ohos_setup

MMKV is an efficient, small, easy-to-use mobile key-value storage framework used in the WeChat application. It's now available on HarmonyOS NEXT.

MMKV for HarmonyOS NEXT

Features

• Efficient. MMKV uses mmap to keep memory synced with files, and protobuf to encode/decode values, making the most of the native platform to achieve the best performance.

  • Multi-Process concurrency: MMKV supports concurrent read-read and read-write access between processes.

• Easy-to-use. You can use MMKV as you go. All changes are saved immediately, no sync, no flush calls are needed.

• Small.

  • A handful of files: MMKV contains process locks, encode/decode helpers and mmap logics and nothing more. It's really tidy.
  • About 600K in binary size: MMKV adds about 600K per architecture on App size, and much less when zipped (HAR/HAP).

Getting Started

Prerequisites

• Apps using MMKV can target: HarmonyOS Next (3.0.0.13) or later.
• ARM64 & x86_64 architecture.
• DevEco Studio NEXT Developer Beta1 (5.0.3.100) or later.

Installation

Via OHPM (recommended):

This is the fastest and most recommended way to add MMKV to your project.

ohpm install @tencent/mmkv

Or, you can add it to your project manually.

• Add the following lines to oh-package.json5 on your app module.

  "dependencies": {
      "@tencent/mmkv": "2.0.0",
  }

• Then run

  ohpm install

Via Source:

You can integrate MMKV by source.

• Checkout MMKV source code:

  git clone https://github.com/Tencent/MMKV.git

  Note:

  • you may consider adding MMKV as git submodule.
  • By default, MMKV links libc++ statically. If you want smaller binary size and you know what you're doing, you can edit the file build-profile.json5 of the MMKV module, change the line to "arguments": "-DOHOS_STL=c++_shared", and "cppFlags": "-DMMKV_STL_SHARED=1.
  • If you want to access MMKV's C++ interface directly, you can edit the file build-profile.json5 of the MMKV module, changing the line to "cppFlags": "…… -fvisibility=default ……". Note that this will increase the binary size a little bit.

• Add the following lines to oh-package.json5 on your app module.

  "dependencies": {
      "@tencent/mmkv": "file:path/to/mmkv/OpenHarmony/MMKV",
  }

• Then run

  ohpm install

Via Pre-Built HAR (not recommended):

You can integrate MMKV by pre-built HAR.

• Checkout MMKV source code:

  git clone https://github.com/Tencent/MMKV.git

• Open the MMKV project in path/to/mmkv/OpenHarmony, open the Product window and select Build Mode to "release"; Select MMKV module, in the menu select Build -> Make Module 'MMKV', export the build result as "MMKV.har" to your project folder.

  • you may consider adding MMKV as git submodule.
  • By default, MMKV links libc++ statically. If you want smaller binary size and you know what you're doing, you can edit the file build-profile.json5 of the MMKV module, change the line to "arguments": "-DOHOS_STL=c++_shared", and "cppFlags": "-DMMKV_STL_SHARED=1.
  • If you want to access MMKV's C++ interface directly, you can edit the file build-profile.json5 of the MMKV module, changing the line to "cppFlags": "…… -fvisibility=default ……". Note that this will increase the binary size a little bit.

• Add the following lines to oh-package.json5 on your app module.

  "dependencies": {
      "@tencent/mmkv": "file:path/to/MMKV.har",
  }

• Then run

  ohpm install

Setup

You can use MMKV as you go. All changes are saved immediately, no sync, no apply calls needed.
Setup MMKV on App startup, say your EntryAbility.onCreate() function, add these lines:

import { MMKV } from '@tencent/mmkv';

export default class EntryAbility extends UIAbility {
  onCreate(want: Want, launchParam: AbilityConstant.LaunchParam): void {
    let appCtx = this.context.getApplicationContext();
    let mmkvRootDir = MMKV.initialize(appCtx);
    console.info('mmkv rootDir: ', mmkvRootDir);
    ……
  }

