1# @ohos.data.storage (Lightweight Data Storage) 2 3The **DataStorage** module provides applications with data processing capability and allows applications to perform lightweight data storage and query. Data is stored in key-value (KV) pairs. Keys are of the string type, and values can be of the number, string, or Boolean type. 4 5 6> **NOTE** 7> 8> - The initial APIs of this module are supported since API version 6. Newly added APIs will be marked with a superscript to indicate their earliest API version. 9> 10> - The APIs of this module are no longer maintained since API version 9. You are advised to use [@ohos.data.preferences](js-apis-data-preferences.md). 11> 12> - The APIs of this module can be used only in the FA model. 13 14 15## Modules to Import 16 17```js 18import data_storage from '@ohos.data.storage'; 19``` 20 21## Constants 22 23**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 24 25| Name | Type| Readable| Writable| Description | 26| ---------------- | -------- | ---- | ---- | ------------------------------------- | 27| MAX_KEY_LENGTH | number | Yes | No | Maximum length of a key, which is 80 bytes. | 28| MAX_VALUE_LENGTH | number | Yes | No | Maximum length of a value, which is 8192 bytes.| 29 30 31## data_storage.getStorageSync 32 33getStorageSync(path: string): Storage 34 35Reads the specified file and loads its data to the **Storage** instance for data operations. 36 37**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 38 39**Parameters** 40 41| Name| Type | Mandatory| Description | 42| ------ | ------ | ---- | -------------------------- | 43| path | string | Yes | Path of the target file.| 44 45**Return value** 46 47| Type | Description | 48| ------------------- | ------------------------------------------------- | 49| [Storage](#storage) | **Storage** instance used for data storage operations.| 50 51**Example** 52 53```js 54import featureAbility from '@ohos.ability.featureAbility'; 55 56let path; 57let context = featureAbility.getContext(); 58context.getFilesDir().then((filePath) => { 59 path = filePath; 60 console.info("======================>getFilesDirPromise====================>"); 61 62 let storage = data_storage.getStorageSync(path + '/mystore'); 63 storage.putSync('startup', 'auto'); 64 storage.flushSync(); 65}); 66``` 67 68 69## data_storage.getStorage 70 71getStorage(path: string, callback: AsyncCallback<Storage>): void 72 73Reads the specified file and loads its data to the **Storage** instance for data operations. This API uses an asynchronous callback to return the result. 74 75**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 76 77**Parameters** 78 79| Name | Type | Mandatory| Description | 80| -------- | ---------------------------------------- | ---- | -------------------------- | 81| path | string | Yes | Path of the target file.| 82| callback | AsyncCallback<[Storage](#storage)> | Yes | Callback invoked to return the result. | 83 84**Example** 85 86```js 87import featureAbility from '@ohos.ability.featureAbility'; 88 89let path; 90let context = featureAbility.getContext(); 91context.getFilesDir().then((filePath) => { 92 path = filePath; 93 console.info("======================>getFilesDirPromise====================>"); 94 95 data_storage.getStorage(path + '/mystore', function (err, storage) { 96 if (err) { 97 console.info("Failed to get the storage. path: " + path + '/mystore'); 98 return; 99 } 100 storage.putSync('startup', 'auto'); 101 storage.flushSync(); 102 }) 103}); 104``` 105 106 107## data_storage.getStorage 108 109getStorage(path: string): Promise<Storage> 110 111Reads the specified file and loads its data to the **Storage** instance for data operations. This API uses a promise to return the result. 112 113**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 114 115**Parameters** 116 117| Name| Type | Mandatory| Description | 118| ------ | ------ | ---- | -------------------------- | 119| path | string | Yes | Path of the target file.| 120 121**Return value** 122 123| Type | Description | 124| ---------------------------------- | ------------------------------- | 125| Promise<[Storage](#storage)> | Promise used to return the result.| 126 127**Example** 128 129```js 130import featureAbility from '@ohos.ability.featureAbility'; 131 132let path; 133let context = featureAbility.getContext(); 134context.getFilesDir().then((filePath) => { 135 path = filePath; 136 console.info("======================>getFilesDirPromise====================>"); 137 138 let getPromise = data_storage.getStorage(path + '/mystore'); 139 getPromise.then((storage) => { 140 storage.putSync('startup', 'auto'); 141 storage.flushSync(); 142 }).catch((err) => { 143 console.info("Failed to get the storage. path: " + path + '/mystore'); 144 }) 145}); 146``` 147 148 149## data_storage.deleteStorageSync 150 151deleteStorageSync(path: string): void 152 153Deletes the singleton **Storage** instance of a file from the memory, and deletes the specified file, its backup file, and damaged files. After the specified files are deleted, the **Storage** instance cannot be used for data operations. Otherwise, data inconsistency will occur. 