【Android-Jetpack进阶】5、Room 数据库、ViewModel 内通过 Database 查到 LiveData 并通知变化、用 createFromAsset() 预填充数据库

五、Room 操作数据库

5.1 用 Entity、Dao、Database 操作数据库

数据库的每个表，都对应一个 Entity，一个 Dao（Dao 负责增删改查操作）

新建项目 Jetpack5RoomTest，在 build.gradle 添加如下依赖：

plugins {
    
    
    id 'com.android.application'
    id 'org.jetbrains.kotlin.android'
    id 'kotlin-kapt'
}

dependencies {
    
    
    def room_version = "2.4.3"
    implementation "androidx.room:room-runtime:$room_version"
    annotationProcessor "androidx.room:room-compiler:$room_version"
    kapt "androidx.room:room-compiler:$room_version"
}

首先，新建 database/Student.kt，代码如下：

package com.bignerdranch.android.jetpack5roomtest.database

import androidx.room.ColumnInfo
import androidx.room.Entity
import androidx.room.Ignore
import androidx.room.PrimaryKey


@Entity(tableName = "student")
class Student {
    
    
    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "id", typeAffinity = ColumnInfo.INTEGER)
    var id = 0

    @ColumnInfo(name = "name", typeAffinity = ColumnInfo.TEXT)
    var name: String

    @ColumnInfo(name = "age", typeAffinity = ColumnInfo.TEXT)
    var age: String

    /**
     * Room会使用这个构造器来存储数据，也就是当你从表中得到Student对象时候，Room会使用这个构造器
     */
    constructor(id: Int, name: String, age: String) {
    
    
        this.id = id
        this.name = name
        this.age = age
    }

    /**
     * 由于Room只能识别和使用一个构造器，如果希望定义多个构造器，你可以使用Ignore标签，让Room忽略这个构造器
     * 同样，@Ignore标签还可用于字段，使用@Ignore标签标记过的字段，Room不会持久化该字段的数据
     */
    @Ignore
    constructor(name: String, age: String) {
    
    
        this.name = name
        this.age = age
    }
}

然后，新建 database/StudentDao.kt，封装增删改查操作，代码如下：

package com.bignerdranch.android.jetpack5roomtest.database

import androidx.room.*

@Dao
interface StudentDao {
    
    
    @Insert
    fun insertStudent(student: Student?)

    @Delete
    fun deleteStudent(student: Student?)

    @Update
    fun updateStudent(student: Student?)

    @get:Query("SELECT * FROM student")
    val studentList: List<Student?>?

    @Query("SELECT * FROM student WHERE id = :id")
    fun getStudentById(id: Int): Student?
}

然后，新建 database/MyDatabase.kt，其中有 studentDao 属性，代码如下：

package com.bignerdranch.android.jetpack5roomtest.database

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

@Database(entities = [Student::class], version = 1)
abstract class MyDatabase : RoomDatabase() {
    
    
    abstract fun studentDao(): StudentDao?

    companion object {
    
    
        private const val DATABASE_NAME = "my_db"
        private var databaseInstance: MyDatabase? = null

        @Synchronized
        fun getInstance(context: Context): MyDatabase? {
    
    
            if (databaseInstance == null) {
    
    
                databaseInstance = Room.databaseBuilder(context.applicationContext, MyDatabase::class.java, DATABASE_NAME).build()
            }
            return databaseInstance
        }
    }
}

activity_main.xml 的布局如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/btnInsertStudent"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="12dp"
        android:layout_gravity="center_horizontal"
        android:textAllCaps="false"
        android:text="Add a Student"/>

    <ListView
        android:id="@+id/lvStudent"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

</LinearLayout>

activity_main.xml 的布局如下：

然后，新建 list_item_student.xml 布局，布局如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="horizontal"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
    android:paddingTop="12dp"
    android:paddingBottom="12dp">

    <TextView
        android:id="@+id/tvId"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:layout_weight="1"/>

    <TextView
        android:id="@+id/tvName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:layout_weight="1"/>

    <TextView
        android:id="@+id/tvAge"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:layout_weight="1"/>

</LinearLayout>

list_item_student.xml 的布局如下：

新建 dialog_layout_student.xml，布局如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <EditText
        android:id="@+id/etName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Name"
        android:layout_weight="1"
        android:autofillHints="" />

