Android(安卓)Room联合AsyncListUtil实现RecyclerView分页加载ORM数据

Android Room联合AsyncListUtil实现RecyclerView分页加载ORM数据
我之前写了一系列关于AsyncListUtil实现RecyclerView和ListView的分页加载机制和技术路线，见附录文章4,5。同时也写了一些列文章介绍Android官方推出的ORM数据库：Room技术，见附录文章1,2。现在结合Android分页加载框架AsyncListUtil，以及Android官方ORM数据库Room，实现数据库数据分页加载到RecyclerView里面。
先给出一个例子，实现一个简单功能，在Android Room数据库中增加一批数据，然后在RecyclerView滚动时候，触发分页加载逻辑，把数据库中的数据分页分段的加载出来，本例中存储的数据为User。
MainActivity.java：

package zhangphil.demo;import android.arch.persistence.room.Room;import android.os.SystemClock;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.support.v7.util.AsyncListUtil;import android.support.v7.widget.LinearLayoutManager;import android.support.v7.widget.RecyclerView;import android.util.Log;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.widget.LinearLayout;import android.widget.TextView;import java.util.List;public class MainActivity extends AppCompatActivity {    private final String TAG = "输出";    private RecyclerView.Adapter mAdapter;    private LinearLayoutManager mLinearLayoutManager;    private AsyncListUtil mAsyncListUtil;    private final int LIMIT = 10;    private UserDatabase mUserDatabase;    private UserDao mUserDao;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        mUserDatabase = Room.databaseBuilder(getApplicationContext(), UserDatabase.class, "users").build();        mUserDao = mUserDatabase.getUserDao();        setContentView(R.layout.activity_main);        RecyclerView mRecyclerView = findViewById(R.id.recycler_view);        mLinearLayoutManager = new LinearLayoutManager(this);        mLinearLayoutManager.setOrientation(LinearLayout.VERTICAL);        mRecyclerView.setLayoutManager(mLinearLayoutManager);        mAdapter = new MyAdapter();        mRecyclerView.setAdapter(mAdapter);        MyViewCallback mViewCallback = new MyViewCallback();        MyDataCallback mDataCallback = new MyDataCallback();        mAsyncListUtil = new AsyncListUtil<>(User.class, LIMIT, mDataCallback, mViewCallback);        mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {            @Override            public void onScrollStateChanged(RecyclerView recyclerView, int newState) {                super.onScrollStateChanged(recyclerView, newState);                Log.d(TAG + "onScrollStateChanged", "onRangeChanged");                mAsyncListUtil.onRangeChanged();            }        });        findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Log.d(TAG + "刷新", "refresh");                mAsyncListUtil.refresh();            }        });        findViewById(R.id.add_data).setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                new Thread(new Runnable() {                    @Override                    public void run() {                        writeDatabase();                    }                }).start();            }        });        //主动刷新数据。        new Thread(new Runnable() {            @Override            public void run() {                SystemClock.sleep(500);                mAsyncListUtil.refresh();            }        }).start();    }    private class MyDataCallback extends AsyncListUtil.DataCallback {        @Override        public int refreshData() {            //更新数据的元素个数。            Log.d(TAG + "refreshData", Integer.MAX_VALUE + "");            return Integer.MAX_VALUE;        }        /**         * 在这里完成耗时的数据加载的耗时任务。         *         * @param data         * @param startPosition         * @param itemCount         */        @Override        public void fillData(User[] data, int startPosition, int itemCount) {            Log.d(TAG + "fillData", startPosition + " , " + itemCount);            List list = mUserDao.getUserWhereUserIdBigThan(startPosition, itemCount);            for (int i = 0; i < list.size(); i++) {                data[i] = list.get(i);            }        }    }    private class MyViewCallback extends AsyncListUtil.ViewCallback {        @Override        public void getItemRangeInto(int[] outRange) {            outRange[0] = mLinearLayoutManager.findFirstVisibleItemPosition();            outRange[1] = mLinearLayoutManager.findLastVisibleItemPosition();            Log.d(TAG + "getItemRangeInto", outRange[0] + " ~ " + outRange[1]);        }        @Override        public void onDataRefresh() {            mAdapter.notifyItemRangeChanged(mLinearLayoutManager.findFirstVisibleItemPosition(), LIMIT);            Log.d(TAG + "onDataRefresh", mLinearLayoutManager.findFirstVisibleItemPosition() + "," + mLinearLayoutManager.findLastVisibleItemPosition());        }        @Override        public void onItemLoaded(int position) {            mAdapter.notifyItemChanged(position);            Log.d(TAG + "onItemLoaded", String.valueOf(position));        }    }    private class MyAdapter extends RecyclerView.Adapter {        public MyAdapter() {            super();        }        @Override        public MyViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {            View view = LayoutInflater.from(getApplicationContext()).inflate(R.layout.item_layout, viewGroup, false);            MyViewHolder holder = new MyViewHolder(view);            return holder;        }        @Override        public void onBindViewHolder(MyViewHolder viewHolder, int i) {            User u = mAsyncListUtil.getItem(i);            viewHolder.setData(u);        }        @Override        public int getItemCount() {            return mAsyncListUtil.getItemCount();        }    }    private class MyViewHolder extends RecyclerView.ViewHolder {        public TextView userId;        public TextView userName;        public TextView userAge;        public MyViewHolder(View itemView) {            super(itemView);            userId = itemView.findViewById(R.id.user_id);            userName = itemView.findViewById(R.id.user_name);            userAge = itemView.findViewById(R.id.user_age);        }        public void setData(User u) {            if (u != null) {                userId.setText("id:" + String.valueOf(u.userId));                userName.setText("姓名:" + String.valueOf(u.name));                userAge.setText("年龄:" + String.valueOf(u.age));            }        }    }    private void writeDatabase() {        Log.d(TAG + "writeDatabase", "开始写入数据...");        for (int i = 0; i < 100; i++) {            User user = new User();            user.name = "张" + i;            user.age = (int) (Math.random() * 100);            user.updateTime = System.currentTimeMillis();            mUserDao.insertUser(user);        }        Log.d(TAG + "writeDatabase", "写入数据库完毕.");    }    @Override    protected void onDestroy() {        super.onDestroy();        mUserDatabase.close();    }}