154 155**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 156 157**Parameters** 158 159| Name| Type | Mandatory| Description | 160| ------ | ------ | ---- | -------------------------- | 161| path | string | Yes | Path of the target file.| 162 163**Example** 164 165```js 166import featureAbility from '@ohos.ability.featureAbility'; 167 168let path; 169let context = featureAbility.getContext(); 170context.getFilesDir().then((filePath) => { 171 path = filePath; 172 console.info("======================>getFilesDirPromise====================>"); 173 174 data_storage.deleteStorageSync(path + '/mystore'); 175}); 176``` 177 178## data_storage.deleteStorage 179 180deleteStorage(path: string, callback: AsyncCallback<void>): void 181 182Deletes the singleton **Storage** instance of a file from the memory, and deletes the specified file, its backup file, and damaged files. After the specified files are deleted, the **Storage** instance cannot be used for data operations. Otherwise, data inconsistency will occur. This API uses an asynchronous callback to return the result. 183 184**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 185 186**Parameters** 187 188| Name | Type | Mandatory| Description | 189| -------- | ------------------------- | ---- | -------------------------- | 190| path | string | Yes | Path of the target file.| 191| callback | AsyncCallback<void> | Yes | Callback that returns no value. | 192 193**Example** 194 195```js 196import featureAbility from '@ohos.ability.featureAbility'; 197 198let path; 199let context = featureAbility.getContext(); 200context.getFilesDir().then((filePath) => { 201 path = filePath; 202 console.info("======================>getFilesDirPromise====================>"); 203 204 data_storage.deleteStorage(path + '/mystore', function (err) { 205 if (err) { 206 console.info("Failed to delete the storage with err: " + err); 207 return; 208 } 209 console.info("Succeeded in deleting the storage."); 210 }) 211}); 212``` 213 214 215## data_storage.deleteStorage 216 217deleteStorage(path: string): Promise<void> 218 219Deletes the singleton **Storage** instance of a file from the memory, and deletes the specified file, its backup file, and damaged files. After the specified files are deleted, the **Storage** instance cannot be used for data operations. Otherwise, data inconsistency will occur. This API uses a promise to return the result. 220 221**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 222 223**Parameters** 224 225| Name| Type | Mandatory| Description | 226| ------ | ------ | ---- | -------------------------- | 227| path | string | Yes | Path of the target file.| 228 229**Return value** 230 231| Type | Description | 232| ------------------- | ------------------------------- | 233| Promise<void> | Promise that returns no value. | 234 235**Example** 236 237```js 238import featureAbility from '@ohos.ability.featureAbility'; 239 240let path; 241let context = featureAbility.getContext(); 242context.getFilesDir().then((filePath) => { 243 path = filePath; 244 console.info("======================>getFilesDirPromise====================>"); 245 246 let promisedelSt = data_storage.deleteStorage(path + '/mystore'); 247 promisedelSt.then(() => { 248 console.info("Succeeded in deleting the storage."); 249 }).catch((err) => { 250 console.info("Failed to delete the storage with err: " + err); 251 }) 252}); 253``` 254 255 256## data_storage.removeStorageFromCacheSync 257 258removeStorageFromCacheSync(path: string): void 259 260Removes the singleton **Storage** instance of a file from the cache. The removed instance cannot be used for data operations. Otherwise, data inconsistency will occur. 261 262**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 263 264**Parameters** 265| Name| Type | Mandatory| Description | 266| ------ | ------ | ---- | -------------------------- | 267| path | string | Yes | Path of the target file.| 268 269**Example** 270 271```js 272import featureAbility from '@ohos.ability.featureAbility'; 273 274let path; 275let context = featureAbility.getContext(); 276context.getFilesDir().then((filePath) => { 277 path = filePath; 278 console.info("======================>getFilesDirPromise====================>"); 279 280 data_storage.removeStorageFromCacheSync(path + '/mystore'); 281}); 282``` 283 284 285## data_storage.removeStorageFromCache 286 287removeStorageFromCache(path: string, callback: AsyncCallback<void>): void 288 289Removes the singleton **Storage** instance of a file from the cache. The removed instance cannot be used for data operations. Otherwise, data inconsistency will occur. This API uses an asynchronous callback to return the result. 