HarmonyOS应用五之轻量化数据存储持久化

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1、数据存储场景持久化方式

用户首选项（Preferences）：为应用提供Key-Value键值型的数据处理能力，通常用于保存应用的配置信息。数据通过文本的形式保存在设备中，应用使用过程中会将文本中的数据全量加载到内存中，所以访问速度快、效率高，但不适合需要存储大量数据的场景。
关系型数据库（RelationalStore）：一种关系型数据库，以行和列的形式存储数据，广泛用于应用中的关系型数据的处理，包括一系列的增、删、改、查等接口，开发者也可以运行自己定义的SQL语句来满足复杂业务场景的需要。
键值型数据库（KV-Store）：一种非关系型数据库，其数据以“键值”对的形式进行组织、索引和存储，其中“键”作为唯一标识符。适合很少数据关系和业务关系的业务数据存储，同时因其在分布式场景中降低了解决数据库版本兼容问题的复杂度，和数据同步过程中冲突解决的复杂度而被广泛使用。相比于关系型数据库，更容易做到跨设备跨版本兼容。

详细介绍以及使用流程

2、数据存储

/** Copyright (c) 2022 Huawei Device Co., Ltd.* Licensed under the Apache License, Version 2.0 (the "License");* you may not use this file except in compliance with the License.* You may obtain a copy of the License at**     http://www.apache.org/licenses/LICENSE-2.0** Unless required by applicable law or agreed to in writing, software* distributed under the License is distributed on an "AS IS" BASIS,* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.* See the License for the specific language governing permissions and* limitations under the License.*/import { promptAction } from '@kit.ArkUI';
import { preferences } from '@kit.ArkData';
import Logger from '../common/utils/Logger';
import CommonConstants from '../common/constants/CommonConstants';
import Fruit from '../viewmodel/Fruit';let context = getContext(this);
let preference: preferences.Preferences;
let preferenceTemp: preferences.Preferences;/*** Preference model.** @param fruitData Fruit data.*/
class PreferenceModel {private fruitData: Fruit = new Fruit();/*** Read the specified Preferences persistence file and load the data into the Preferences instance.*/async getPreferencesFromStorage() {try {preference = await preferences.getPreferences(context, CommonConstants.PREFERENCES_NAME);} catch (err) {Logger.error(CommonConstants.TAG, `Failed to get preferences, Cause: ${err}`);}}/*** Deletes the specified Preferences persistence file from memory and removes the Preferences instance.*/async deletePreferences() {try {await preferences.deletePreferences(context, CommonConstants.PREFERENCES_NAME);} catch(err) {Logger.error(CommonConstants.TAG, `Failed to delete preferences, Cause: ${err}`);};preference = preferenceTemp;this.showToastMessage($r('app.string.delete_success_msg'));}/*** Save the data to the Preferences.** @param fruit Fruit data.*/async putPreference(fruit: Fruit) {if (!preference) {await this.getPreferencesFromStorage();}// The fruit name and fruit quantity data entered by the user are saved to the cached Preference instance.try {await preference.put(CommonConstants.KEY_NAME, JSON.stringify(fruit));} catch (err) {Logger.error(CommonConstants.TAG, `Failed to put value, Cause: ${err}`);}// Store the Preference instance in the preference persistence fileawait preference.flush();}/*** Get preference data.*/async getPreference() {let fruit = '';if (!preference) {await this.getPreferencesFromStorage();}try {// fruit = <string> await preference.get(CommonConstants.KEY_NAME, '');fruit = (await preference.get(CommonConstants.KEY_NAME, '')).toString();} catch (err) {Logger.error(CommonConstants.TAG, `Failed to get value, Cause: ${err}`);}// If the data is empty, a message is displayed indicating that data needs to be written.if (fruit === '') {this.showToastMessage($r('app.string.data_is_null_msg'));return;}this.showToastMessage($r('app.string.read_success_msg'));return JSON.parse(fruit);}/*** Process the data obtained from the database.*/async getFruitData() {this.fruitData = await this.getPreference();return this.fruitData;}/*** Verifies that the data entered is not empty.** @param fruit Fruit data.*/checkFruitData(fruit: Fruit) {if (fruit.fruitName === '' || fruit.fruitNum === '') {this.showToastMessage($r('app.string.fruit_input_null_msg'));return true;}return false;}/*** write data.** @param fruit  Fruit data.*/writeData(fruit: Fruit) {// Check whether the data is null.let isDataNull = this.checkFruitData(fruit);if (isDataNull) {return;}// The data is inserted into the preferences database if it is not empty.this.putPreference(fruit);this.showToastMessage($r('app.string.write_success_msg'));}/*** Popup window prompt message.** @param message Prompt message.*/showToastMessage(message: Resource) {promptAction.showToast({message: message,duration: CommonConstants.DURATION});};
}export default new PreferenceModel();