    <EditText
        android:id="@+id/etAge"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Age"
        android:layout_weight="1"
        android:autofillHints="" />

</LinearLayout>

dialog_layout_student.xml 的效果如下：

接下来，在 MainActivity 中可通过 database 来操作数据库，代码如下：

package com.bignerdranch.android.jetpack5roomtest

import android.content.DialogInterface
import android.os.AsyncTask
import android.os.Bundle
import android.text.TextUtils
import android.view.View
import android.widget.AdapterView.OnItemLongClickListener
import android.widget.EditText
import android.widget.ListView
import android.widget.Toast
import androidx.appcompat.app.AlertDialog
import androidx.appcompat.app.AppCompatActivity
import com.bignerdranch.android.jetpack5roomtest.database.MyDatabase
import com.bignerdranch.android.jetpack5roomtest.database.Student


class MainActivity : AppCompatActivity() {
    
    
    private var myDatabase: MyDatabase? = null
    private var studentList: MutableList<Student?>? = ArrayList()
    private var studentAdapter: StudentAdapter? = null
    override fun onCreate(savedInstanceState: Bundle?) {
    
    
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        findViewById<View>(R.id.btnInsertStudent).setOnClickListener {
    
     openAddStudentDialog() }
        val lvStudent: ListView = findViewById(R.id.lvStudent)
        studentAdapter = StudentAdapter(this@MainActivity, studentList)
        lvStudent.adapter = studentAdapter
        lvStudent.onItemLongClickListener = OnItemLongClickListener {
    
     _, _, position, _ ->
            updateOrDeleteDialog((studentList as ArrayList<Student?>)[position])
            false
        }
        myDatabase = MyDatabase.getInstance(this)
        QueryStudentTask().execute()
    }

    private fun updateOrDeleteDialog(student: Student?) {
    
    
        val options = arrayOf("更新", "删除")
        AlertDialog.Builder(this@MainActivity)
            .setTitle("")
            .setItems(options, DialogInterface.OnClickListener {
    
     _, which ->
                if (which == 0) {
    
    
                    openUpdateStudentDialog(student)
                } else if (which == 1) {
    
    
                    if (student != null) {
    
    
                        DeleteStudentTask(student).execute()
                    }
                }
            }).show()
    }

    private fun openAddStudentDialog() {
    
    
        val customView: View = this.layoutInflater.inflate(R.layout.dialog_layout_student, null)
        val etName: EditText = customView.findViewById(R.id.etName)
        val etAge: EditText = customView.findViewById(R.id.etAge)
        val dialog: AlertDialog = AlertDialog.Builder(this@MainActivity).create()
        dialog.setTitle("Add Student")
        dialog.setButton(DialogInterface.BUTTON_POSITIVE, "OK", DialogInterface.OnClickListener {
    
     _, _ ->
            if (TextUtils.isEmpty(etName.text.toString()) || TextUtils.isEmpty(etAge.text.toString())) {
    
    
                Toast.makeText(this@MainActivity, "输入不能为空", Toast.LENGTH_SHORT).show()
            } else {
    
    
                InsertStudentTask(etName.text.toString(), etAge.text.toString()).execute()
            }
        })
        dialog.setButton(DialogInterface.BUTTON_NEGATIVE, "CANCEL", DialogInterface.OnClickListener {
    
     _, _ -> dialog.dismiss() })
        dialog.setView(customView)
        dialog.show()
    }

    private fun openUpdateStudentDialog(student: Student?) {
    
    
        if (student == null) {
    
    
            return
        }
        val customView: View = this.layoutInflater.inflate(R.layout.dialog_layout_student, null)
        val etName: EditText = customView.findViewById(R.id.etName)
        val etAge: EditText = customView.findViewById(R.id.etAge)
        etName.setText(student.name)
        etAge.setText(student.age)
        val dialog: AlertDialog = AlertDialog.Builder(this@MainActivity).create()
        dialog.setTitle("Update Student")
        dialog.setButton(DialogInterface.BUTTON_POSITIVE, "OK", DialogInterface.OnClickListener {
    
     _, _ ->
            if (TextUtils.isEmpty(etName.text.toString()) || TextUtils.isEmpty(etAge.text.toString())) {
    
    
                Toast.makeText(this@MainActivity, "输入不能为空", Toast.LENGTH_SHORT).show()
            } else {
    
