data-persistence-by-preferences.md

# Persisting Preferences Data


## When to Use

The **Preferences** module provides APIs for processing data in the form of key-value (KV) pairs, and supports persistence of the KV pairs when required, as well as modification and query of the data. You can use **Preferences** when you want a unique storage for global data. **Preferences** caches data in the memory, which allows fast access when the data is required. **Preferences** is recommended for storing small amount of data, such as personalized settings (font size and whether to enable the night mode) of applications.


## Working Principles

User applications call **Preference** through the JS interface to read and write data files. You can load the data of a **Preferences** persistence file to a **Preferences** instance. Each file uniquely corresponds to an instance. The system stores the instance in memory through a static container until the instance is removed from the memory or the file is deleted. The following figure illustrates how **Preference** works.

The preference persistent file of an application is stored in the application sandbox. You can use **context** to obtain the file path. For details, see [Obtaining Application File Paths](../application-models/application-context-stage.md#obtaining-application-file-paths).

**Figure 1** Preferences working mechanism 

![preferences](figures/preferences.jpg)


## Constraints

- The key in a KV pair must be a string and cannot be empty or exceed 80 bytes.

- If the value is of the string type, use the UTF-8 encoding format. It can be empty or a string not longer than 8192 bytes.

- The memory usage increases with the amount of **Preferences** data. The maximum number of data records recommended is 10,000. Otherwise, high memory overheads will be caused.


## Available APIs

The following table lists the APIs used for persisting user preference data. For more information about the APIs, see [User Preferences](../reference/apis/js-apis-data-preferences.md).

| API                                                          | Description                                                  |
| ------------------------------------------------------------ | ------------------------------------------------------------ |
| getPreferences(context: Context, name: string, callback: AsyncCallback<Preferences>): void | Obtains a **Preferences** instance.                          |
| putSync(key: string, value: ValueType): void                 | Writes data to the Preferences instance. You can use **flush()** to persist the **Preferences** instance data. An asynchronous API is also provided. |
| hasSync(key: string): void                                   | Checks whether the **Preferences** instance contains a KV pair with the given key. The key cannot be empty. An asynchronous API is also provided. |
| getSync(key: string, defValue: ValueType): void              | Obtains the value of the specified key. If the value is null or not of the default value type, **defValue** is returned. An asynchronous API is also provided. |
| deleteSync(key: string): void                                | Deletes the KV pair with the given key from the **Preferences** instance. An asynchronous API is also provided. |
| flush(callback: AsyncCallback<void>): void             | Flushes the data of this **Preferences** instance to a file for data persistence. |
| on(type: 'change', callback: Callback<{ key : string }>): void | Subscribes to data changes of the specified key. When the value of the specified key is changed and saved by **flush()**, a callback will be invoked to return the new data. |
| off(type: 'change', callback?: Callback<{ key : string }>): void | Unsubscribes from data changes.                              |
| deletePreferences(context: Context, name: string, callback: AsyncCallback<void>): void | Deletes a **Preferences** instance from memory. If the **Preferences** instance has a persistent file, this API also deletes the persistent file. |


## How to Develop

1. Import the **@ohos.data.preferences** module.
   
   ```js
   import dataPreferences from '@ohos.data.preferences';
   ```

2. Obtain a **Preferences** instance. Read data from a file and load the data to a **Preferences** instance for data operations.

   Stage model:

   
   ```js
   import UIAbility from '@ohos.app.ability.UIAbility';
   import { BusinessError } from '@ohos.base';
   import window from '@ohos.window';
   
   class EntryAbility extends UIAbility {
     onWindowStageCreate(windowStage: window.WindowStage) {
       try {
         dataPreferences.getPreferences(this.context, 'myStore', (err: BusinessError, preferences: dataPreferences.Preferences) => {
           if (err) {
             console.error(`Failed to get preferences. Code:${err.code},message:${err.message}`);
             return;
           }
           console.info('Succeeded in getting preferences.');
           // Before performing related data operations, obtain a Preferences instance.
         })
       } catch (err) {
         console.error(`Failed to get preferences. Code:${err.code},message:${err.message}`);
       }
     }
   }
   ```

   FA model:

   
   ```js
   import featureAbility from '@ohos.ability.featureAbility';
   import { BusinessError } from '@ohos.base';
   
   // Obtain the context.
   let context = featureAbility.getContext();
   
   try {
     dataPreferences.getPreferences(this.context, 'myStore', (err: BusinessError, preferences: dataPreferences.Preferences) => {
       if (err) {
         console.error(`Failed to get preferences. Code:${err.code},message:${err.message}`);
         return;
       }
       console.info('Succeeded in getting preferences.');
       // Before performing related data operations, obtain a Preferences instance.
     })
   } catch (err) {
     console.error(`Failed to get preferences. Code is ${err.code},message:${err.message}`);
   }
   ```

3. Write data.

   Use **putSync()** to save data to the cached **Preferences** instance. After data is written, you can use **flush()** to persist the **Preferences** instance data to a file if necessary.

   > **NOTE**
   >
   > If the specified key already exists, the **putSync()** method changes the value. You can use **hasSync()** to check whether the KV pair exists.

   Example:

   
   ```js
   try {
     if (preferences.hasSync('startup')) {
       console.info("The key 'startup' is contained.");
     } else {
       console.info("The key 'startup' does not contain.");
       // Add a KV pair.
       preferences.putSync('startup', 'auto');
     }
   } catch (err) {
     console.error(`Failed to check the key 'startup'. Code:${err.code}, message:${err.message}`);
   }
   ```

4. Read data.

     Use **getSync()** to obtain the value of the specified key. If the value is null or is not of the default value type, the default data is returned. Example:
     
   ```js
   try {
     let val = preferences.getSync('startup', 'default');
     console.info(`Succeeded in getting value of 'startup'. val: ${val}.`);
   } catch (err) {
     console.error(`Failed to get value of 'startup'. Code:${err.code}, message:${err.message}`);
   }
   ```

5. Delete data.

   Use **deleteSync()** to delete a KV pair.<br>Example:

   
   ```js
   try {
     preferences.deleteSync('startup');
   } catch (err) {
     console.error(`Failed to delete the key 'startup'. Code:${err.code}, message:${err.message}`);
   }
   ```

6. Persist data.

     You can use **flush()** to persist the data held in a **Preferences** instance to a file. Example:
     
   ```js
   try {
     preferences.flush((err: BusinessError) => {
       if (err) {
         console.error(`Failed to flush. Code:${err.code}, message:${err.message}`);
         return;
       }
       console.info('Succeeded in flushing.');
     })
   } catch (err) {
     console.error(`Failed to flush. Code:${err.code}, message:${err.message}`);
   }
   ```

7. Subscribe to data changes.

     Specify an observer as the callback to return the data changes for an application. When the value of the subscribed key is changed and saved by **flush()**, the observer callback will be invoked to return the new data. Example:
     
   ```js
   interface observer {
      key: string
   }
   preferences.on('change', (key: observer) => {
     console.info('The key' + key + 'changed.');
   });
   // The data is changed from 'auto' to 'manual'.
   preferences.put('startup', 'manual', (err: BusinessError) => {
     if (err) {
       console.error(`Failed to put the value of 'startup'. Code:${err.code},message:${err.message}`);
       return;
     }
     console.info("Succeeded in putting the value of 'startup'.");
     preferences.flush((err: BusinessError) => {
       if (err) {
         console.error(`Failed to flush. Code:${err.code}, message:${err.message}`);
         return;
       }
       console.info('Succeeded in flushing.');
     })
   })
   ```

8. Delete a **Preferences** instance from the memory.

   Use **deletePreferences()** to delete a **Preferences** instance from the memory. If the **Preferences** instance has a persistent file, the persistent file and its backup and corrupted files will also be deleted.

   > **NOTE**
   >
   > - The deleted **Preferences** instance cannot be used for data operations. Otherwise, data inconsistency will be caused.
   > 
   > - The deleted data and files cannot be restored.

   Example:

   
   ```js
   try {
     dataPreferences.deletePreferences(this.context, 'myStore', (err: BusinessError) => {
       if (err) {
         console.error(`Failed to delete preferences. Code:${err.code}, message:${err.message}`);
         return;
       }
       console.info('Succeeded in deleting preferences.');
     })
   } catch (err) {
     console.error(`Failed to delete preferences. Code:${err.code}, message:${err.message}`);
   }
   ```