Android要求不要在主线程上查询，但是异步查询正在延迟执行。

huangapple
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英文:

Android asks for not to query on main thread, but asyc queries are delaying

问题

正如标题所说，Android 需要在主线程之外执行查询，否则会抛出 java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time 错误。因此，我按照许多教程的解释，成功地实现了异步查询，但从目前来看，这并没有太多意义。

  1. public class NewDetalleDiarioActivity extends AppCompatActivity {
  3.     @Override
  4.     protected void onStart() {
  5.         super.onStart();
  6.         db = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, "database").build();
  7.         findPeriodo();
  8.         findDiario();
  9.     }
  11.     private void findPeriodo() {
  12.         periodo = Diarios.getPeriodo(db);
  13.         if (periodo == null) {
  14.             Intent intent = new Intent(NewDetalleDiarioActivity.this, NewPeriodoActivity.class);
  15.             startActivity(intent);
  16.         }
  17.     }
  18. }

问题/错误：

如果 periodo 为空，会启动另一个活动，否则该活动将继续执行线程。
问题在于，当我调试它时（这会减慢进程，当然），periodo 会返回数据库中的实例，但当我不调试代码运行时，periodo 为空。

  1. public class Diarios {
  2.     public static Periodo getPeriodo(AppDatabase db) {
  3.         return Factory.getIntPeriodo().getPeriodo(db);
  4.     }
  5. }
  7. public class Factory {
  8.     private static IntPeriodo intPeriodo;
  9.     public static IntPeriodo getIntPeriodo() {
  10.         return (intPeriodo == null) ? intPeriodo = new BusPeriodo() : intPeriodo;
  11.     }
  12. }
  14. public class BusPeriodo implements IntPeriodo {
  15.     // 我认为没有必要贴出接口部分...
  16.     @Override
  17.     public Periodo getPeriodo(final AppDatabase db) {
  18.         final Periodo[] periodo = new Periodo[1];
  19.         AsyncTask.execute(new Runnable() {
  20.             @Override
  21.             public void run() { // 那个让我发狂的异步查询。
  22.                 periodo[0] = db.periodoDao().getPeriodo(new Date());
  23.             }
  24.         });
  25.         return periodo[0];
  26.     }
  27. }

有没有正确的方法可以在不延迟查询的情况下执行 select 查询呢？
select 查询确实有效，我认为不需要贴出它（因为在 调试 时返回唯一结果），但是在没有调试的情况下运行代码时，它会返回 null！！请帮忙解决。

英文:

As the title says, android needs queries out of main thread since it will trhow java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time otherwise. So I managed to make async queries as many tutorials explain, but it doesn't make so much sense (so far) as I could achieve.

  1. public class NewDetalleDiarioActivity extends AppCompatActivity {
  3.     @Override
  4.     protected void onStart() {
  5.         super.onStart();
  6.         db = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, "database").build();
  7.         findPeriodo();
  8.         findDiario();
  9.     }
  11.     private void findPeriodo() {
  12.         periodo = Diarios.getPeriodo(db);
  13.         if (periodo == null) {
  14.             Intent intent = new Intent(NewDetalleDiarioActivity.this, NewPeriodoActivity.class);
  15.             startActivity(intent);
  16.         }
  17.     }

PROBLEM/ERROR:

If periodo is null, another activity is started, otherwise this one continues its thread.
The problem is that, when I debug it (which slows proceses, of course) periodo returns an instance from the database, but when I run the code without debugging, periodo is null.

  1. public class Diarios {
  2.     public static Periodo getPeriodo(AppDatabase db) {
  3.         return Factory.getIntPeriodo().getPeriodo(db);
  4.     }
  5. }

.

  1. public class Factory {
  2.     private static IntPeriodo intPeriodo;
  3.     public static IntPeriodo getIntPeriodo() {
  4.         return (intPeriodo == null) ? intPeriodo = new BusPeriodo() : intPeriodo;
  5.     }
  6. }

.

