Android DataStore 使用详解

文章目录

概述

官方文档：​​https://developer.android.com/topic/libraries/architecture/datastore​​

Jetpack DataStore 是一种数据存储解决方案，允许您使用协议缓冲区存储键值对或类型化对象。DataStore 使用 Kotlin 协程和 Flow 以异步、一致的事务方式存储数据。

使用

添加依赖：

implementation "androidx.datastore:datastore-preferences:1.0.0"

定义dataStore

//定义DataStore
val Context.dataStore: DataStore<Preferences> by preferencesDataStore(name = "user_info")

//定义key 
val keyName = stringPreferencesKey("name")
val keyAge = intPreferencesKey("age")

保存数据：

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.datastore.preferences.core.edit
import androidx.lifecycle.lifecycleScope
import kotlinx.coroutines.launch

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        lifecycleScope.launch {
            saveData("zhaoyanjun", 18)
        }
    }

    //dataStore保存数据
    suspend fun saveData(name: String, age: Int) {
        dataStore.edit {
            it[keyName] = name //保存字符串
            it[keyAge] = age //保存int
        }
    }

}

获取数据

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
     
        lifecycleScope.launch {
            getData()
        }

    }

    //dataStore获取数据，collect 是一个挂起函数，所以会一直挂起，只要name的值发起变更，collect 就会回调
    suspend fun getData() {
        val nameFlow = dataStore.data.map {
            it[keyName]
        }
        nameFlow.collect { name ->
            Log.d("getData ", "name $name")
        }
    }
}

​​dataStore.data ​​​ 是一个 ​​Flow​​​ 对象，使用一个 ​​collect ​​​操作符 可以接受 值的变化，一旦值发生变化，​​collect { }​​ 就会回调，可以实现数据驱动 UI 的效果。

DataStore 本地数据

DataStore 文件在 ​​files/datastore/​​ 目录，完整路径是

​​/data/data/com.zyj.exoplayerdemo/files/datastore/user_info.preferences_pb​​

查看DataStore 文件

双击 ​​user_info.preferences_pb​​ 在 AS 里打开

发现是乱码。

点击右键把 ​​user_info.preferences_pb​​ 导出到桌面

在 mac appStore 下载安装 ​​Protobuf Viewer​​

用 ​​Protobuf Viewer​​ 打开我们导出的 pb 文件

Key 的枚举

在文中我们用到了

stringPreferencesKey("name")
intPreferencesKey("age")

除此之外，DataStore 还提供了其他类型的 Key

@JvmName("intKey")
public fun intPreferencesKey(name: String): Preferences.Key<Int> = Preferences.Key(name)

@JvmName("doubleKey")
public fun doublePreferencesKey(name: String): Preferences.Key<Double> = Preferences.Key(name)

@JvmName("stringKey")
public fun stringPreferencesKey(name: String): Preferences.Key<String> = Preferences.Key(name)

@JvmName("booleanKey")
public fun booleanPreferencesKey(name: String): Preferences.Key<Boolean> = Preferences.Key(name)

@JvmName("floatKey")
public fun floatPreferencesKey(name: String): Preferences.Key<Float> = Preferences.Key(name)

@JvmName("longKey")
public fun longPreferencesKey(name: String): Preferences.Key<Long> = Preferences.Key(name)

@JvmName("stringSetKey")
public fun stringSetPreferencesKey(name: String): Preferences.Key<Set<String>> =
    Preferences.Key(name)

同步API

在上面的演示过程中，我们使用 ​​Flow​​​ 的 ​​collect { }​​​ 操作符 , 但是 ​​collect { }​​ 会一直处于挂起状态，只要值发生变化，我们就会收到通知，符合数据驱动 UI 的设计模式。

但是在现实开发中，我们往往需要一个同步 api , 仅仅获取当前一次值，我们只关注本次的值是什么，至于以后得值变化，我们不关心。DataStore 提供了 同步api 来供我们使用 。

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
  
        lifecycleScope.launch {
            //同步api
            val first = dataStore.data.first()
            val name = first[keyName]
            val age = first[keyAge]
            Log.d("getData ", "name $name")
            Log.d("getData ", "age $age")
        }
    }

如果是 pb 文件里面没有值，那么就会返回 null

com.zyj.exoplayerdemo D/getData: name null
com.zyj.exoplayerdemo D/getData: age null

所以我们可以把获取名字，封装成一个同步方法

suspend fun getNameData(): String? {
    val nameFlow = dataStore.data.map {
            it[keyName]
    }
    return nameFlow.first()
}

SharedPreferences 数据迁移

如果你原来是用 SharedPreferences ， 想换到 DataStore 上，DataStore 提供了一键迁移，就一行代码就搞定了。

val Context.dataStore: DataStore<Preferences> by preferencesDataStore(
    name = "user_info",
    produceMigrations = { context ->
        listOf(SharedPreferencesMigration(context, "sp_file_name"))
    })

DataStore 源码

DataStore 是一个接口

public interface DataStore<T> {
    /**
     * Provides efficient, cached (when possible) access to the latest durably persisted state.
     * The flow will always either emit a value or throw an exception encountered when attempting
     * to read from disk. If an exception is encountered, collecting again will attempt to read the
     * data again.
     *
     * Do not layer a cache on top of this API: it will be be impossible to guarantee consistency.
     * Instead, use data.first() to access a single snapshot.
     *
     * @return a flow representing the current state of the data
     * @throws IOException when an exception is encountered when reading data
     */
    public val data: Flow<T>

    /**
     * Updates the data transactionally in an atomic read-modify-write operation. All operations
     * are serialized, and the transform itself is a coroutine so it can perform heavy work
     * such as RPCs.
     *
     * The coroutine completes when the data has been persisted durably to disk (after which
     * [data] will reflect the update). If the transform or write to disk fails, the
     * transaction is aborted and an exception is thrown.
     *
     * @return the snapshot returned by the transform
     * @throws IOException when an exception is encountered when writing data to disk
     * @throws Exception when thrown by the transform function
     */
    public suspend fun updateData(transform: suspend (t: T) -> T): T
}