CRUD Operations

• MMKV has a global instance, that can be used directly:

  import { MMKV } from '@tencent/mmkv';
      
  let mmkv = MMKV.defaultMMKV();
  mmkv.encodeBool('bool', true);
  console.info('bool = ', mmkv.decodeBool('bool'));
  
  mmkv.encodeInt32('int32', Math.pow(2, 31) - 1);
  console.info('max int32 = ', mmkv.decodeInt32('int32'));
  
  mmkv.encodeInt64('int', BigInt(2**63) - BigInt(1));
  console.info('max int64 = ', mmkv.decodeInt64('int'));
  
  let str: string = 'Hello OpenHarmony from MMKV';
  mmkv.encodeString('string', str);
  console.info('string = ', mmkv.decodeString('string'));
  
  let arrayBuffer: ArrayBuffer = StringToArrayBuffer('Hello OpenHarmony from MMKV with bytes');
  mmkv.encodeBytes('bytes', arrayBuffer);
  let bytes = mmkv.decodeBytes('bytes');
  console.info('bytes = ', ArrayBufferToString(bytes));

  As you can see, MMKV is quite easy to use.

• Deleting & Querying:

  mmkv.removeValueForKey('bool');
  console.info('contains "bool"', mmkv.containsKey('bool'));
  
  mmkv.removeValuesForKeys(['int32', 'int']);
  console.info('all keys: ', mmkv.allKeys().join());

• If different modules/logic need isolated storage, you can also create your own MMKV instance separately:

  var mmkv = MMKV.mmkvWithID('test');
  mmkv.encodeBool('bool', true);
  console.info('bool = ', mmkv.decodeBool('bool'));

• If multi-process accessing is needed，you can set MMKV.MULTI_PROCESS_MODE on MMKV initialization:

  var mmkv = MMKV.mmkvWithID('test-multi-process', MMKV.MULTI_PROCESS_MODE);
  mmkv.encodeBool('bool', true);
  console.info('bool = ', mmkv.decodeBool('bool'));

Supported Types

• Primitive Types:

  • boolean, number, bigint, string

• Classes & Collections:

  • boolean[], number[], string[], ArrayBuffer

• Custom Classes:
  We don't support custom classes directly because there's no Parcel replacement in OHOS. You can encode/decode to JSON first. Or use whatever encoding lib you like such as protobuf.

  let user = {
    username: 'dummy_user',
    age: 23
  }
  
  // Serialize the object into a JSON string
  mmkv.encodeString('user', JSON.stringify(user))
  
  // Deserialize the JSON string into an object
  let jsonUser = mmkv.decodeString('user') // { 'username': 'dummy_user', 'age': 23 }
  let userObject = JSON.parse(jsonUser)

Log

• By default, MMKV prints log to hilog, which is not convenient for diagnosing online issues. You can setup MMKV log redirecting on App startup on the native interface of MMKV. Checkout how to do it on C++. Due to the current limitation of NAPI runtime, we can't efficiently redirect log to the JavaScript side.

• You can turn off MMKV's logging once and for all on initialization (which we strongly disrecommend).

  import { MMKV, MMKVLogLevel } from '@tencent/mmkv';
  
  MMKV.initialize(appCtx, MMKVLogLevel.None);

Encryption

• By default MMKV stores all key-values in plain text on file, relying on Android's/iOS's sandbox to make sure the file is encrypted. Should you worry about information leaking, you can choose to encrypt MMKV.

  let encryptKey = 'MyEncryptKey';
  let mmkv = MMKV.mmkvWithID('test-encryption', MMKV.SINGLE_PROCESS_MODE, encryptKey);

• You can change the encryption key later as you like. You can also change an existing MMKV instance from encrypted to unencrypted, or vice versa.

  // an unencrypted MMKV instance
  let mmkv = MMKV.mmkvWithID('test-encryption');
  
  // change from unencrypted to encrypted
  mmkv.reKey('Key_seq_1');
  
  // change encryption key
  mmkv.reKey('Key_seq_2');
  
  // change from encrypted to unencrypted
  kmmkv.reKey();

Customize location

• By default, MMKV stores file inside $(FilesDir)/mmkv/. You can customize MMKV's root directory on App startup:

  let appCtx = this.context.getApplicationContext();
  let rootDir = appCtx.filesDir + '/mmkv_2';
  let cacheDir = appCtx.cacheDir;
  MMKV.initializeWithPath(rootDir, cacheDir);

• You can even customize any MMKV instance's location:

  let appCtx = this.context.getApplicationContext();
  let rootDir = appCtx.filesDir + '/mmkv_3';
  var mmkv = MMKV.mmkvWithID('testCustomDir', MMKV.SINGLE_PROCESS_MODE, null, rootDir);

  Note: It's recommended to store MMKV files inside your App's sandbox path. DO NOT store them on external storage(aka SD card).