290 291**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 292 293**Parameters** 294 295| Name | Type | Mandatory| Description | 296| -------- | ------------------------- | ---- | -------------------------- | 297| path | string | Yes | Path of the target file.| 298| callback | AsyncCallback<void> | Yes | Callback that returns no value. | 299 300**Example** 301 302```js 303import featureAbility from '@ohos.ability.featureAbility'; 304 305let path; 306let context = featureAbility.getContext(); 307context.getFilesDir().then((filePath) => { 308 path = filePath; 309 console.info("======================>getFilesDirPromise====================>"); 310 311 data_storage.removeStorageFromCache(path + '/mystore', function (err) { 312 if (err) { 313 console.info("Failed to remove storage from cache with err: " + err); 314 return; 315 } 316 console.info("Succeeded in removing storage from cache."); 317 }) 318}); 319``` 320 321 322## data_storage.removeStorageFromCache 323 324removeStorageFromCache(path: string): Promise<void> 325 326Removes the singleton **Storage** instance of a file from the cache. The removed instance cannot be used for data operations. Otherwise, data inconsistency will occur. This API uses a promise to return the result. 327 328**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 329 330**Parameters** 331 332| Name| Type | Mandatory| Description | 333| ------ | ------ | ---- | -------------------------- | 334| path | string | Yes | Path of the target file.| 335 336**Return value** 337 338| Type | Description | 339| ------------------- | ------------------------------- | 340| Promise<void> | Promise that returns no value. | 341 342**Example** 343 344```js 345import featureAbility from '@ohos.ability.featureAbility'; 346 347let path; 348let context = featureAbility.getContext(); 349context.getFilesDir().then((filePath) => { 350 path = filePath; 351 console.info("======================>getFilesDirPromise====================>"); 352 353 let promiserevSt = data_storage.removeStorageFromCache(path + '/mystore') 354 promiserevSt.then(() => { 355 console.info("Succeeded in removing storage from cache."); 356 }).catch((err) => { 357 console.info("Failed to remove storage from cache with err: " + err); 358 }) 359}); 360``` 361 362## Storage 363 364Provides APIs for obtaining and modifying storage data. 365 366### getSync 367 368getSync(key: string, defValue: ValueType): ValueType 369 370Obtains the value corresponding to a key. If the value is null or not of the default value type, **defValue** is returned. 371 372**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 373 374**Parameters** 375 376| Name | Type | Mandatory| Description | 377| -------- | ----------------------- | ---- | ------------------------------------------------------------ | 378| key | string | Yes | Key of the data. It cannot be empty. | 379| defValue | [ValueType](#valuetype) | Yes | Default value to be returned if the value of the specified key does not exist. It can be a number, string, or Boolean value.| 380 381**Return value** 382 383| Type | Description | 384| --------- | -------------------------------------------------------- | 385| ValueType | Value corresponding to the specified key. If the value is null or not in the default value format, the default value is returned.| 386 387**Example** 388 389```js 390let value = storage.getSync('startup', 'default'); 391console.info("The value of startup is " + value); 392``` 393 394 395### get 396 397get(key: string, defValue: ValueType, callback: AsyncCallback<ValueType>): void 398 399Obtains the value corresponding to a key. If the value is null or not of the default value type, **defValue** is returned. This API uses an asynchronous callback to return the result. 400 401**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 402 403**Parameters** 404 405| Name | Type | Mandatory| Description | 406| -------- | ------------------------------ | ---- | ----------------------------------------- | 407| key | string | Yes | Key of the data. It cannot be empty. | 408| defValue | [ValueType](#valuetype) | Yes | Default value to be returned. It can be a number, string, or Boolean value.| 409| callback | AsyncCallback<ValueType> | Yes | Callback invoked to return the result. | 410 411**Example** 412 413```js 414storage.get('startup', 'default', function(err, value) { 415 if (err) { 416 console.info("Failed to get the value of startup with err: " + err); 417 return; 418 } 419 console.info("The value of startup is " + value); 420}) 421``` 422 423 424### get 425 426get(key: string, defValue: ValueType): Promise<ValueType> 427 428Obtains the value corresponding to a key. If the value is null or not of the default value type, **defValue** is returned. This API uses a promise to return the result. 