如何在单个活动中加载多个 AndroidViewModel 以访问使用 Room 从多个表返回值的 LiveData

Question

我正在 android 中开发一个电子商务应用程序，我必须在其中创建多个表，在其上插入数据并从该表中获取活动/片段中的数据（Spinner 中的加载值，基于运行条件的子查询等）。为此，我声明了 2 个表。地址，银行如下

银行.java

@Entity
public class Bank {

    @PrimaryKey(autoGenerate = true)
    public int id;
    public String bankId;
    public String bankName;
    public String bankType;

    public Bank() {
    }

    public String getBankId() {
        return bankId;
    }

    public String getBankName() {
        return bankName;
    }

    public String getBankType() {
        return bankType;
    }
}

地址.java

@Entity
public class Address {

    @PrimaryKey(autoGenerate = true)
    public int id;
    public String state;
    public String district;
    public String block;
    public String id_panchayat;
    public String panchayat;

    public Address() {
    }

    public String getState() {
        return state;
    }

    public String getDistrict() {
        return district;
    }

    public String getBlock() {
        return block;
    }

    public String getId_panchayat() {
        return id_panchayat;
    }

    public String getPanchayat() {
        return panchayat;
    }
}

这两个表的 Dao 类是：

银行道

@Dao
public interface BankDao {

    @Insert(onConflict = REPLACE)
    void insertSingleBank (Bank bank);

    @Insert(onConflict = REPLACE)
    void insertMultipleBank (List<Bank> bankList);

    @Query("SELECT * FROM Bank")
    LiveData<List<Bank>> getAllBank();

    @Query("SELECT bankName FROM Bank")
    List<String> getAllBankData ();

    @Query("SELECT * FROM Bank WHERE bankName = :bankName")
    Bank getBankSubData (String bankName);

    @Query("DELETE FROM Bank")
    void deleteBank();
}

AddressDao.java

@Dao
public interface AddressDao {

    @Insert(onConflict = REPLACE)
    void insertSingleAddress(Address address);

    @Insert(onConflict = REPLACE)
    void insertMultipleAddress(List<Address> addressList);

    @Query("SELECT * FROM Address")
    LiveData<List<Address>> getAllAddress();

    @Query("SELECT state FROM Address")
    List<String> getAllStateList();

    @Query("SELECT district FROM Address WHERE state = :state")
    List<String> getDistrictListAll(String state);

    @Query("SELECT block FROM Address WHERE state = :state AND district = :district")
    List<String> getBlockListAll(String state, String district);

    @Query("SELECT panchayat FROM Address WHERE state = :state AND district = :district AND block = :block")
    List<String> getPanchayatListAll(String state, String district, String block);

    @Query("DELETE FROM Address")
    void deleteAddress();
}

这两个类的 ViewModel 是

地址模型

public class AddressModel extends AndroidViewModel {

    private RoomDatabaseHelper roomDatabase;
    private final LiveData<List<Address>> getAllAddress;

    public AddressModel(Application application) {
        super(application);
        roomDatabase = RoomDatabaseHelper.getInstance(this.getApplication());
        getAllAddress = roomDatabase.daoAddress().getAllAddress();
    }

    public void insertSingleAddress(Address address) {
        new InsertSingleAddressAsyncTask(roomDatabase).execute(address);
    }

    private static class InsertSingleAddressAsyncTask extends AsyncTask<Address, Void, Void> {

        private RoomDatabaseHelper db;

        InsertSingleAddressAsyncTask(RoomDatabaseHelper userDatabase) {
            db = userDatabase;
        }

        @Override
        protected Void doInBackground(Address...params) {
            db.daoAddress().insertSingleAddress(params[0]);
            return null;
        }
    }

    public void insertMultipleAddress(List<Address> registrationList) {
        new InsertMultipleAddressAsyncTask(roomDatabase).execute(registrationList);
    }

    private static class InsertMultipleAddressAsyncTask extends AsyncTask<List<Address>, Void, Void> {

        private RoomDatabaseHelper db;

        InsertMultipleAddressAsyncTask(RoomDatabaseHelper userDatabase) {
            db = userDatabase;
        }

