# PersistentStorage: Persisting Application State


During application development, you may want selected attributes to persist even when the application is closed. In this case, you'll need PersistentStorage.


PersistentStorage is an optional singleton object within an application. Its purpose is to persist selected AppStorage attributes so that their values are the same upon application re-start as they were when the application was closed.


## Overview

PersistentStorage retains the selected AppStorage attributes on the device. The application uses the API to determine which AppStorage attributes should be persisted with PersistentStorage. The UI and business logic do not directly access attributes in PersistentStorage. All attribute access is to AppStorage. Changes in AppStorage are automatically synchronized to PersistentStorage.

PersistentStorage creates a two-way synchronization with attributes in AppStorage. A frequently used API function is to access AppStorage through PersistentStorage. Additional API functions include managing persisted attributes. The business logic always obtains or sets attributes through AppStorage.

## Constraints

PersistentStorage accepts the following types and values:

- Primitive types such as number, string, boolean, and enum.
- Objects that can be reconstructed by **JSON.stringify()** and **JSON.parse()**, but member methods of the objects are not supported.
- Map type since API version 12: The overall value changes of the Map instance can be observed; you can call the **set**, **clear**, and **delete** APIs to update the instance; the updated value is persisted. For details, see [Decorating Variables of the Map Type](#decorating-variables-of-the-map-type).
- Set type since API version 12: The overall value changes of the Set instance can be observed; you can call the **add**, **clear**, and **delete** APIs to update the instance; the updated value is persisted. For details, see [Decorating Variables of the Set Type](#decorating-variables-of-the-set-type).
- Date type since API version 12: The overall value changes of the Date instance can be observed; you can call the following APIs to update the Date properties: **setFullYear**, **setMonth**, **setDate**, **setHours**, **setMinutes**, **setSeconds**, **setMilliseconds**, **setTime**, **setUTCFullYear**, **setUTCMonth**, **setUTCDate**, **setUTCHours**, **setUTCMinutes**, **setUTCSeconds**, and **setUTCMilliseconds**. the updated value is persisted. For details, see [Decorating Variables of the Date Type](#decorating-variables-of-the-date-type).
- **undefined** and **null** since API version 12.
- [Union types](#union-types) since API version 12.

PersistentStorage does not accept the following types and values:

- Nested objects (object arrays and object attributes), because the framework cannot detect the value changes of nested objects (including arrays) in AppStorage.

Data persistence is a time-consuming operation. As such, avoid the following situations whenever possible:

- Persistence of large data sets

- Persistence of variables that change frequently

It is recommended that the persistent variables of PersistentStorage be less than 2 KB. As PersistentStorage flushes data synchronously, a large amount of persistent data may result in simultaneous time-consuming read and write operations in the UI thread, affecting UI rendering performance. If you need to store a large amount of data, consider using the database API.

PersistentStorage is associated with UI instances. Data persistence can succeed only when a UI instance has been initialized (that is, when the callback passed in by [loadContent](../reference/apis-arkui/js-apis-window.md#loadcontent9-2) is called).

```ts
// EntryAbility.ets
onWindowStageCreate(windowStage: window.WindowStage): void {
  windowStage.loadContent('pages/Index', (err) => {
    if (err.code) {
      return;
    }
    PersistentStorage.persistProp('aProp', 47);
  });
}
```

## Use Scenarios


### Accessing PersistentStorage Initialized Attribute from AppStorage

1. Initialize the PersistentStorage instance.

   ```ts
   PersistentStorage.persistProp('aProp', 47);
   ```

2. Obtain the corresponding attribute from AppStorage.

   ```ts
   AppStorage.get<number>('aProp'); // returns 47
   ```

   Alternatively, apply local definition within the component:
   
   
      ```ts
      @StorageLink('aProp') aProp: number = 48;
      ```

The complete code is as follows:


```ts
PersistentStorage.persistProp('aProp', 47);

@Entry
@Component
struct Index {
  @State message: string = 'Hello World'
  @StorageLink('aProp') aProp: number = 48

  build() {
    Row() {
      Column() {
        Text(this.message)
        // The current result is saved when the application exits. After the restart, the last saved result is displayed.
        Text(`${this.aProp}`)
          .onClick(() => {
            this.aProp += 1;
          })
      }
    }
  }
}
```

- First running after fresh application installation:
  1. **persistProp** is called to initialize PersistentStorage. A search for the **aProp** attribute in PersistentStorage returns no result, because the application has just been installed.
  2. A search for the attribute **aProp** in AppStorage still returns no result.
  3. Create the **aProp** attribute of the number type in AppStorge and initialize it with the value 47.
  4. PersistentStorage writes the **aProp** attribute and its value **47** to the local device. The value of **aProp** in AppStorage and its subsequent changes are persisted.
  5. In the **Index** component, create the state variable **\@StorageLink('aProp') aProp**, which creates a two-way synchronization with the **aProp** attribute in AppStorage. During the creation, the search in AppStorage for the **aProp** attribute is successful, and therefore, the state variable is initialized with the value **47** found in AppStorage.

