从适配器访问SQLite帮助器

以下内容可能会引起您的某些问题。就是说，您应该将该方法传递给MesagesAdapter，但就目前而言，没有理由将任何内容传递给getAllQuestions3方法。

public Cursor getAllQuestions3(MessagesAdapter usageSettings2) {
    return this.getWritableDatabase().query(TABLE_CHAT_DATA,null,null,null,null,null,null);
}

尝试将以上内容更改为

public Cursor getAllQuestions3() {
    return this.getWritableDatabase().query(TABLE_CHAT_DATA,null,null,null,null,null,null);
}

和使用

ChatData mHelper = new ChatData(this);
Cursor csr = mHelper.getAllQuestions3();

代替

final ChatData mHelper = new ChatData(this,ChatData.DATABASE_NAME,null,ChatData.DATABASE_VERSION);
final Cursor csr = mHelper.getAllQuestions3(this);

请注意，以上几行应位于Activity的onCreate方法或从onCreate调用的方法中，以便您具有有效的上下文（即this）。

编辑1

请注意，以上内容已根据ChatData进行了更改，您需要提供4个用于实例化ChatData对象的参数。

我建议将构造函数更改为：-

public ChatData(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
} 

然后您可以使用ChatData mHelper = new ChatData(this);。

工作实例

以下是基于您提供的代码的工作示例。

数据库助手ChatData.java

public class ChatData extends SQLiteOpenHelper {
    private static final int DATABASE_VERSION = 1;
    public static final String DATABASE_NAME = "MessagePlus";
    public static final String TABLE_CHAT_DATA = "chatdata";
    public static final String COL_CHATDATA_ID = BaseColumns._ID;
    public static final String COL_CHATDATA_TIMESTAMP = "timestamp";
    public static final String COL_CHATDATA_MESSAGE = "message";
    public static final String COL_CHATDATA_USER = "user";

    public ChatData(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }



    @Override
    public void onCreate(SQLiteDatabase db) {
        String crt_sql = "CREATE TABLE IF NOT EXISTS " + TABLE_CHAT_DATA + "(" +
                COL_CHATDATA_ID + " INTEGER PRIMARY KEY, " +
                COL_CHATDATA_TIMESTAMP + " TEXT DEFAULT CURRENT_TIMESTAMP," +
                COL_CHATDATA_MESSAGE + " TEXT, " +
                COL_CHATDATA_USER + " INTEGER" +
                ")";
        db.execSQL(crt_sql);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }
    public Cursor getAllQuestions3() {
        return this.getWritableDatabase().query(TABLE_CHAT_DATA,null,null,null,null,null,null);
    }

    public long addMessage(String message, long user) {
        ContentValues cv = new ContentValues();
        cv.put(COL_CHATDATA_MESSAGE,message);
        cv.put(COL_CHATDATA_USER,user);
        SQLiteDatabase db = this.getWritableDatabase();
        return db.insert(TABLE_CHAT_DATA,null,cv);
    }
}

• 注意已修改以创建表并允许将行添加到表中。
• 注意构造函数。

定制适配器（游标适配器）MessageAdapter.java

public class MessageAdapter extends CursorAdapter {
    public MessageAdapter(Context context, Cursor c, boolean autoRequery) {
        super(context, c, autoRequery);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        return super.getView(position, convertView, parent);
    }

    @Override
    public View newView(Context context, Cursor cursor, ViewGroup parent) {
        View rv = LayoutInflater.from(context).inflate(
                R.layout.messagelist_item,
                parent,
                false
        );
        return rv;
    }

    @Override
    public void bindView(View view, Context context, Cursor cursor) {

        TextView mUser = view.findViewById(R.id.user);
        TextView mTimestamp = view.findViewById(R.id.timestamp);
        TextView mMessage = view.findViewById(R.id.message);

        mUser.setText(cursor.getString(cursor.getColumnIndex(ChatData.COL_CHATDATA_USER)));
        mTimestamp.setText(cursor.getString(cursor.getColumnIndex(ChatData.COL_CHATDATA_TIMESTAMP)));
        mMessage.setText(cursor.getString(cursor.getColumnIndex(ChatData.COL_CHATDATA_MESSAGE)));
    }
}

列表messagelist_item.xml中使用的布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/user"
        android:layout_width="0dp"
        android:layout_weight="1"
        android:layout_height="wrap_content" />
    <TextView
        android:id="@+id/timestamp"
        android:layout_width="0dp"
        android:layout_weight="1"
        android:layout_height="wrap_content"
        />
    <TextView
        android:id="@+id/message"
        android:layout_width="0dp"
        android:layout_weight="8"
        android:layout_height="wrap_content" />
</LinearLayout>

活动中用于通过适配器列出消息的代码（MainActivty.java）

public class MainActivity extends AppCompatActivity {

    ChatData mDBHlpr;
    Cursor mCsr;
    MessageAdapter mMesaageAdapter;
    ListView mMessageList;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mMessageList = this.findViewById(R.id.messagelist);
        mDBHlpr = new ChatData(this);
        addSomeData();
        mCsr = mDBHlpr.getAllQuestions3();
        mMesaageAdapter = new MessageAdapter(this,mCsr,false);
        mMessageList.setAdapter(mMesaageAdapter);

    }

    private void addSomeData() {
        mDBHlpr.addMessage("Hello",1);
        mDBHlpr.addMessage("Hi",2);
        mDBHlpr.addMessage("How are you?",1);
        mDBHlpr.addMessage("I'm OK thanks, and you?",2);
        mDBHlpr.addMessage("Good.",1);
    }
}

结果