MainActivity需要的布局文件activity_main.xml：

<?xml version="1.0" encoding="utf-8"?>

item_layout.xml：

<?xml version="1.0" encoding="utf-8"?>

涉及到Android Room的Model，表，Dao。
User.java：

package zhangphil.demo;import android.arch.persistence.room.ColumnInfo;import android.arch.persistence.room.Entity;import android.arch.persistence.room.PrimaryKey;/** * Created by Phil on 2017/11/22. */@Entity(tableName = "user_table")public class User {    @PrimaryKey(autoGenerate = true)    public int userId;    @ColumnInfo(name = "userName")    public String name;    @ColumnInfo(name = "userAge")    public int age = -1;    @ColumnInfo(name = "updateTime")    public long updateTime = -1;}

UserDao.java：

package zhangphil.demo;import android.arch.persistence.room.Dao;import android.arch.persistence.room.Delete;import android.arch.persistence.room.Insert;import android.arch.persistence.room.OnConflictStrategy;import android.arch.persistence.room.Query;import android.arch.persistence.room.Update;import java.util.List;/** * Created by Phil on 2017/11/22. */@Daopublic interface UserDao {    @Query("SELECT * FROM user_table")    public List getAllUsers();    @Query("SELECT * FROM user_table WHERE userId >:uid ORDER BY userId ASC LIMIT :limit")    public List getUserWhereUserIdBigThan(int uid, int limit);    @Query("SELECT * FROM user_table WHERE userId =:uid")    public List getUserWhereUserIdEqual(int uid);    @Query("SELECT * FROM user_table WHERE userId BETWEEN :minId AND :maxId  ORDER BY userId ASC")    public List getUserIdBetween(int minId, int maxId);    @Insert(onConflict = OnConflictStrategy.REPLACE)    public void insertUser(User... users);    @Update    public void updateUser(User... users);    @Delete    public void deleteUser(User... users);}

UserDatabase.java：

package zhangphil.demo;import android.arch.persistence.room.Database;import android.arch.persistence.room.RoomDatabase;/** * Created by Phil on 2017/11/22. */@Database(entities = {User.class}, version = 1, exportSchema = false)public abstract  class UserDatabase  extends RoomDatabase {    public abstract UserDao getUserDao();}

代码运行结果，初始化：

当点击“添加数据”添加完数据库数据后，点击“更新”按钮后：

小结：
（一）在本例中，使用Android Room获得分页数据时候，和之前的UserDao相比，增加了SQL查询约束LIMIT，关于LIMIT，详情见附录文章3。从SQL取数据时，就事先作为分页，把数据分块，而不像以前那样一次性无脑取出数据库表中的全部数据。从而提高了性能。
（二）AsyncListUtil需要延迟的主动refresh()，才能再初始化后且无滚动RecyclerView时加载出来数据。本例中在MainActivity的onCreate最后，开启一个线程，该线程故意延迟一定时间，然后才启动AsyncListUtil的refresh()。之所以这么做，有部分原因是因为AsyncListUtil和RecyclerView之间，UI绘制和数据的更新没有同步，导致第一次初始化加载后，无法刷出来数据。明显的现象就是：如果数据库中有数据，初次打开整个程序，RecyclerView加载出来的每个item是空的，但是明明此时数据库中有数据，按照道理应该初始化加载出来，但是没有。追踪代码就会发现，导致这一现象发生，极大程度上是AsyncListUtil的回调先于RecyclerView完成初始化，导致onDataRefresh和getItemRangeInto
捕捉和使用的第一个和最后一个RecyclerView可见item position均为-1引起。

附录：
1，《Android官方ORM数据库Room技术解决方案：@Embedded内嵌对象（二）》链接：http://blog.csdn.net/zhangphil/article/details/78621009
2，《Android官方ORM数据库Room技术解决方案简介（一）》链接：http://blog.csdn.net/zhangphil/article/details/78611632
3，《SQL数据库查询LIMIT 数据分页》链接：http://blog.csdn.net/zhangphil/article/details/78653677
4，《基于Android官方AsyncListUtil优化改进RecyclerView分页加载机制（一）》链接：http://blog.csdn.net/zhangphil/article/details/78603499
5，《基于Android官方AsyncListUtil优化经典ListView分页加载机制（二）》链接：http://blog.csdn.net/zhangphil/article/details/78645089

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