  1. public class BusPeriodo implements IntPeriodo {
  2.     // I don't think it's necessary to post the interface...
  3.     @Override
  4.     public Periodo getPeriodo(final AppDatabase db) {
  5.         final Periodo[] periodo = new Periodo[1];
  6.         AsyncTask.execute(new Runnable() {
  7.             @Override
  8.             public void run() { //the async query that is driving me mad.
  9.                 periodo[0] = db.periodoDao().getPeriodo(new Date());
  10.             }
  11.         });
  12.         return periodo[0];
  13.     }
  14. }

What's the proper way to make select queries without getting them delayed?
The select query is indeed working, I don't think is necessary to post it (because it is returning an unique result when I debug), but it returns null when I run the code without debugging!! Please help.

答案1

得分: 0

SOLUTION:

正如 @user7041125 所建议的，但我创建了一个新的类，使用接口将方法回调到活动中，就像这样：

  1. public class PeriodoBridge extends AsyncTask<Void, Void, Periodo> implements IntPeriodoBridge {
  3.     private WeakReference<Activity> weakActivity;
  4.     private IntPeriodoBridge caller; //在需要查询的活动中实现这个接口
  5.     private AppDatabase db;
  6.     private Periodo periodo;
  8.     public PeriodoBridge(Activity activity, IntPeriodoBridge caller, AppDatabase db) {
  9.         weakActivity = new WeakReference<>(activity);
  10.         this.caller = caller; //将活动实例分配给本地接口实例
  11.         this.db = db;
  12.         executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
  13.     }
  15.     @Override
  16.     protected Periodo doInBackground(Void... voids) {
  17.         periodo = Diarios.getPeriodo(db);
  18.         return periodo;
  19.     }
  21.     @Override
  22.     protected void onPostExecute(Periodo periodo) {
  23.         Activity activity = weakActivity.get();
  24.         if (activity == null) {
  25.             return;
  26.         }
  27.         if (periodo == null) {
  28.             Intent intent = new Intent(activity, NewPeriodoActivity.class);
  29.             activity.startActivity(intent);
  30.         } else {
  31.             setPeriodo(periodo);
  32.         }
  33.     }
  35.     @Override  //这是一个接口方法（IntPeriodoBridge）
  36.     public void setPeriodo(Periodo periodo) {
  37.         caller.setPeriodo(periodo);  //我可以通过这个方法将查询结果返回给活动类
  38.     }
  39. }
  41. 调用这个类的 init 方法。活动实现了 `IntPeriodoBridge`，这样我就可以将查询结果对象设置到活动类中。
英文:

SOLUTION:

As @user7041125 suggested, but instead I made a new class with an interface to call methods back to the activity, like this:

  1. public class PeriodoBridge extends AsyncTask&lt;Void, Void, Periodo&gt; implements IntPeriodoBridge {
  3.     private WeakReference&lt;Activity&gt; weakActivity;
  4.     private IntPeriodoBridge caller; //implement this interface in the activity which needs to query
  5.     private AppDatabase db;
  6.     private Periodo periodo;
  8.     public PeriodoBridge(Activity activity, IntPeriodoBridge caller, AppDatabase db) {
  9.         weakActivity = new WeakReference&lt;&gt;(activity);
  10.         this.caller = caller; // assign activity instance to the local interface instance
  11.         this.db = db;
  12.         executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
  13.     }
  15.     @Override
  16.     protected Periodo doInBackground(Void... voids) {
  17.         periodo = Diarios.getPeriodo(db);
  18.         return periodo;
  19.     }
  21.     @Override
  22.     protected void onPostExecute(Periodo periodo) {
  23.         Activity activity = weakActivity.get();
  24.         if (activity == null) {
  25.             return;
  26.         }
  27.         if (periodo == null) {
  28.             Intent intent = new Intent(activity, NewPeriodoActivity.class);
  29.             activity.startActivity(intent);
  30.         } else {
  31.             setPeriodo(periodo);
  32.         }
  33.     }
  35.     @Override  //this is an interface method (IntPeriodoBridge)
  36.     public void setPeriodo(Periodo periodo) {
  37.         caller.setPeriodo(periodo);  //I can set the query result back to the activity class with this
  38.     }

Call the init method of this class. The activity implements IntPeriodoBridge and in that way I can set the query result object to the activity class.

huangapple
  • 本文由 发表于 2020年5月2日 12:35:45
  • 转载请务必保留本文链接：https://java.coder-hub.com/61554518.html
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