Lightweight Data Store Development

When to Use

The lightweight data store is ideal for storing lightweight and frequently used data, but not for storing a large amount of data or data with frequent changes. The application data is persistently stored on a device in the form of files. Note that the instance accessed by an application contains all data of the file. The data is always loaded to the memory of the device until the application removes it from the memory. The application can perform data operations using the Storage APIs.

Available APIs

The lightweight data store provides applications with data processing capability and allows applications to perform lightweight data storage and query. Data is stored in key-value pairs. Keys are of the string type, and values can be of the number, string, or Boolean type.

Creating a Storage Instance

Create a Storage instance for data operations. A Storage instance is created after data is read from a specified file and loaded to the instance.

Table 1 API for creating a Storage instance

Writing Data

Call the put() method to add or modify data in a Storage instance.

Table 2 API for writing data

Reading Data

Call the get() method to read data from a Storage instance.

Table 3 API for reading data

Storing Data Persistently

Call the flush() method to write the cached data back to its text file for persistent storage.

Table 4 API for data persistence

Observing Data Changes

Specify StorageObserver as the callback to subscribe to data changes. When the value of the subscribed key is changed and the flush() method is executed, StorageObserver will be invoked.

Table 5 APIs for subscribing to data changes

Deleting Data

Use the following APIs to delete a Storage instance or data file.

Table 6 APIs for deleting data

How to Develop

1. Import @ohos.data.storage and related modules to the development environment.

   import dataStorage from '@ohos.data.storage'
   import featureAbility from '@ohos.ability.featureAbility'  // Used to obtain the file storage path.
   

2. Create a Storage instance.

   Read the specified file and load its data to the Storage instance for data operations.

   var context = featureAbility.getContext()
   var path = await context.getFilesDir()
   let promise = dataStorage.getStorage(path + '/mystore')
   

3. Write data.

   Use the put() method of the Storage class to write data to the cached Storage instance.

   promise.then((storage) => {
       let getPromise = storage.put('startup', 'auto') // Save data to the Storage instance.
       getPromise.then(() => {
           console.info("Put the value of startup successfully.")
       }).catch((err) => {
           console.info("Put the value of startup failed with err: " + err)
       })
   }).catch((err) => {
       console.info("Get the storage failed")
   })
   

4. Read data.

   Use the get() method of the Storage class to read data.

   promise.then((storage) => {
       let getPromise = storage.get('startup', 'default')
       getPromise.then((value) => {
           console.info("The value of startup is " + value)
       }).catch((err) => {
           console.info("Get the value of startup failed with err: " + err)
       })
   }).catch((err) => {
       console.info("Get the storage failed")
   })
   

5. Store data persistently.

   Use the flush or flushSync method to flush data in the Storage instance to its file.

   storage.flush();
   

6. Observe data changes.

   Specify StorageObserver as the callback to subscribe to data changes for an application. When the value of the subscribed key is changed and the flush() method is executed, StorageObserver will be invoked. Unregister the StorageObserver when it is no longer required.

   promise.then((storage) => {
       var observer = function (key) {
           console.info("The key of " + key + " changed.")
       }
       storage.on('change', observer)
       storage.putSync('startup', 'auto') // Modify data in the Storage instance.
       storage.flushSync() // Trigger the StorageObserver callback.
   
       storage.off(...change..., observer) // Unsubscribe from the data changes.
   }).catch((err) => {
       console.info("Get the storage failed")
   })
   

7. Delete the specified file.

   Use the deleteStorage method to delete the Storage singleton of the specified file from the memory, and delete the specified file, its backup file, and damaged files. After the specified files are deleted, the application cannot use that instance to perform any data operation. Otherwise, data inconsistency will occur. The deleted data and files cannot be restored.

   let promise = dataStorage.deleteStorage(path + '/mystore')
   promise.then(() => {
       console.info("Deleted successfully.")
   }).catch((err) => {
       console.info("Deleted failed with err: " + err)
   })