数据存储调用代码：

 await preference.put(CommonConstants.KEY_NAME, JSON.stringify(fruit));

3、数据读取

 new ButtonItemData($r('app.string.read_data_btn_text'),() => {// Get data from the preferences database.PreferenceModel.getFruitData().then((resultData: Fruit) => {if (resultData) {this.fruit = resultData;}});}),

 async getFruitData() {this.fruitData = await this.getPreference();return this.fruitData;}

 async getPreference() {let fruit = '';if (!preference) {await this.getPreferencesFromStorage();}try {// fruit = <string> await preference.get(CommonConstants.KEY_NAME, '');fruit = (await preference.get(CommonConstants.KEY_NAME, '')).toString();} catch (err) {Logger.error(CommonConstants.TAG, `Failed to get value, Cause: ${err}`);}// If the data is empty, a message is displayed indicating that data needs to be written.if (fruit === '') {this.showToastMessage($r('app.string.data_is_null_msg'));return;}this.showToastMessage($r('app.string.read_success_msg'));return JSON.parse(fruit);}

根据debug流程发现请求如下：
PreferenceModel.getFruitData()-》await this.getPreference()（异步）-》await preference.get(CommonConstants.KEY_NAME, ‘’)).toString()（异步）-》然后原路返回PreferenceModel.getFruitData()走完-》this.showToastMessage（判断里面）-》getFruitData()
在这里插入图片描述
再执行完await preference.get(CommonConstants.KEY_NAME, ‘’)).toString()（异步）返回时走了then里面已经成功拿到数据了。

4、删除数据

 new ButtonItemData($r('app.string.delete_data_file_btn_text'),() => {// Delete database files.PreferenceModel.deletePreferences();// After a database file is deleted, the corresponding data is left blank.this.fruit.fruitName = '';this.fruit.fruitNum = ''})

  async deletePreferences() {try {await preferences.deletePreferences(context, CommonConstants.PREFERENCES_NAME);} catch(err) {Logger.error(CommonConstants.TAG, `Failed to delete preferences, Cause: ${err}`);};preference = preferenceTemp;this.showToastMessage($r('app.string.delete_success_msg'));}

await preferences.deletePreferences(context, CommonConstants.PREFERENCES_NAME);是根据实例名称删除指定的实例。

参考链接

5、应用内字体大小调节

  Slider({value: this.changeFontSize === CommonConstants.SET_SIZE_HUGE? CommonConstants.SET_SLIDER_MAX : this.changeFontSize,min: CommonConstants.SET_SLIDER_MIN,max: CommonConstants.SET_SLIDER_MAX,step: CommonConstants.SET_SLIDER_STEP,style: SliderStyle.InSet}).showSteps(true).width(StyleConstants.SLIDER_WIDTH_PERCENT).onChange(async (value: number) => {if (this.changeFontSize === 0) {this.changeFontSize = await PreferencesUtil.getChangeFontSize();this.fontSizeText = SetViewModel.getTextByFontSize(value);return;}this.changeFontSize = (value === CommonConstants.SET_SLIDER_MAX ? CommonConstants.SET_SIZE_HUGE : value);this.fontSizeText = SetViewModel.getTextByFontSize(this.changeFontSize);PreferencesUtil.saveChangeFontSize(this.changeFontSize);})