Paging

介绍

Paging的主要功能就是一次加载小部分数据，可以减少网络带宽和系统资源的使用

使用

本次的Demo结合了之前的数据库Room，然后那个刚好是用Kotlin写的，这次就使用Java写一遍，当然引入Paging后代码会有点出入

1. 添加·Room·和·Paging·依赖

def room_version = "2.2.1"
implementation "androidx.room:room-runtime:$room_version"
annotationProcessor "androidx.room:room-compiler:$room_version" // For Kotlin use kapt instead of annotationProcesso

def paging_version = "2.1.0"
implementation "androidx.paging:paging-runtime:$paging_version" // For Kotlin use paging-runtime-kt

2. 创建User实体类

代码还是和之前一样

package com.example.pagingdemo;

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

@Entity(tableName = "user")
public class User {

    @PrimaryKey(autoGenerate = true)
    private int id;
    @ColumnInfo(name = "name")
    private String name;

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

2. 创建UserDao接口类

这部分要注意，查询全部的返回值从LiveData>改成了DataSource.Factory

package com.example.pagingdemo;

import androidx.paging.DataSource;
import androidx.room.Dao;
import androidx.room.Insert;
import androidx.room.Query;

@Dao
public interface UserDao {

    @Insert
    void insertUser(User... users);

    @Query("DELETE FROM user")
    void deleteUsers();

    @Query("SELECT * FROM user")
    DataSource.Factory getAllUsers();
}

3. 创建DataBase抽象类

这部分也和之前一样保持不变

@Database(entities = {User.class}, version = 1)
public abstract class AppDataBase extends RoomDatabase {

    private static AppDataBase dataBase;

    static synchronized AppDataBase getInstance(Context context) {
        if (dataBase == null) {
            dataBase = Room.databaseBuilder(context, AppDataBase.class, "user_database").build();
        }
        return dataBase;
    }

    abstract UserDao getUserDao();
}

4. 创建RecyclerView的适配器

数据还是和之前的一样，用RecyclerView展示，但是使用的适配器从ListAdapter换成了PageLitAdapter

package com.example.pagingdemo;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.paging.PagedListAdapter;
import androidx.recyclerview.widget.DiffUtil;
import androidx.recyclerview.widget.RecyclerView;

public class RecyclerViewAdapter extends PagedListAdapter {


    protected RecyclerViewAdapter() {
        super(new DiffUtil.ItemCallback() {
            @Override
            public boolean areItemsTheSame(@NonNull User oldItem, @NonNull User newItem) {
                return oldItem.getId() == newItem.getId();
            }

            @Override
            public boolean areContentsTheSame(@NonNull User oldItem, @NonNull User newItem) {
                return oldItem.getName().equals(newItem.getName());
            }
        });
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.rv_item, parent, false);
        return new MyViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        User user = getItem(position);
        if (user == null) {
            holder.textView.setText("loading ... ...");
        } else {
            holder.textView.setText(user.getName());
        }
    }

    class MyViewHolder extends RecyclerView.ViewHolder {

        TextView textView;

        MyViewHolder(@NonNull View itemView) {
            super(itemView);
            textView = itemView.findViewById(R.id.textView);
        }
    }
}

这里在onBindViewHolder中做了非空判断，为了就是当用户快速滑动视图时，数据可能还没加载出来，就给一个默认文本，提升用户体验

5. 修改MainActivity

代码有点多，但实际上和之前的相比只是发生了一点点变化

因为刚才在创建UserDao接口类的时候说过，查询全部的返回值从LiveData>改成了DataSource.Factory，所以想要实现数据观察就需要得到LiveData，而实现的方式就是：

LiveData> liveUserList = new LivePagedListBuilder<>(userDao.getAllUsers(), 10).build();

package com.example.pagingdemo;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.Observer;
import androidx.paging.LivePagedListBuilder;
import androidx.paging.PagedList;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    private AppDataBase dataBase;
    private LiveData> liveUserList;
    private UserDao userDao;
    private RecyclerView recyclerView;
    private RecyclerViewAdapter adapter;
    private Button loadBtn, clearBtn;

    @Override

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

        recyclerView = findViewById(R.id.recyclerView);
        recyclerView.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false));
        adapter = new RecyclerViewAdapter();
        recyclerView.setAdapter(adapter);
        dataBase = AppDataBase.getInstance(this);
        userDao = dataBase.getUserDao();
        liveUserList = new LivePagedListBuilder<>(userDao.getAllUsers(), 10).build();
        liveUserList.observe(this, new Observer>() {
            @Override
            public void onChanged(PagedList users) {
                adapter.submitList(users);
            }
        });

        loadBtn = findViewById(R.id.load);
        clearBtn = findViewById(R.id.clear);
        loadBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                for (int i = 0; i < 1000; i++) {
                    User user = new User();
                    user.setName("用户" + (i + 1));
                    new InsertAsyncTask(userDao).execute(user);
                }
            }
        });

        clearBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new ClearAsyncTask(userDao).execute();
            }
        });
    }

    static class InsertAsyncTask extends AsyncTask {

        UserDao userDao;

        InsertAsyncTask(UserDao userDao) {
            this.userDao = userDao;
        }
        @Override
        protected Void doInBackground(User... users) {
            userDao.insertUser(users);
            return null;
        }
    }

    static class ClearAsyncTask extends AsyncTask {
        UserDao userDao;

        ClearAsyncTask(UserDao userDao) {
            this.userDao = userDao;
        }
        @Override
        protected Void doInBackground(Void... voids) {
            userDao.deleteUsers();
            return null;
        }
    }
}

到此就完成了Paging的简单使用