLite 数据库中已经保存的项目的 ID 值来做到这一点。我担心的一件事是databaseHelper = new DatabaseHelper(recyclerViewHolder.tv_name.getContext()); 经常被调用。问题 1. 这是一个问题吗？

为了减轻我的顾虑，我尝试通过构造函数传递上下文并使 databaseHelper 成为类对象，但是无论我传递什么上下文都会导致我的 SQLite 数据库方法在 databaseHelper 类中崩溃。我尝试传递 Activity.this、应用程序上下文、基本上下文等。我只成功传递了 viewHolder 项上下文，如图所示。就像我说的代码作为书面作品，但我担心它很昂贵。

这是我试过的代码：

 public class MtbListAdapter extends RecyclerView.Adapter<MtbListAdapter.RecyclerViewHolder> implements Filterable {
    private Context context;
    private ArrayList<ApiObject> arrayList;
    private ArrayList<ApiObject> arrayListFiltered;
    private DatabaseHelper databaseHelper = new DatabaseHelper(context);

public MtbListAdapter(ArrayList<ApiObject> arrayList, Context context) {
    this.arrayList =arrayList;
    this.arrayListFiltered =arrayList;
    this.context =context;
}

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

@Override
public void onBindViewHolder(RecyclerViewHolder recyclerViewHolder, int position){

    //load texts to card
    recyclerViewHolder.tv_name.setText(arrayListFiltered.get(position).getName());
    recyclerViewHolder.tv_length.setText(arrayListFiltered.get(position).getLength());

    //is card already saved?
    String s_identifier = arrayListFiltered.get(position).getId();
    boolean isSaved = databaseHelper.isSaved(s_identifier);

    if (isSaved) recyclerViewHolder.iv_isSaved.setVisibility(View.VISIBLE);
    else recyclerViewHolder.iv_isSaved.setVisibility(View.INVISIBLE);
}

这是我的 SQLite 方法：

 //returns true if is already saved
public boolean isSaved(String s) {
    SQLiteDatabase db = getReadableDatabase();
    String[] columns = new String[] {KEY_IDENTIFIER};
    String where = KEY_IDENTIFIER + " = ?";
    String[] whereArgs = new String[] {s};

    // select column_word from table where column_word = 's' limit 1;
    try (Cursor c = db.query(TABLE_NAME, columns, where, whereArgs, null, null, null, "1")){
        return c.moveToFirst();
    }
}

这是我的错误（我只收到通过构造函数传递上下文的代码）。否则一切正常。

java.lang.NullPointerException: 尝试调用虚拟方法 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android. database.DatabaseErrorHandler)' 在空对象引用上

Answer 1

您可以通过构造函数包含 上下文，然后在构造函数中创建助手的实例。

例如而不是：-

public MtbListAdapter(ArrayList<ApiObject> arrayList) {
    this.arrayList =arrayList;
    this.arrayListFiltered =arrayList;
}

使用：-

public MtbListAdapter(Context context, ArrayList<ApiObject> arrayList) {
    this.arrayList =arrayList;
    this.arrayListFiltered =arrayList;
    databaseHelper = new DatabaseHelper(context);
}

并删除该行：-

    databaseHelper = new DatabaseHelper(recyclerViewHolder.tv_name.getContext());

我担心的一件事是 databaseHelper = new DatabaseHelper(recyclerViewHolder.tv_name.getContext());被称为 很多。问题 1. 这是一个问题吗？

这不是最好的方法，因为您正在重新构建实例。关于打开数据库，实际上不是在构造时完成，而是在调用getReableDatabase 或getWriteableDatabase 时完成，这与调用isSaved 方法时所做的相同。 get????ableDatabase 旨在应对多次调用。所以，我相信没什么好担心的。

但是，您不会在 isSaved 方法中关闭光标。这应该引起关注，因为如果您打开了太多游标，则会发生异常。

你可能想改变：-

//returns true if is already saved
public boolean isSaved(String s) {
    SQLiteDatabase db = getReadableDatabase();
    String[] columns = new String[] {KEY_IDENTIFIER};
    String where = KEY_IDENTIFIER + " = ?";
    String[] whereArgs = new String[] {s};

    // select column_word from table where column_word = 's' limit 1;
    try (Cursor c = db.query(TABLE_NAME, columns, where, whereArgs, null, null, null, "1")){
        return c.moveToFirst();
    }
}

类似于：-

//returns true if is already saved
public boolean isSaved(String s) {
    boolean rv = false;
    SQLiteDatabase db = getReadableDatabase();
    String[] columns = new String[] {KEY_IDENTIFIER};
    String where = KEY_IDENTIFIER + " = ?";
    String[] whereArgs = new String[] {s};

    // select column_word from table where column_word = 's' limit 1;
    (Cursor c = db.query(TABLE_NAME, columns, where, whereArgs, null, null, null, "1"));
    if(c.moveToFirst) {
            rv = true;
    }
    c.close()
    return rv;
}