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，如果是，请在卡上放置"已保存"图像。 我通过根据 SQLite 数据库中已保存的项目检查 ID 值来做到这一点。 我担心的一件事是databaseHelper = new DatabaseHelper(recyclerViewHolder.tv_name.getContext());经常被召唤。 问题1. 这是一个问题吗？

为了减轻我的担忧，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 项目上下文，如图所示。 就像我说的代码是书面作品，但我担心它的成本很高。

这是我尝试过的代码：

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(我只得到通过构造器传递上下文的发布代码)。 否则一切都有效。

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

Answer 1

您可以通过构造器包含上下文，然后在构造函数中创建帮助程序的实例。

例如，代替 ：-

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

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());

我担心的一件事是数据库助手=新的 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，因为您正在重建实例。关于打开数据库，这实际上不是在构造时完成的，而是在调用getReableDatabase或getWriteableDatabase时完成的，这与调用isSaved方法时相同。得到????ableDatabase 旨在应对多次调用。所以，我相信没有什么可担心的。

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

您可能想更改：-

//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();
}
}

像这样的东西：-

//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;
}