    
                UpdateStudentTask(student.id, etName.text.toString(), etAge.text.toString()).execute()
            }
        })
        dialog.setButton(DialogInterface.BUTTON_NEGATIVE, "CANCEL", DialogInterface.OnClickListener {
    
     dialog, _ -> dialog.dismiss() })
        dialog.setView(customView)
        dialog.show()
    }

    private inner class InsertStudentTask(var name: String, var age: String) : AsyncTask<Void?, Void?, Void?>() {
    
    
        override fun doInBackground(vararg params: Void?): Void? {
    
    
            myDatabase!!.studentDao()!!.insertStudent(Student(name, age))
            studentList!!.clear()
            studentList!!.addAll(myDatabase!!.studentDao()!!.studentList!!)
            return null
        }

        override fun onPostExecute(result: Void?) {
    
    
            super.onPostExecute(result)
            studentAdapter?.notifyDataSetChanged()
        }
    }

    private inner class UpdateStudentTask(var id: Int, var name: String, var age: String) : AsyncTask<Void?, Void?, Void?>() {
    
    
        override fun onPostExecute(result: Void?) {
    
    
            super.onPostExecute(result)
            studentAdapter?.notifyDataSetChanged()
        }

        override fun doInBackground(vararg params: Void?): Void? {
    
    
            myDatabase!!.studentDao()!!.updateStudent(Student(id, name, age))
            studentList!!.clear()
            studentList!!.addAll(myDatabase!!.studentDao()!!.studentList!!)
            return null
        }
    }

    private inner class DeleteStudentTask(var student: Student) : AsyncTask<Void?, Void?, Void?>() {
    
    
        override fun onPostExecute(result: Void?) {
    
    
            super.onPostExecute(result)
            studentAdapter?.notifyDataSetChanged()
        }

        override fun doInBackground(vararg params: Void?): Void? {
    
    
            myDatabase!!.studentDao()!!.deleteStudent(student)
            studentList!!.clear()
            studentList!!.addAll(myDatabase!!.studentDao()!!.studentList!!)
            return null
        }
    }

    private inner class QueryStudentTask : AsyncTask<Void?, Void?, Void?>() {
    
    
        override fun onPostExecute(result: Void?) {
    
    
            super.onPostExecute(result)
            studentAdapter?.notifyDataSetChanged()
        }

        override fun doInBackground(vararg params: Void?): Void? {
    
    
            studentList!!.clear()
            studentList!!.addAll(myDatabase!!.studentDao()!!.studentList!!)
            return null
        }
    }
}

新建 StudentAdapter.kt，绑定 activity_main.xml 的 ListView，代码如下：

package com.bignerdranch.android.jetpack5roomtest

import android.content.Context
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.BaseAdapter
import android.widget.TextView
import com.bignerdranch.android.jetpack5roomtest.database.Student


class StudentAdapter(context: Context, private val data: MutableList<Student?>?) : BaseAdapter() {
    
    
    private val layoutInflater: LayoutInflater

    internal inner class ViewHolder {
    
    
        var tvId: TextView? = null
        var tvName: TextView? = null
        var tvAge: TextView? = null
    }

    override fun getView(position: Int, convertView: View?, parent: ViewGroup?): View? {
    
    
        var convertView: View? = convertView
        val viewHolder: ViewHolder
        if (convertView == null) {
    
    
            convertView = layoutInflater.inflate(R.layout.list_item_student, null)
            viewHolder = ViewHolder()
            viewHolder.tvId = convertView.findViewById(R.id.tvId)
            viewHolder.tvName = convertView.findViewById(R.id.tvName)
            viewHolder.tvAge = convertView.findViewById(R.id.tvAge)
            convertView.tag = viewHolder
        } else {
    
    
            viewHolder = convertView.tag as ViewHolder
        }
        viewHolder.tvId!!.text = (data!![position]?.id).toString()
        viewHolder.tvName!!.text = data[position]!!.name
        viewHolder.tvAge!!.text = data[position]!!.age
        return convertView
    }

    override fun getCount(): Int {
    
    
        return data?.size ?: 0
    }

    override fun getItem(position: Int): Student? {
    
    
        return data?.get(position)
    }

    override fun getItemId(position: Int): Long {
    
    
        return position.toLong()
    }

    init {
    
    
        layoutInflater = context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) as LayoutInflater
    }
}

