关于Android Room中的一些代码与View示例项目的混淆

在 Android 开发中，Room 是一个持久性库，它提供了一个抽象层来访问 SQLite 数据库。结合 ViewModel 和 LiveData，可以实现一个高效且响应式的数据层。以下是一个示例项目，展示了如何在 Android 应用中使用 Room、ViewModel 和 LiveData。

1. 添加依赖

首先，在你的 build.gradle 文件中添加 Room、ViewModel 和 LiveData 的依赖：

dependencies {
    def room_version = "2.4.2"
    def lifecycle_version = "2.4.1"

    implementation "androidx.room:room-runtime:$room_version"
    annotationProcessor "androidx.room:room-compiler:$room_version"
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
    implementation "androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version"
}

2. 创建实体类

创建一个实体类来表示数据库中的表。例如，一个简单的用户表：

import androidx.room.Entity;
import androidx.room.PrimaryKey;

@Entity(tableName = "user_table")
public class User {
    @PrimaryKey(autoGenerate = true)
    private int id;
    private String name;

    public User(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }
}

3. 创建 DAO 接口

创建一个 DAO 接口来定义访问数据库的方法：

import androidx.lifecycle.LiveData;
import androidx.room.Dao;
import androidx.room.Insert;
import androidx.room.Query;

import java.util.List;

@Dao
public interface UserDao {
    @Insert
    void insert(User user);

    @Query("SELECT * FROM user_table")
    LiveData<List<User>> getAllUsers();
}

4. 创建数据库类

创建一个数据库类来持有数据库并作为应用的主要访问点：

import android.content.Context;
import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;

@Database(entities = {User.class}, version = 1, exportSchema = false)
public abstract class UserDatabase extends RoomDatabase {
    public abstract UserDao userDao();

    private static volatile UserDatabase INSTANCE;

    public static UserDatabase getDatabase(final Context context) {
        if (INSTANCE == null) {
            synchronized (UserDatabase.class) {
                if (INSTANCE == null) {
                    INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                            UserDatabase.class, "user_database")
                            .build();
                }
            }
        }
        return INSTANCE;
    }
}

5. 创建 Repository 类

创建一个 Repository 类来管理数据操作：

import android.app.Application;
import androidx.lifecycle.LiveData;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class UserRepository {
    private UserDao userDao;
    private LiveData<List<User>> allUsers;
    private static final int NUMBER_OF_THREADS = 4;
    static final ExecutorService databaseWriteExecutor =
            Executors.newFixedThreadPool(NUMBER_OF_THREADS);

    public UserRepository(Application application) {
        UserDatabase db = UserDatabase.getDatabase(application);
        userDao = db.userDao();
        allUsers = userDao.getAllUsers();
    }

    public LiveData<List<User>> getAllUsers() {
        return allUsers;
    }

    public void insert(User user) {
        databaseWriteExecutor.execute(() -> {
            userDao.insert(user);
        });
    }
}

6. 创建 ViewModel 类

创建一个 ViewModel 类来提供数据给 UI 并且生存期比 UI 长：

import android.app.Application;
import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;
import java.util.List;

public class UserViewModel extends AndroidViewModel {
    private UserRepository repository;
    private LiveData<List<User>> allUsers;

    public UserViewModel(Application application) {
        super(application);
        repository = new UserRepository(application);
        allUsers = repository.getAllUsers();
    }

    public LiveData<List<User>> getAllUsers() {
        return allUsers;
    }

    public void insert(User user) {
        repository.insert(user);
    }
}

7. 创建 UI

在你的 Activity 或 Fragment 中使用 ViewModel 和 LiveData 来显示数据：

import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import java.util.List;

public class MainActivity extends AppCompatActivity {
    private UserViewModel userViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        RecyclerView recyclerView = findViewById(R.id.recyclerview);
        final UserListAdapter adapter = new UserListAdapter(this);
        recyclerView.setAdapter(adapter);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));

        userViewModel = new ViewModelProvider(this).get(UserViewModel.class);
        userViewModel.getAllUsers().observe(this, new Observer<List<User>>() {
            @Override
            public void onChanged(List<User> users) {
                adapter.setUsers(users);
            }
        });

        // Example of inserting a new user
        findViewById(R.id.button_add).setOnClickListener(v -> {
            User user = new User("New User");
            userViewModel.insert(user);
        });
    }
}

8. 创建 RecyclerView Adapter

创建一个 RecyclerView Adapter 来显示用户列表：

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.recyclerview.widget.RecyclerView;
import java.util.List;

public class UserListAdapter extends RecyclerView.Adapter<UserListAdapter.UserViewHolder> {
    private final LayoutInflater mInflater;
    private List<User> mUsers;

    UserListAdapter(Context context) {
        mInflater = LayoutInflater.from(context);
    }

    @Override
    public UserViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemView = mInflater.inflate(R.layout.recyclerview_item, parent, false);
        return new UserViewHolder(itemView);
    }

    @Override
    public void onBindViewHolder(UserViewHolder holder, int position) {
        if (mUsers != null) {
            User current = mUsers.get(position);
            holder.userItemView.setText(current.getName());
        } else {
            holder.userItemView.setText("No User");
        }
    }

    void setUsers(List<User> users) {
        mUsers = users;
        notifyDataSetChanged();
    }

    @Override
    public int getItemCount() {
        if (mUsers != null)
            return mUsers.size();
        else return 0;
    }

    class UserViewHolder extends RecyclerView.ViewHolder {
        private final TextView userItemView;

        private UserViewHolder(View itemView) {
            super(itemView);
            userItemView = itemView.findViewById(R.id.textView);
        }
    }
}

9. 布局文件

创建相应的布局文件：

activity_main.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button_add"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Add User" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerview"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

recyclerview_item.xml:

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/textView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="16dp"
    android:textSize="20sp" />

通过以上步骤，你可以在 Android 应用中使用 Room、ViewModel 和 LiveData 来实现一个简单的用户管理系统。这个示例展示了如何在 Room 数据库中存储和检索数据，并使用 ViewModel 和 LiveData 在 UI 中显示这些数据。