        @Override
        protected Void doInBackground(List<Address>...params) {
            db.daoAddress().insertMultipleAddress(params[0]);
            return null;
        }
    }

    public LiveData<List<Address>> getAllAddress() {
        return getAllAddress;
    }
}

银行模型

public class BankModel extends AndroidViewModel {

    private RoomDatabaseHelper roomDatabase;
    private final LiveData<List<Bank>> getAllBank;

    public BankModel(Application application) {
        super(application);
        roomDatabase = RoomDatabaseHelper.getInstance(this.getApplication());
        getAllBank = roomDatabase.daoBank().getAllBank();
    }

    public void insertSingleBank(Bank bank) {
        new InsertSingleBankAsyncTask(roomDatabase).execute(bank);
    }

    private static class InsertSingleBankAsyncTask extends AsyncTask<Bank, Void, Void> {

        private RoomDatabaseHelper db;

        InsertSingleBankAsyncTask(RoomDatabaseHelper userDatabase) {
            db = userDatabase;
        }

        @Override
        protected Void doInBackground(Bank...params) {
            db.daoBank().insertSingleBank(params[0]);
            return null;
        }
    }

    public void insertMultipleBank(List<Bank> registrationList) {
        new InsertMultipleBankAsyncTask(roomDatabase).execute(registrationList);
    }

    private static class InsertMultipleBankAsyncTask extends AsyncTask<List<Bank>, Void, Void> {

        private RoomDatabaseHelper db;

        InsertMultipleBankAsyncTask(RoomDatabaseHelper userDatabase) {
            db = userDatabase;
        }

        @Override
        protected Void doInBackground(List<Bank>...params) {
            db.daoBank().insertMultipleBank(params[0]);
            return null;
        }
    }

    public LiveData<List<Bank>> getAllBank() {
        return getAllBank;
    }
}

RoomDatabase 类是

RoomDatabaseHelper.java

@Database(entities = {Address.class, Bank.class}, version = 1, exportSchema = false)
public abstract class RoomDatabaseHelper extends RoomDatabase {

    private static RoomDatabaseHelper sInstance;

    public static synchronized RoomDatabaseHelper getInstance(Context context) {
        if (sInstance == null) {
            sInstance = Room.databaseBuilder(context.getApplicationContext(), RoomDatabaseHelper.class, "DemoTable").build();
        }
        return sInstance;
    }

    public abstract AddressDao daoAddress();

    public abstract BankDao daoBank();
}

在我的片段中：

BankModel bankModel = ViewModelProviders.of(this).get(BankModel.class);
AddressModel addressModel = ViewModelProviders.of(this).get(AddressModel.class);

现在根据文档访问getAllBank()和getAllAddress()来自两个模型类我必须这样定义

addressModel.getAllAddress().observe(mActivity, new Observer<List<Address>>() {
    @Override
    public void onChanged(List<Address> addressList) {
    }
});

addressModel.getAllBank().observe(mActivity, new Observer<List<Bank>>() {
    @Override
    public void onChanged(final List<Bank> bankList) {
    }
});

现在这两种方法都是AsyncTask一样的。所以在活动负载上我不能改变 UI 相关的东西。我必须将整个 UI 放在onChange. 如何同时访问两个观察者。因为嵌套观察者不是正确的解决方案。而且我还需要许多复杂的数据库查询。因此，为此我必须定义这么多观察者或者是否有任何其他解决方案。

Answer 1

首先，ViewModel 应该用作片段或活动的助手，以检索和处理视图所需的数据。因此，一个 Activity/Fragment 应该有一个视图模型。在这里，您将它们用作某种存储库。

如果你仍然想这样使用它们： How can I access both observer at the same

在 onChanged 上调用一个方法，例如tryUpdateUi()检查getAllBank().getValue()和getAllAddress().getValue()是否不为空。如果是这样，您可以进行更新/计算等。

尽管我更喜欢在 ViewModel 中进行此检查和数据处理，但使用另一个 LiveData 将处理后的模型传递给 Fragment。它可以通过 MediatorLiveData 来实现，文档