运行后，可以通过 Room 对数据库做添加、更新、删除、查询，效果如下：

项目github代码详见

5.2 ViewModel 内：通过 Room.Database 查到 LiveData 数据，在外部监听 LiveData

当 Room 变化时，通过 ViewModel 内的 LiveData 通知页面数据的变化，架构如下：

有两种实例化数据库对象的方式：

• 在 Application 中实例化数据库，ViewModel 调用它
• 用 AndroidViewModel 的构造函数参数的 Application 参数（作为 Context 的子类），实例化数据库

新建项目（拷贝上文5.1节的项目），项目github详见。

首先，修改 StudentDao.kt，用 LiveData 将 List 包装起来，代码如下：

package com.bignerdranch.android.jetpack5roomwithlivedataandviewmodeltest.database

import androidx.lifecycle.LiveData
import androidx.room.*

@Dao
interface StudentDao {
    
    
    @Insert
    fun insertStudent(student: Student?)

    @Delete
    fun deleteStudent(student: Student?)

    @Update
    fun updateStudent(student: Student?)

    @Query("SELECT * FROM student")
    fun getStudentList(): LiveData<List<Student?>?>? //希望监听学生表的变化，为其加上LiveData

    @Query("SELECT * FROM student WHERE id = :id")
    fun getStudentById(id: Int): Student?
}

其次，新建 StudentViewModel 类，该类继承自 AndroidViewModel，其中有 Database 和 LiveData，代码如下：

package com.bignerdranch.android.jetpack5roomwithlivedataandviewmodeltest

import android.app.Application
import androidx.lifecycle.AndroidViewModel
import androidx.lifecycle.LiveData
import com.bignerdranch.android.jetpack5roomwithlivedataandviewmodeltest.database.MyDatabase
import com.bignerdranch.android.jetpack5roomwithlivedataandviewmodeltest.database.Student


class StudentViewModel(application: Application) : AndroidViewModel(application) {
    
    
    private val myDatabase: MyDatabase?
    val liveDataStudent: LiveData<List<Student?>?>?

    init {
    
    
        myDatabase = MyDatabase.getInstance(application)
        liveDataStudent = myDatabase!!.studentDao()!!.getStudentList()
    }
}

然后，在 MainActivity 中初始化 List，实例化 StudentViewModel 并监听其 LiveData 的变化，代码如下：

package com.bignerdranch.android.jetpack5roomwithlivedataandviewmodeltest

import android.content.DialogInterface
import android.os.AsyncTask
import android.os.Bundle
import android.text.TextUtils
import android.view.View
import android.widget.AdapterView.OnItemLongClickListener
import android.widget.EditText
import android.widget.ListView
import android.widget.Toast
import androidx.appcompat.app.AlertDialog
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.ViewModelProvider
import com.bignerdranch.android.jetpack5roomwithlivedataandviewmodeltest.database.MyDatabase
import com.bignerdranch.android.jetpack5roomwithlivedataandviewmodeltest.database.Student


class MainActivity : AppCompatActivity() {
    
    
    private var myDatabase: MyDatabase? = null
    private var studentList: MutableList<Student?>? = null
    private var studentAdapter: StudentAdapter? = null
    override fun onCreate(savedInstanceState: Bundle?) {
    
    
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        findViewById<View>(R.id.btnInsertStudent).setOnClickListener {
    
     openAddStudentDialog() }
        val lvStudent = findViewById<ListView>(R.id.lvStudent)
        studentList = ArrayList()
        studentAdapter = StudentAdapter(this@MainActivity, studentList)
        lvStudent.adapter = studentAdapter
        lvStudent.onItemLongClickListener = OnItemLongClickListener {
    
     parent, view, position, id ->
            updateOrDeleteDialog((studentList as ArrayList<Student?>)[position])
            false
        }
        myDatabase = MyDatabase.getInstance(this)
        val studentViewModel: StudentViewModel = ViewModelProvider(this)[StudentViewModel::class.java]
        studentViewModel.liveDataStudent!!.observe(this) {
    
     students ->
            (studentList as ArrayList<Student?>).clear()
            (studentList as ArrayList<Student?>).addAll(students!!)
            studentAdapter!!.notifyDataSetChanged()
        }
    }

    private fun updateOrDeleteDialog(student: Student?) {
    
    
        val options = arrayOf("更新", "删除")
        AlertDialog.Builder(this@MainActivity)
            .setTitle("")
            .setItems(options) {
    
