如何在主线程上的onCreate/中获取房间列表的大小? [英] How to get size of Room list in onCreate/ on main thread?

问题描述

如何轻松获得数据库大小的计数，然后根据数据库是否为空来执行适当的操作?

How can I easily get the count of the database size so I can then do the appropriate actions based on whether the DB is empty or not?

我有一个App DB，View Model，Repository，Dao和所有其他组件，并且我的对象插入了...但是我无法调用onCreate()数据库中列表的大小.当我尝试获取mAppDatabase.vehicleDao().getAll().getValue().size()或mVehicleViewModel.getAll().getValue().size()时，会出现空指针异常.

I have an App DB, View Model, Repository, Dao, and all the other pieces, and my objects insert... But I cannot call in onCreate() the size of the list in the DB. When I ever try to get mAppDatabase.vehicleDao().getAll().getValue().size(), or mVehicleViewModel.getAll().getValue().size(), I get null pointer exceptions.

但是我知道我的对象正在插入，因为当我运行一个可观察对象时，我可以记录它们的信息...但是我无法在onCreate的主线程/上获得计数.帮助！下面的示例代码:

However I know my objects are inserting because when I run an observable, I can log their information... but I cannot get the count on the main thread/ in onCreate. Help! Example code below:

protected void onCreate(Bundle savedInstanceState) {
...
mAppDatabase = AppDatabase.getInstance(MyActivity.this);
Log.d("LISTSIZEAPP", String.valueOf(mAppDatabase.myDao().getAll().getValue().size()));
ObserveItems();

OR

protected void onCreate(Bundle savedInstanceState) {
...
mViewModel = ViewModelProviders.of(this).get(MyViewModel.class);
Log.d("LISTSIZEVM",  String.valueOf(mViewModel.getAll().getValue().size()));
ObserveItems();

两者的

^ Null指针异常(尝试调用接口方法 空对象上的'java.lang.Object [] java.util.List.toArray()' 参考)...

^Null pointer exception for both (Attempt to invoke interface method 'java.lang.Object[] java.util.List.toArray()' on a null object reference)...

但是:

private void ObserveItems() {
    mViewModel.getAll().observe(this, new Observer<List<Foo>>() {
        @Override
        public void onChanged(@Nullable final List<Foo> foos) {
            mFoos= foos;
            for (Vehicle v: mFoos) {
                Log.d("ROOM INFO - FOOS", v.getFooTitle());
            }
        }
    });
}

我可以记录我想要的所有信息.这样就可以清楚地插入项目了.是什么赋予了?我想念什么?谢谢.

I can log all the info I want. So the items are CLEARLY inserted. What gives? What am I missing? Thank you.

推荐答案

这是从LiveData同步访问数据:

This is accessing data synchronously from the LiveData:

Log.d("LISTSIZEVM",  String.valueOf(mViewModel.getAll().getValue().size()));

但是此数据可能尚未到达.

But this data might no have arrived yet.

尝试将此查询添加到您的Dao中，请参见

Try adding this query to your Dao, see google samples:

@Query("SELECT COUNT(column) FROM table")
int getDataCount();

正如@CommonsWare指出的那样，不能从主UI线程调用此方法，因为它是数据库操作，它将阻塞UI线程并引发错误.您可以在主线程上调用它，也可以返回LiveData<Integer>并观察值，就像处理数据列表一样.

As pointed out by @CommonsWare this cannot be called from the main UI thread because it's a database opearation that will block the UI thread and will throw an error. You can either call this off the main thread, or return a LiveData<Integer> and observe the value, like you did with the list of data.

@Query("SELECT COUNT(column) FROM table")
LiveData<Integer> getDataCount();

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