Native Buffer

• Typically, when a string or ArrayBuffer value is getting from MMKV, there's a memory copying from native to JSVM. And if that value is passed to another native library(NAPI) immediately, another memory-copying from JSVM to native happens. The whole process wastes too much if that value's size is large. Here comes the Native Buffer to the rescue.
  Native Buffer is a memory buffer created in native, wrapped as NativeBuffer in JavaScript, which can be passed to another native library transparently. This process saves memory-copying to & from JSVM. Example code:

  let sizeNeeded = mmkv.getValueSize('bytes', true);
  let nativeBuffer = MMKV.createNativeBuffer(sizeNeeded);
  if (nativeBuffer != null) {
      let size = mmkv.writeValueToNativeBuffer('bytes', nativeBuffer);
      console.info('NativeBuffer: size Needed = ', sizeNeeded, ',  written size = ', size);
  
      // pass nativeBuffer to another native library
      // ...
  
      // destroy when you're done
      MMKV.destroyNativeBuffer(nativeBuffer);
  }

Backup & Restore

• You can use MMKV's backup & restore API to backup your data to somewhere else, and restore them later.

  let rootDir = ...;
  let backupRootDir = rootDir + '/mmkv_backup';
  // backup one instance
  let ret = MMKV.backupOneToDirectory(mmapID, backupRootDir);
  // backup all instances
  let count = MMKV.backupAllToDirectory(backupRootDir);
  
  // restore one instance
  ret = MMKV.restoreOneMMKVFromDirectory(mmapID, backupRootDir);
  // restore all instances
  count = MMKV.restoreAllFromDirectory(backupRootDir);

Auto Expiration

• You can upgrade MMKV to auto key expiration. Note that this is a breaking change. Once upgraded to auto key expiration, the file is not valid for any older version of MMKV (<= v1.2.16) to operate correctly.

• Global Expiration. The most simple way to do it is to turn on auto key expiration for all keys in the whole file.

  // expire in a day
  mmkv.enableAutoKeyExpire(MMKV.ExpireInDay); // MMKV.ExpireInDay = 24 * 60 * 60

  Or, if you prefer, you can enable auto key expiration without setting a global expiration duration. In this case, each key is not expired by default.

  // enable auto key expiration without global duration
  mmkv.enableAutoKeyExpire(MMKV.ExpireNever); // MMKV.ExpireNever = 0

• Individual Expiration. You can set a special expiration duration for a key, regardless with the file has a global duration or not. Note that you have to enable auto key expiration first.

  // enable auto key expiration with an hour duration
  mmkv.enableAutoKeyExpire(MMKV.ExpireInHour); // MMKV.ExpireInHour = 60 * 60
  
  // set a key with the file's global expiration duration, aka MMKV.ExpireInHour
  mmkv.encodeString('key_1', 'some value');
  
  // set a special key that expires in two hours
  mmkv.encodeString('key_2', 'some value', 2 * 60 * 60);
  
  // set a special key that never expires
  mmkv.encodeString('key_3', 'some value', MMKV.ExpireNever);

  Or, if you prefer, you can enable auto key expiration without setting a global expiration duration. In this case, each key is not expired by default.

  // enable auto key expiration without global duration
  mmkv.enableAutoKeyExpire(MMKV.ExpireNever); // MMKV.ExpireNever = 0
  
  // set a key that never expires
  mmkv.encodeString('key_1', 'some value');
  
  // set a special key that expires in an hour
  mmkv.encodeString('key_2', 'some value', MMKV.ExpireInHour);

• The expire duration is counted in seconds. MMKV has some pre-defined duration for your convenience. You can use any other duration you prefer. For example, expiration in a week is 7 * 24 * 60 * 60.

  static const int ExpireNever = 0;
  static const int ExpireInMinute = 60;
  static const int ExpireInHour = 60 * 60;
  static const int ExpireInDay = 24 * 60 * 60;
  static const int ExpireInMonth = 30 * 24 * 60 * 60;
  static const int ExpireInYear = 365 * 30 * 24 * 60 * 60;

Change Log

Check out the CHANGELOG.md for details of change history.