     _, which ->
                if (which == 0) {
    
    
                    openUpdateStudentDialog(student)
                } else if (which == 1) {
    
    
                    if (student != null) {
    
    
                        DeleteStudentTask(student).execute()
                    }
                }
            }.show()
    }

    private fun openAddStudentDialog() {
    
    
        val customView: View = this.layoutInflater.inflate(R.layout.dialog_layout_student, null)
        val etName = customView.findViewById<EditText>(R.id.etName)
        val etAge = customView.findViewById<EditText>(R.id.etAge)
        val builder = AlertDialog.Builder(this@MainActivity)
        val dialog = builder.create()
        dialog.setTitle("Add Student")
        dialog.setButton(DialogInterface.BUTTON_POSITIVE, "OK") {
    
     dialog, which ->
            if (TextUtils.isEmpty(etName.text.toString()) || TextUtils.isEmpty(etAge.text.toString())) {
    
    
                Toast.makeText(this@MainActivity, "输入不能为空", Toast.LENGTH_SHORT).show()
            } else {
    
    
                InsertStudentTask(etName.text.toString(), etAge.text.toString()).execute()
            }
        }
        dialog.setButton(DialogInterface.BUTTON_NEGATIVE, "CANCEL") {
    
     dialog, which -> dialog.dismiss() }
        dialog.setView(customView)
        dialog.show()
    }

    private fun openUpdateStudentDialog(student: Student?) {
    
    
        if (student == null) {
    
    
            return
        }
        val customView: View = this.layoutInflater.inflate(R.layout.dialog_layout_student, null)
        val etName = customView.findViewById<EditText>(R.id.etName)
        val etAge = customView.findViewById<EditText>(R.id.etAge)
        etName.setText(student.name)
        etAge.setText(student.age)
        val builder = AlertDialog.Builder(this@MainActivity)
        val dialog = builder.create()
        dialog.setTitle("Update Student")
        dialog.setButton(DialogInterface.BUTTON_POSITIVE, "OK") {
    
     dialog, which ->
            if (TextUtils.isEmpty(etName.text.toString()) || TextUtils.isEmpty(etAge.text.toString())) {
    
    
                Toast.makeText(this@MainActivity, "输入不能为空", Toast.LENGTH_SHORT).show()
            } else {
    
    
                UpdateStudentTask(student.id, etName.text.toString(), etAge.text.toString()).execute()
            }
        }
        dialog.setButton(DialogInterface.BUTTON_NEGATIVE, "CANCEL") {
    
     dialog, which -> dialog.dismiss() }
        dialog.setView(customView)
        dialog.show()
    }

    private inner class InsertStudentTask(var name: String, var age: String) :
        AsyncTask<Void?, Void?, Void?>() {
    
    
        override fun doInBackground(vararg params: Void?): Void? {
    
    
            myDatabase!!.studentDao()!!.insertStudent(Student(name, age))
            return null
        }
    }

    private inner class UpdateStudentTask(var id: Int, var name: String, var age: String) :
        AsyncTask<Void?, Void?, Void?>() {
    
    
        override fun doInBackground(vararg params: Void?): Void? {
    
    
            myDatabase!!.studentDao()!!.updateStudent(Student(id, name, age))
            return null
        }
    }

    private inner class DeleteStudentTask(var student: Student) : AsyncTask<Void?, Void?, Void?>() {
    
    
        override fun doInBackground(vararg params: Void?): Void? {
    
    
            myDatabase!!.studentDao()!!.deleteStudent(student)
            return null
        }
    }
}

运行后，当 LiveData 数据变化时，更新 UI 即可，而不需要每次增删改后都必须用 QueryStudentTask() 来手动查询一次数据库，简化了系统，效果如下：

5.3 Room 数据库 version 升级

5.3.1 用 Migration 升级

在 MyDatabase.kt 中用 Migration 升级数据库，代码如下：

@Database(entities = [Student::class], exportSchema = false, version = 1)
abstract class MyDatabase() : RoomDatabase() {
    
    
    abstract fun studentDao(): StudentDao?

