房间数据库查询 [英] Room Database Query

问题描述

我是 Room 的新手，我正在尝试 query 我的 database 以获取 row从中.我尝试通过 querying 使用 primary key 这是 id 来这样做，但问题是我不知道如何返回目标 object来自 repository.

I'm new to Room and i'm trying to query my database to get a row from it. I attempted doing so by querying it with the primary key which is id but the problem is i don't know how to return the target object from the repository.

这是Dao

@Query("SELECT * FROM targets WHERE id = :id LIMIT 1")
Targets findTargetById(int id);

这是存储库类

 public Targets findTarget (int id) {
    new findTargetByIDAsyncTask(mTargetsDao).execute(id);

}

   private static class findTargetByIDAsyncTask extends AsyncTask<Integer, Void, Targets> {

    private TargetsDao mAsyncTaskDao;

    findTargetByIDAsyncTask(TargetsDao dao) {
        mAsyncTaskDao = dao;
    }


    @Override
    protected Targets doInBackground(Integer... integers) {

        return mAsyncTaskDao.findTargetById(integers[0]);
    }

    @Override
    protected void onPostExecute(Targets targets) {
        super.onPostExecute(targets);
    }
}

推荐答案

返回结果有两种方式.

第一种方法是调用 AsyncTask.get() 方法，但如果任务超过 5 秒，它仍然会持有一个导致 ANR 的 MainThread:

The first way is to call AsyncTask.get() method, but it will still hold a MainThread what leads to ANR if a task will longer than 5 seconds:

public Targets findTarget (int id) {
    return new findTargetByIDAsyncTask(mTargetsDao).execute(id).get();
}

第二种方式更复杂，但它不会保存主线程.您应该添加一个回调类:

The second way is more complicated but it will not hold the MainThread. You should add a Callback class:

public interface Callback {
        void onSuccess(Targets targets);
    }

存储库中的每个方法都如下所示:

Each method of your repository will look like that:

public void findTarget (Callback callback, int id) {
        new findTargetByIDAsyncTask(mTargetsDao, callback).execute(id);
    }

AsynTask 看起来像这样:

And AsynTask will look like that:

private static class FindTargetByIDAsyncTask extends AsyncTask<Integer, Void, Targets> {

    private final TargetsDao mAsyncTaskDao;
    private final Callback callback;

    FindTargetByIDAsyncTask(TargetsDao dao, Callback callback) {
        mAsyncTaskDao = dao;
        this.callback = callback;
    }


    @Override
    protected Targets doInBackground(Integer... integers) {
        return mAsyncTaskDao.findTargetById(integers[0]);
    }

    @Override
    protected void onPostExecute(Targets targets) {
        callback.onSuccess(targets);
    }
}

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