android - 如何从 Android LiveData> 获取简单的 List？

Question

我是 Android 的新手，通过开发简单的应用程序来学习相同的知识，该应用程序由我使用 android Room 数据库访问的单个客户表组成。

客户实体类是

@Entity(tableName = "Customers")
public class CustomerEntity {
@PrimaryKey(autoGenerate = true)
private int customerId;
private String customerName;
private String customerAddress;
private String customerZipCode;
private String customerEMailId;
}

Customer Dao 接口(interface)是

@Dao
public interface CustomerDao {
@Insert
    public void insertCustomer( CustomerEntity customerEntity );
    @Update
    public void updateCustomer( CustomerEntity customerEntity );
    @Delete
    public void deleteCustomer( CustomerEntity customerEntity) ;
    @Query("SELECT * FROM Customers")
    LiveData<List<CustomerEntity>> getAllCustomers();
    @Query("SELECT * FROM Customers WHERE customerZipCode == :givenZipCode ")
    LiveData<List<CustomerEntity>> getGivenZipCodeCustomer( String givenZipCode);
    @Query("SELECT * FROM Customers WHERE customerZipCode == :givenZipCode ")
    List<CustomerEntity> getGivenZipCodeCustomerList( String givenZipCode);
    @Query("SELECT * FROM Customers WHERE customerZipCode == :givenZipCode ")
    Cursor getGivenZipCodeCustomerCursor(String givenZipCode
}

Customer Repository 类(部分显示)

public List<CustomerEntity> getGivenZipCodeCustomersList(CustomerEntity customerEntity){
            CustomerRepository.CustomerZipCodeAsyncTask customerZipCodeAsyncTask ;
            customerZipCodeAsyncTask = new CustomerRepository.CustomerZipCodeAsyncTask( customerDao );
            givenZipCodeCustomersList = customerZipCodeAsyncTask.doInBackground( customerEntity );
            return givenZipCodeCustomersList ;
        }

        private static class CustomerZipCodeAsyncTask extends AsyncTask< CustomerEntity ,
                    Void , List < CustomerEntity > > {
            private CustomerDao customerDao;
            private CustomerZipCodeAsyncTask ( CustomerDao customerDao ){
                this.customerDao = customerDao;
            }

            @Override
            protected List < CustomerEntity > doInBackground(CustomerEntity... customerEntities) {
                String zipCode = customerEntities[0].getCustomerZipCode();
                return ( customerDao.getGivenZipCodeCustomerList( zipCode ) ) ;
            }
        }

当我尝试从应用程序的其他部分获取客户列表时，我收到消息

"java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time."

另一方面，如果我尝试通过执行另一个异步进程来获取客户列表，该进程成功返回 LiveData 列表，但当我在 LiveData 上使用 getValue() 时返回 null。

在我执行此任务的应用程序部分，我不希望检索到的列表会发生变化，也不需要将其呈现给用户。所以我不需要观察这个列表。我需要一个简单的列表，我可以从中访问列表项并进一步处理它们。

我使用的是 Android Studio 3.4 Canary 9，androidx room_version = "2.1.0-alpha03", androix lifecycle_version = "2.0.0"

Answer 1

通过在 LiveData 上使用 getValue()，您没有使用 LiveData 的全部功能。在您的 CustomerRepository 类中，将发布的方法更改为:

public LiveData<List<CustomerEntity>> getGivenZipCodeCustomersList(CustomerEntity customerEntity) {
  String zipCode = customerEntity.getCustomerZipCode();
  return customerDao.getGivenZipCodeCustomerList(zipCode);
}

那么你的 Activity 可能看起来像这样

public class MainActivity extends AppCompatActivity {

  private CustomerRepository customerRepository;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    customerRepository = ... //init your repo here
    CustomerEntity customerEntity = ... //init your customer entity here

    customerRepository.getGivenZipCodeCustomersList(customerEntity).observe(this, new Observer<List<CustomerEntity>> {
      @Override
      public void onChanged(@Nullable List<CustomerEntity> customerEntities) {
        //do stuff with your customerEntities here
      }
    });

    //.. more code
  }
}

这样做，每当您的客户匹配到您的数据库中 CustomerEntity 中的邮政编码更改时，onChanged 就会触发，以便您可以自动处理更改。

有关详细信息，请参阅 documentation如何使用 LiveData。

希望对您有所帮助!