    companion object {
    
    
        private val DATABASE_NAME = "my_db"
        private var databaseInstance: MyDatabase? = null
        @Synchronized
        fun getInstance(context: Context): MyDatabase? {
    
    
            if (databaseInstance == null) {
    
    
                databaseInstance = Room
                    .databaseBuilder(context.applicationContext, MyDatabase::class.java, DATABASE_NAME)
                    .fallbackToDestructiveMigration()
                    .addMigrations(MIGRATION_1_2, MIGRATION_2_3, MIGRATION_1_3, MIGRATION_3_4)
                    .build()
            }
            return databaseInstance
        }

        val MIGRATION_1_2: Migration = object : Migration(1, 2) {
    
    
            override fun migrate(database: SupportSQLiteDatabase) {
    
    
                //do something
                Log.d("MyDatabase", "MIGRATION_1_2")
            }
        }
        private val MIGRATION_2_3: Migration = object : Migration(2, 3) {
    
    
            override fun migrate(database: SupportSQLiteDatabase) {
    
    
                //do something
                Log.d("MyDatabase", "MIGRATION_2_3")
            }
        }
        private val MIGRATION_1_3: Migration = object : Migration(1, 3) {
    
    
            override fun migrate(database: SupportSQLiteDatabase) {
    
    
                //do something
                Log.d("MyDatabase", "MIGRATION_1_3")
            }
        }
        val MIGRATION_3_4: Migration = object : Migration(3, 4) {
    
    
            override fun migrate(database: SupportSQLiteDatabase) {
    
    
                Log.d("MyDatabase", "MIGRATION_3_4")
                database.execSQL(
                    "CREATE TABLE temp_Student (" +
                            "id INTEGER PRIMARY KEY NOT NULL," +
                            "name TEXT," +
                            "age TEXT)"
                )
                database.execSQL(
                    "INSERT INTO temp_Student (id, name, age) " +
                            "SELECT id, name, age FROM Student"
                )
                database.execSQL("DROP TABLE Student")
                database.execSQL("ALTER TABLE temp_Student RENAME TO Student")
            }
        }
    }
}

然后，通过 .addMigrations(MIGRATION_1_2, MIGRATION_2_3, MIGRATION_1_3, MIGRATION_3_4) 执行升级。通过 @Database(entities = [Student::class], version = 1) 可标记数据库的目标 verison。用 fallbackToDestructiveMigration() 避免升级失败的崩溃。

对于跨 version 的情况，例如如果当前手机设备上 sqlite 的数据库 version 是 1，要安装的 App 的数据库 version 是3，Room 会先找 Migration(1,3)，若无则执行 Migration(1,2) 和 Migration(2,3)

5.3.2 export 各版本数据库的 Sechema 文件

schema 文件记录了数据库的变化，方便我们了解历次数据库操作是否成功。

在 build.gradle(app) 中指定 Schema 文件导出的位置，设置如下：

android {
    
    
    defaultConfig {
    
    
        javaCompileOptions {
    
    
            annotationProcessorOptions {
    
    
                arguments += ["room.schemaLocation": "$projectDir/schemas".toString()]
            }
        }
    }
}

并在 MyDatabase.kt 中设置 exportSchema = true（ @Database(entities = [Student::class], exportSchema = true, version = 2)）。

运行后，即可在 app/schemas 下看到导出的数据库json文件，效果如下：

5.3.3 修改表结构：销毁旧表并重建新表

在 sqlite 修改表结构，例如把 Student 表的 age 字段从 INTEGET 改为 TEXT，需要销毁旧表并重建新表，分为如下步骤：

1. 创建临时表 tmpStudent
2. 将数据从旧表 Student 复制到临时表 tmpStudent
3. 删除旧表 Student
4. 将临时表 tmpStudent 重命名为 Student

MyDatabase.kt 的代码如下：

        val MIGRATION_3_4: Migration = object : Migration(3, 4) {
    
    
            override fun migrate(database: SupportSQLiteDatabase) {
    
    
                Log.d("MyDatabase", "MIGRATION_3_4")
                database.execSQL(
                    "CREATE TABLE temp_Student (" +
                            "id INTEGER PRIMARY KEY NOT NULL," +
                            "name TEXT," +
                            "age TEXT)"
                )
                database.execSQL(
                    "INSERT INTO temp_Student (id, name, age) " +
                            "SELECT id, name, age FROM Student"
                )
                database.execSQL("DROP TABLE Student")
                database.execSQL("ALTER TABLE temp_Student RENAME TO Student")
            }
        }