429 430**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 431 432**Parameters** 433 434| Name | Type | Mandatory| Description | 435| -------- | ----------------------- | ---- | ----------------------------------------- | 436| key | string | Yes | Key of the data. It cannot be empty. | 437| defValue | [ValueType](#valuetype) | Yes | Default value to be returned. It can be a number, string, or Boolean value.| 438 439**Return value** 440 441| Type | Description | 442| ------------------------ | ------------------------------- | 443| Promise<ValueType> | Promise used to return the result.| 444 445**Example** 446 447```js 448let promiseget = storage.get('startup', 'default'); 449promiseget.then((value) => { 450 console.info("The value of startup is " + value) 451}).catch((err) => { 452 console.info("Failed to get the value of startup with err: " + err); 453}) 454``` 455 456 457### putSync 458 459putSync(key: string, value: ValueType): void 460 461Obtains the **Storage** instance corresponding to the specified file, writes data to the **Storage** instance using a **Storage** API, and saves the modification using **flush()** or **flushSync()**. 462 463**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 464 465**Parameters** 466 467| Name| Type | Mandatory| Description | 468| ------ | ----------------------- | ---- | ----------------------------------------- | 469| key | string | Yes | Key of the data. It cannot be empty. | 470| value | [ValueType](#valuetype) | Yes | New value to store. It can be a number, string, or Boolean value.| 471 472**Example** 473 474```js 475storage.putSync('startup', 'auto'); 476``` 477 478 479### put 480 481put(key: string, value: ValueType, callback: AsyncCallback<void>): void 482 483Obtains the **Storage** instance corresponding to the specified file, writes data to the **Storage** instance using a **Storage** API, and saves the modification using **flush()** or **flushSync()**. This API uses an asynchronous callback to return the result. 484 485**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 486 487**Parameters** 488 489| Name | Type | Mandatory| Description | 490| -------- | ------------------------- | ---- | ----------------------------------------- | 491| key | string | Yes | Key of the data. It cannot be empty. | 492| value | [ValueType](#valuetype) | Yes | New value to store. It can be a number, string, or Boolean value.| 493| callback | AsyncCallback<void> | Yes | Callback that returns no value. | 494 495**Example** 496 497```js 498storage.put('startup', 'auto', function (err) { 499 if (err) { 500 console.info("Failed to put the value of startup with err: " + err); 501 return; 502 } 503 console.info("Succeeded in putting the value of startup."); 504}) 505``` 506 507 508### put 509 510put(key: string, value: ValueType): Promise<void> 511 512Obtains the **Storage** instance corresponding to the specified file, writes data to the **Storage** instance using a **Storage** API, and saves the modification using **flush()** or **flushSync()**. This API uses a promise to return the result. 513 514**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 515 516**Parameters** 517 518| Name| Type | Mandatory| Description | 519| ------ | ----------------------- | ---- | ----------------------------------------- | 520| key | string | Yes | Key of the data. It cannot be empty. | 521| value | [ValueType](#valuetype) | Yes | New value to store. It can be a number, string, or Boolean value.| 522 523**Return value** 524 525| Type | Description | 526| ------------------- | --------------------------- | 527| Promise<void> | Promise that returns no value. | 528 529**Example** 530 531```js 532let promiseput = storage.put('startup', 'auto'); 533promiseput.then(() => { 534 console.info("Succeeded in putting the value of startup."); 535}).catch((err) => { 536 console.info("Failed to put the value of startup with err: " + err); 537}) 538``` 539 540 541### hasSync 542 543hasSync(key: string): boolean 544 545Checks whether the storage object contains data with a given key. 546 547**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 548 549**Parameters** 550 551| Name| Type | Mandatory| Description | 552| ------ | ------ | ---- | ------------------------------- | 553| key | string | Yes | Key of the data. It cannot be empty.| 554 555**Return value** 556 557| Type | Description | 558| ------- | ------------------------------------- | 559| boolean | Returns **true** if the storage object contains data with the specified key; returns **false** otherwise.| 560 561**Example** 562 563```js 564let isExist = storage.hasSync('startup'); 565if (isExist) { 566 console.info("The key of startup is contained."); 567} 568``` 569 570 571### has 572 573has(key: string, callback: AsyncCallback<boolean>): boolean 574 575Checks whether the storage object contains data with a given key. This API uses an asynchronous callback to return the result. 