# Room

一个来自Android 开发者官方JetPack的SqLite类库用来存储查询 sql数据非常简单...

# 先来看看如何导入Room

apply from: 'versions.gradle'    buildscript {        ext.kotlin_version = '1.3.31'        repositories {            google()            jcenter()                    }        dependencies {            classpath 'com.android.tools.build:gradle:3.4.1'            classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"            // NOTE: Do not place your application dependencies here; they belong            // in the individual module build.gradle files        }    }        allprojects {        repositories {            google()            jcenter()                    }    }        task clean(type: Delete) {        delete rootProject.buildDir    }复制代码

app/build.gradle 下设置

dependencies {    def dep = rootProject.ext.dep    implementation fileTree(dir: 'libs', include: ['*.jar'])    implementation"org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"    implementation dep.x_appcompat    implementation dep.x_room    implementation dep.gson    kapt dep.x_room_compiler  // kapt kotlin 注解处理器    implementation 'androidx.core:core-ktx:1.0.2'    testImplementation dep.junit    annotationProcessor dep.x_room_compiler    androidTestImplementation dep.x_android_test_runner    androidTestImplementation dep.x_android_test_espresso}复制代码

def dep = [:]def version = [:]version.x_appcompat = "1.0.2"version.junit = "4.12"version.x_android_test_runner = "1.1.1"version.x_android_test_espresso = "3.1.1"version.gson = "2.8.2"version.x_room = "2.1.0-beta01"dep.x_appcompat = "androidx.appcompat:appcompat:$version.x_appcompat"dep.junit = "junit:junit:$version.junit"dep.x_android_test_runner = "androidx.test:runner:$version.x_android_test_runner"dep.x_android_test_espresso = "androidx.test.espresso:espresso-core:3.1.1"dep.gson = "com.google.code.gson:gson:$version.gson"dep.x_room = "androidx.room:room-runtime:$version.x_room"dep.x_room_compiler = "androidx.room:room-compiler:2.1.0-alpha04"复制代码

# Room 常用的注解

@Entity(tableName="xxx") 表注解

@PrimaryKey 主键 主键不能为空

@ColumnInfo(name="xxxx") 列名注解

@Dao Sql 操作接口注解

@TypeConverters 类型转换器

Room 实体类存储List 会报错: Cannot figure out how to save this field into database. You can consider adding a type converter for it.

// example converter for java.util.Date public static class Converters {    @TypeConverter    public Date fromTimestamp(Long value) {        return value == null ? null : new Date(value);    }    @TypeConverter    public Long dateToTimestamp(Date date) {        if (date == null) {            return null;        } else {            return date.getTime();        }    }}复制代码

# 创建表

@Entity(tableName = "class_cache")@TypeConverters(UserConverters::class)data class ClassCache(    @PrimaryKey val id: String,    @ColumnInfo(name = "class_name") val className: String?,    @ColumnInfo(name = "grade") val grade: String?,    @ColumnInfo(name = "users") val users: List
    
     )@Entity(tableName = "user_cache", primaryKeys = ["user_id", "class_id"])data class UserCache(    @ColumnInfo(name = "user_id") val userId: String,    @ColumnInfo(name = "class_id") val classId: String,    @ColumnInfo(name = "user_name") val userName: String?,    @ColumnInfo(name = "sex") val sex: Int)/** * 类型转换器 */class UserConverters {    @TypeConverter    fun stringToObject(value: String): List
     
       {        val listType = object : TypeToken
      
       
        >() {        }.type        return Gson().fromJson(value, listType)    }    @TypeConverter    fun objectToString(list: List
        
         ): String {        val gson = Gson()        return gson.toJson(list)    }}复制代码

# 创建接口

@Daointerface ClassDao {    @Query("select * from class_cache")    fun getAllClass(): List
    
         @Query("select * from class_cache where id=:classId")    fun getClass(classId: String): ClassCache    @Insert    fun insertClass(classList: List
     
      )    @Insert    fun insertClassOne(classCache: ClassCache)    @Delete    fun delClassCache(classList: List
      
       )    @Delete    fun delClassOne(classCache: ClassCache)}复制代码

# 数据库

@Database(entities = arrayOf(ClassCache::class, UserCache::class), version = 1)abstract class AppDatabase : RoomDatabase() {    abstract fun getClassDao(): ClassDao    companion object {        // For Singleton instantiation        @Volatile        private var instance: AppDatabase? = null        fun getInstance(context: Context): AppDatabase {            return instance ?: synchronized(this) {                instance ?: buildDatabase(context).also { instance = it }            }        }        private fun buildDatabase(context: Context): AppDatabase {            return Room.databaseBuilder(context, AppDatabase::class.java, "test-db")                .allowMainThreadQueries()//允许在主线程查询数据                .addMigrations()//迁移数据库使用                .fallbackToDestructiveMigration()//迁移数据库如果发生错误，将会重新创建数据库，而不是发生崩溃                .build()        }    }复制代码

# 数据库升级 Migrations

//数据库升级 迁移    object Migrations {        //        val MIGRATION_1_2 = object : Migration(1, 2) {
            //            override fun migrate(database: SupportSQLiteDatabase) {
            //                database.execSQL(        //                    "CREATE TABLE `Fruit` (`id` INTEGER, `name` TEXT, " +        //                            "PRIMARY KEY(`id`))"        //                )        //            }        //        }    }复制代码