java - Android 要求不要在主线程上查询，但异步查询正在延迟

Question

正如标题所示，android 需要在主线程之外进行查询，因为它会抛出 java.lang.IllegalStateException: Cannot access database on the main thread because it might mightlock the UI for a long period of time 否则。因此，我设法按照许多教程的说明进行异步查询，但(到目前为止)它并没有我所能实现的那么有意义。

public class NewDetalleDiarioActivity extends AppCompatActivity {

    @Override
    protected void onStart() {
        super.onStart();
        db = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, "database").build();
        findPeriodo();
        findDiario();
    }

    private void findPeriodo() {
        periodo = Diarios.getPeriodo(db);
        if (periodo == null) {
            Intent intent = new Intent(NewDetalleDiarioActivity.this, NewPeriodoActivity.class);
            startActivity(intent);
        }
    }

问题/错误:

如果periodo为空，则启动另一个 Activity ，否则该 Activity 将继续其线程。问题是，当我调试它时(当然，这会减慢进程)periodo 从数据库返回一个实例，但是当我在没有调试的情况下运行代码时，periodo 是空。

public class Diarios {
    public static Periodo getPeriodo(AppDatabase db) {
        return Factory.getIntPeriodo().getPeriodo(db);
    }
}

.

public class Factory {
    private static IntPeriodo intPeriodo;
    public static IntPeriodo getIntPeriodo() {
        return (intPeriodo == null) ? intPeriodo = new BusPeriodo() : intPeriodo;
    }
}

.

public class BusPeriodo implements IntPeriodo {
    // I don't think it's necessary to post the interface...
    @Override
    public Periodo getPeriodo(final AppDatabase db) {
        final Periodo[] periodo = new Periodo[1];
        AsyncTask.execute(new Runnable() {
            @Override
            public void run() { //the async query that is driving me mad.
                periodo[0] = db.periodoDao().getPeriodo(new Date());
            }
        });
        return periodo[0];
    }
}

在不延迟查询的情况下进行select查询的正确方法是什么？选择查询确实有效，我认为没有必要发布它(因为当我调试时它返回一个唯一的结果)，但是当我调试时它返回null无需调试即可运行代码!请帮忙。

Answer 1

解决方案:

正如 @user7041125 所建议的，但我创建了一个带有接口(interface)的新类来将方法调用回 Activity ，如下所示:

public class PeriodoBridge extends AsyncTask<Void, Void, Periodo> implements IntPeriodoBridge {

    private WeakReference<Activity> weakActivity;
    private IntPeriodoBridge caller; //implement this interface in the activity which needs to query
    private AppDatabase db;
    private Periodo periodo;

    public PeriodoBridge(Activity activity, IntPeriodoBridge caller, AppDatabase db) {
        weakActivity = new WeakReference<>(activity);
        this.caller = caller; // assign activity instance to the local interface instance
        this.db = db;
        executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
    }

    @Override
    protected Periodo doInBackground(Void... voids) {
        periodo = Diarios.getPeriodo(db);
        return periodo;
    }

    @Override
    protected void onPostExecute(Periodo periodo) {
        Activity activity = weakActivity.get();
        if (activity == null) {
            return;
        }
        if (periodo == null) {
            Intent intent = new Intent(activity, NewPeriodoActivity.class);
            activity.startActivity(intent);
        } else {
            setPeriodo(periodo);
        }
    }

    @Override  //this is an interface method (IntPeriodoBridge)
    public void setPeriodo(Periodo periodo) {
        caller.setPeriodo(periodo);  //I can set the query result back to the activity class with this
    }

调用该类的init方法。该 Activity 实现了 IntPeriodoBridge，这样我就可以将查询结果对象设置为 Activity 类。