576 577**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 578 579**Parameters** 580 581| Name | Type | Mandatory| Description | 582| -------- | ---------------------------- | ---- | ------------------------------- | 583| key | string | Yes | Key of the data. It cannot be empty.| 584| callback | AsyncCallback<boolean> | Yes | Callback invoked to return the result. | 585 586**Return value** 587 588| Type | Description | 589| ------- | ------------------------------- | 590| boolean | Returns **true** if the storage object contains data with the specified key; returns **false** otherwise.| 591 592**Example** 593 594```js 595storage.has('startup', function (err, isExist) { 596 if (err) { 597 console.info("Failed to check the key of startup with err: " + err); 598 return; 599 } 600 if (isExist) { 601 console.info("The key of startup is contained."); 602 } 603}) 604``` 605 606 607### has 608 609has(key: string): Promise<boolean> 610 611Checks whether the storage object contains data with a given key. This API uses a promise to return the result. 612 613**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 614 615**Parameters** 616 617| Name| Type | Mandatory| Description | 618| ------ | ------ | ---- | ------------------------------- | 619| key | string | Yes | Key of the data. It cannot be empty.| 620 621**Return value** 622 623| Type | Description | 624| ---------------------- | --------------------------- | 625| Promise<boolean> | Promise used to return the result.| 626 627**Example** 628 629```js 630let promisehas = storage.has('startup') 631promisehas.then((isExist) => { 632 if (isExist) { 633 console.info("The key of startup is contained."); 634 } 635}).catch((err) => { 636 console.info("Failed to check the key of startup with err: " + err); 637}) 638``` 639 640 641### deleteSync 642 643deleteSync(key: string): void 644 645Deletes data with the specified key from this storage object. 646 647**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 648 649**Parameters** 650 651| Name| Type | Mandatory| Description | 652| ------ | ------ | ---- | --------------------------------- | 653| key | string | Yes | Key of the data. It cannot be empty.| 654 655**Example** 656 657```js 658 storage.deleteSync('startup'); 659``` 660 661 662### delete 663 664delete(key: string, callback: AsyncCallback<void>): void 665 666Deletes data with the specified key from this storage object. This API uses an asynchronous callback to return the result. 667 668**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 669 670**Parameters** 671 672| Name | Type | Mandatory| Description | 673| -------- | ------------------------- | ---- | ------------------------------- | 674| key | string | Yes | Key of the data. It cannot be empty.| 675| callback | AsyncCallback<void> | Yes | Callback that returns no value. | 676 677**Example** 678 679```js 680storage.delete('startup', function (err) { 681 if (err) { 682 console.info("Failed to delete startup key failed err: " + err); 683 return; 684 } 685 console.info("Succeeded in deleting startup key."); 686}) 687``` 688 689 690### delete 691 692delete(key: string): Promise<void> 693 694Deletes data with the specified key from this storage object. This API uses a promise to return the result. 695 696**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 697 698**Parameters** 699 700| Name| Type | Mandatory| Description | 701| ------ | ------ | ---- | --------------------- | 702| key | string | Yes | Key of the data.| 703 704**Return value** 705 706| Type | Description | 707| ------------------- | --------------------------- | 708| Promise<void> | Promise that returns no value. | 709 710**Example** 711 712```js 713let promisedel = storage.delete('startup') 714promisedel.then(() => { 715 console.info("Succeeded in deleting startup key."); 716}).catch((err) => { 717 console.info("Failed to delete startup key failed err: " + err); 718}) 719``` 720 721 722### flushSync 723 724flushSync(): void 725 726Saves the modification of this object to the **Storage** instance and synchronizes the modification to the file. 727 728**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 729 730**Example** 731 732```js 733storage.flushSync(); 734``` 735 736 737### flush 738 739flush(callback: AsyncCallback<void>): void 740 741Saves the modification of this object to the **Storage** instance and synchronizes the modification to the file. This API uses an asynchronous callback to return the result. 