运行后，数据库字段类型变化生效，效果如下：

5.4 用 createFromAsset() 和 createFromFile() 预填充数据库

首先，创建一个 student 的数据库，示例如下：

bash > sqlite3 student.db
sqlite> create table student(id integer not null, name text not null, age text not null, primary key(id autoincrement));
sqlite> insert into student values (1,'zhao','1'),(2,'qian','2'),(3,'sun','3');

设置 Student.kt 都为 NotNull，与上文的 student.db 的数据类型匹配，代码如下：

@Entity(tableName = "student")
class Student {
    
    
    @NotNull
    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "id", typeAffinity = ColumnInfo.INTEGER)
    var id = 0

    @NotNull
    @ColumnInfo(name = "name", typeAffinity = ColumnInfo.TEXT)
    var name: String

    @NotNull
    @ColumnInfo(name = "age", typeAffinity = ColumnInfo.TEXT)
    var age: String

    /**
     * Room会使用这个构造器来存储数据，也就是当你从表中得到Student对象时候，Room会使用这个构造器
     */
    constructor(id: Int, name: String, age: String) {
    
    
        this.id = id
        this.name = name
        this.age = age
    }

    /**
     * 由于Room只能识别和使用一个构造器，如果希望定义多个构造器，你可以使用Ignore标签，让Room忽略这个构造器
     * 同样，@Ignore标签还可用于字段，使用@Ignore标签标记过的字段，Room不会持久化该字段的数据
     */
    @Ignore
    constructor(name: String, age: String) {
    
    
        this.name = name
        this.age = age
    }
}

其次，新建 assets folder 目录，示例如下：

然后，新建 app/assets/databases，并在其中放置 student.db 文件，其位置如下：

然后，在 MyDatabase.kt 中，用 createFromAsset() 方法从 assets/database/students.db 创建 Room 数据库，代码如下：

package com.bignerdranch.android.jetpack5roomwithlivedataandviewmodeltest.database

import android.content.Context
import android.util.Log
import androidx.room.Database
import androidx.room.Room
import androidx.room.RoomDatabase
import androidx.room.migration.Migration
import androidx.sqlite.db.SupportSQLiteDatabase


@Database(entities = [Student::class], exportSchema = false, version = 1)
abstract class MyDatabase() : RoomDatabase() {
    
    
    abstract fun studentDao(): StudentDao?

    companion object {
    
    
        private val DATABASE_NAME = "my_db"
        private var databaseInstance: MyDatabase? = null

        @Synchronized
        fun getInstance(context: Context): MyDatabase? {
    
    
            if (databaseInstance == null) {
    
    
                databaseInstance = Room
                    .databaseBuilder(context.applicationContext, MyDatabase::class.java, DATABASE_NAME)
                    .createFromAsset("databases/student.db")
                    .fallbackToDestructiveMigration()
//                    .addMigrations(MIGRATION_1_2, MIGRATION_2_3, MIGRATION_1_3, MIGRATION_3_4)
                    .build()
            }
            return databaseInstance
        }

        val MIGRATION_1_2: Migration = object : Migration(1, 2) {
    
    
            override fun migrate(database: SupportSQLiteDatabase) {
    
    
                //do something
                Log.d("MyDatabase", "MIGRATION_1_2")
            }
        }
        private val MIGRATION_2_3: Migration = object : Migration(2, 3) {
    
    
            override fun migrate(database: SupportSQLiteDatabase) {
    
    
                //do something
                Log.d("MyDatabase", "MIGRATION_2_3")
            }
        }
        private val MIGRATION_1_3: Migration = object : Migration(1, 3) {
    
    
            override fun migrate(database: SupportSQLiteDatabase) {
    
    
                //do something
                Log.d("MyDatabase", "MIGRATION_1_3")
            }
        }
        val MIGRATION_3_4: Migration = object : Migration(3, 4) {
    
    
            override fun migrate(database: SupportSQLiteDatabase) {
    
    
                Log.d("MyDatabase", "MIGRATION_3_4")
                database.execSQL(
                    "CREATE TABLE temp_Student (" +
                            "id INTEGER PRIMARY KEY NOT NULL," +
                            "name TEXT," +
                            "age TEXT)"
                )
                database.execSQL(
                    "INSERT INTO temp_Student (id, name, age) " +
                            "SELECT id, name, age FROM Student"
                )
                database.execSQL("DROP TABLE Student")
                database.execSQL("ALTER TABLE temp_Student RENAME TO Student")
            }
        }
    }
}

运行后，从指定的 student.db 文件创建了数据库，示例如下：