如何检查微调器是否为空？

Question

我需要检查我的微调器是否为空。但是当微调器内部有数据时，它仍然向我显示吐司它是空的，不包含任何数据。主要问题是微调器已经包含数据库中的数据，但程序在再次打开应用程序后不断添加新数据。

主要活动

public class MainActivity extends AppCompatActivity implements AdapterView.OnItemSelectedListener {
Spinner spinner;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    spinner = (Spinner) findViewById(R.id.spinner);
    databaseHelper db = new databaseHelper(getApplicationContext());


    if (spinner.getCount() == 0){
        Toast.makeText(MainActivity.this,"Spinner will be populated now",Toast.LENGTH_LONG).show();
        db.insertData();
    }else {
        Toast.makeText(MainActivity.this,"Spinners is already populated",Toast.LENGTH_LONG).show();
    }
    loadData();
}

public void loadData(){
    databaseHelper db = new databaseHelper(getApplicationContext());
    List<String> labels = db.getAllLabels();
    ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_spinner_item, labels);
    dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    spinner.setAdapter(dataAdapter);
}

@Override
public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
    String label = adapterView.getItemAtPosition(i).toString();
    Toast.makeText(adapterView.getContext(),"Selected: "+label,Toast.LENGTH_LONG).show();
}

@Override
public void onNothingSelected(AdapterView<?> adapterView) {

}
}

数据库

public databaseHelper(Context context) {
    super(context, DATABASE_NAME, null, 1);
}

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("create table " + TABLE_NAME + " (ID INTEGER PRIMARY KEY AUTOINCREMENT," + COLUMN_2 + " TEXT," + COLUMN_3 + " TEXT," + COLUMN_4 + " TEXT)");

}

@Override
public void onUpgrade(SQLiteDatabase db, int i, int i1) {
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
    onCreate(db);
}

public boolean insertData() {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(COLUMN_2, "1");
    contentValues.put(COLUMN_3, "2");
    contentValues.put(COLUMN_4, "3");
    long result = db.insert(TABLE_NAME, null, contentValues);

    if (result == -1) {
        return false;
    } else
        return true;
}

public List<String> getAllLabels() {
    List<String> labels = new ArrayList<String>();

    String selectQuery = "SELECT  * FROM " + TABLE_NAME;

    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);
    if (cursor.moveToFirst()) {
        do {
            labels.add(cursor.getString(1));
            labels.add(cursor.getString(2));
            labels.add(cursor.getString(3));
        } while (cursor.moveToNext());
    }
    cursor.close();
    db.close();

    return labels;
}
}

Answer 1

你不应该做一些耗时的工作，比如从主应用线程调用 db。异步调用 db 例如使用AsyncTask。

您可以像这样获取微调器项目数：

int count = spinner.getAdapter() != null ? spinner.getAdapter().getCount() : 0;

Answer 2

我已经检查过你的代码并且总是在调用 db.insertData() 因为一开始的微调器 getCount 总是为 0。

因此，我让您在 MainActivity 中进行了一些更改，并在 Database 类中创建了一个方法来了解何时需要填充数据库。

MainActivity

public class MainActivity extends AppCompatActivity implements Spinner.OnItemSelectedListener{
    private Spinner spinner;
    private Database db;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

            spinner = findViewById(R.id.spinner);
            db  = new Database(getApplicationContext());

            if(db.shouldPopulate()){
                db.insertData();
            }

            loadData();
        }

    public void loadData(){
        List<String> labels = db.getAllLabels();

        ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,
                android.R.layout.simple_spinner_item, labels);

        dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinner.setOnItemSelectedListener(this);
        spinner.setAdapter(dataAdapter);

    }

        @Override
        public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
            String label = adapterView.getItemAtPosition(i).toString();
            Toast.makeText(adapterView.getContext(),"Selected: "+label,Toast.LENGTH_LONG).show();
        }

        @Override
        public void onNothingSelected(AdapterView<?> adapterView) {

        }
}

数据库 这里我将insertData方法中结果的条件改为插入的结果。

我创建了一个方法来知道你应该何时插入数据。

public boolean insertData() {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put(COLUMN_2, "1");
        contentValues.put(COLUMN_3, "2");
        contentValues.put(COLUMN_4, "3");

        return db.insert(TABLE_NAME, null, contentValues) != -1;
    }

    public boolean shouldPopulate(){
        return  getAllLabels().isEmpty();
    }

希望这段代码对你有所帮助。

重要

您应该将您的数据库调用包装在后台处理程序中，这对于 Android 应用程序的性能和良好做法非常重要。

Answer 3

您在哪里向微调器添加数据？你只需声明它：

spinner = (Spinner) findViewById(R.id.spinner);

您应该使用 Adapter 将数据添加到您的 Spinner：

// Create an ArrayAdapter using the string array and a default spinner layout
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this,
        R.array.planets_array, android.R.layout.simple_spinner_item);
// Specify the layout to use when the list of choices appears
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
// Apply the adapter to the spinner
spinner.setAdapter(adapter);

spinner.getCount() 是什么意思？可能一定是指adapter？