742 743**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 744 745**Parameters** 746 747| Name | Type | Mandatory| Description | 748| -------- | ------------------------- | ---- | ---------- | 749| callback | AsyncCallback<void> | Yes | Callback that returns no value. | 750 751**Example** 752 753```js 754storage.flush(function (err) { 755 if (err) { 756 console.info("Failed to flush to file with err: " + err); 757 return; 758 } 759 console.info("Succeeded in flushing to file."); 760}) 761``` 762 763 764### flush 765 766flush(): Promise<void> 767 768Saves the modification of this object to the **Storage** instance and synchronizes the modification to the file. This API uses a promise to return the result. 769 770**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 771 772**Return value** 773 774| Type | Description | 775| ------------------- | --------------------------- | 776| Promise<void> | Promise that returns no value. | 777 778**Example** 779 780```js 781let promiseflush = storage.flush(); 782promiseflush.then(() => { 783 console.info("Succeeded in flushing to file."); 784}).catch((err) => { 785 console.info("Failed to flush to file with err: " + err); 786}) 787``` 788 789 790### clearSync 791 792clearSync(): void 793 794Clears this **Storage** object. 795 796**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 797 798**Example** 799 800```js 801storage.clearSync(); 802``` 803 804 805### clear 806 807clear(callback: AsyncCallback<void>): void 808 809Clears this **Storage** object. This API uses an asynchronous callback to return the result. 810 811**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 812 813**Parameters** 814 815| Name | Type | Mandatory| Description | 816| -------- | ------------------------- | ---- | ---------- | 817| callback | AsyncCallback<void> | Yes | Callback that returns no value. | 818 819**Example** 820 821```js 822storage.clear(function (err) { 823 if (err) { 824 console.info("Failed to clear the storage with err: " + err); 825 return; 826 } 827 console.info("Succeeded in clearing the storage."); 828}) 829``` 830 831 832### clear 833 834clear(): Promise<void> 835 836Clears this **Storage** object. This API uses a promise to return the result. 837 838**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 839 840**Return value** 841| Type | Description | 842| ------------------- | --------------------------- | 843| Promise<void> | Promise that returns no value. | 844 845**Example** 846 847```js 848let promiseclear = storage.clear(); 849promiseclear.then(() => { 850 console.info("Succeeded in clearing the storage."); 851}).catch((err) => { 852 console.info("Failed to clear the storage with err: " + err); 853}) 854``` 855 856 857### on('change') 858 859on(type: 'change', callback: Callback<StorageObserver>): void 860 861Subscribes to data changes. The **StorageObserver** needs to be implemented. When the value of the key subscribed to is changed, a callback will be invoked after **flush()** or **flushSync()** is executed. 862 863**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 864 865**Parameters** 866 867| Name | Type | Mandatory| Description | 868| -------- | --------------------------------------------------- | ------ |---------------------------------------- | 869| type | string |Yes| Event type. The value **change** indicates data change events.| 870| callback | Callback<[StorageObserver](#storageobserver)> | Yes|Callback invoked to return the data change. | 871 872**Example** 873 874```js 875let observer = function (key) { 876 console.info("The key of " + key + " changed."); 877} 878storage.on('change', observer); 879storage.putSync('startup', 'auto'); 880storage.flushSync(); // observer will be called. 881``` 882 883 884### off('change') 885 886off(type: 'change', callback: Callback<StorageObserver>): void 887 888Unsubscribes from data changes. 889 890**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 891 892**Parameters** 893 894| Name | Type | Mandatory| Description | 895| -------- | --------------------------------------------------- | ------ |---------------------------------------- | 896| type | string |Yes| Event type. The value **change** indicates data change events.| 897| callback | Callback<[StorageObserver](#storageobserver)> | Yes|Callback for the data change. | 898 899**Example** 900 901```js 902let observer = function (key) { 903 console.info("The key of " + key + " changed."); 904} 905storage.off('change', observer); 906``` 907 908 909## StorageObserver 910 911**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 912 913| Name| Type| Mandatory| Description | 914| ---- | -------- | ---- | ---------------- | 915| key | string | No | Data changed.| 916 917## ValueType 918 919Enumerates the value types. 920 921**System capability**: SystemCapability.DistributedDataManager.Preferences.Core 922 923| Type | Description | 924| ------- | -------------------- | 925| number | The value is a number. | 926| string | The value is a string. | 927| boolean | The value is of Boolean type.| 928