  **Figure 1** PersistProp initialization process 

![en-us_image_0000001553348833](figures/en-us_image_0000001553348833.png)

- After a click event is triggered:
  1. The state variable **\@StorageLink('aProp') aProp** is updated, triggering the **Text** component to be re-rendered.
  2. The two-way synchronization between the \@StorageLink decorated variable and AppStorage results in the change of the **\@StorageLink('aProp') aProp** being synchronized back to AppStorage.
  3. The change of the **aProp** attribute in AppStorage triggers any other one-way or two-way bound variables to be updated. (In this example, there are no such other variables.)
  4. Because the attribute corresponding to **aProp** has been persisted, the change of the **aProp** attribute in AppStorage triggers PersistentStorage to write the attribute and its new value to the device.

- Subsequent application running:
  1. **PersistentStorage.persistProp('aProp', 47)** is called. A search for the **aProp** attribute in PersistentStorage succeeds.
  2. The attribute is added to AppStorage with the value found in PersistentStorage.
  3. In the **Index** component, the value of the @StorageLink decorated **aProp** attribute is the value written by PersistentStorage to AppStorage, that is, the value stored when the application was closed last time.


### Accessing an Attribute in AppStorage Before PersistentStorage

This example is an incorrect use. It is incorrect to use the API to access the attributes in AppStorage before calling **PersistentStorage.persistProp** or **persistProps**, because such a call sequence will result in loss of the attribute values used in the previous application run:


```ts
let aProp = AppStorage.setOrCreate('aProp', 47);
PersistentStorage.persistProp('aProp', 48);
```

**AppStorage.setOrCreate('aProp', 47)**: The **aProp** attribute of the number type is created in AppStorage, and its value is set to the specified default value **47**. **aProp** is a persisted attribute. Therefore, it is written back to PersistentStorage, and the value stored in PersistentStorage from the previous run is lost.

PersistentStorage.persistProp('aProp', 48): An attribute with the name **aProp** and value **47** – set through the API in AppStorage – is found in PersistentStorage.

### Accessing an Attribute in AppStorage After PersistentStorage

If you do not want to overwrite the values saved in PersistentStorage during the previous application run, make sure any access to attributes in AppStorage is made after a call to a PersistentStorage API.

```ts
PersistentStorage.persistProp('aProp', 48);
if (AppStorage.get('aProp') > 50) {
    // If the value stored in PersistentStorage exceeds 50, set the value to 47.
    AppStorage.setOrCreate('aProp',47);
}
```

After reading the data stored in PersistentStorage, the sample code checks whether the value of **aProp** is greater than 50 and, if it is, sets **aProp** to **47** through an API in AppStorage.


### Union Types

PersistentStorage supports union types, **undefined**, and **null**. In the following example, the **persistProp** API is used to initialize **"P"** to **undefined**. **@StorageLink("P")** is used to bind variable **p** of the **number | undefined | null** type to the component. After the button is clicked, the value of **P** changes, and the UI is re-rendered. In addition, the value of **P** is persisted.

```ts
PersistentStorage.persistProp("P", undefined);

@Entry
@Component
struct TestCase6 {
  @StorageLink("P") p: number | undefined | null = 10;

  build() {
    Row() {
      Column() {
        Text(this.p + "")
          .fontSize(50)
          .fontWeight(FontWeight.Bold)
        Button("changeToNumber").onClick(() => {
          this.p = 10;
        })
        Button("changeTo undefined").onClick(() => {
          this.p = undefined;
        })
        Button("changeTo null").onClick(() => {
          this.p = null;
        })
      }  
      .width('100%')
    }
    .height('100%')
  }
}
```


### Decorating Variables of the Date Type

In this example, the **persistedDate** variable decorated by @StorageLink is of the Date type. After the button is clicked, the value of **persistedDate** changes, and the UI is re-rendered. In addition, the value of **persistedDate** is persisted.

```ts
PersistentStorage.persistProp("persistedDate", new Date());

@Entry
@Component
struct PersistedDate {
  @StorageLink("persistedDate") persistedDate: Date = new Date();

  updateDate() {
    this.persistedDate = new Date();
  }

  build() {
    List() {
      ListItem() {
        Column() {
          Text(`Persisted Date is ${this.persistedDate.toString()}`)
            .margin(20)

          Text(`Persisted Date month is ${this.persistedDate.getMonth()}`)
            .margin(20)

          Text(`Persisted Date day is ${this.persistedDate.getDay()}`)
            .margin(20)

          Text(`Persisted Date time is ${this.persistedDate.toLocaleTimeString()}`)
            .margin(20)

          Button() {
            Text('Update Date')
              .fontSize(25)
              .fontWeight(FontWeight.Bold)
              .fontColor(Color.White)
          }
          .type(ButtonType.Capsule)
          .margin({
            top: 20
          })
          .backgroundColor('#0D9FFB')
          .width('60%')
          .height('5%')
          .onClick(() => {
            this.updateDate();
          })

        }.width('100%')
      }
    }
  }
}
```

### Decorating Variables of the Map Type

In this example, the **persistedMapString** variable decorated by @StorageLink is of the Map\<number, string\> type. After the button is clicked, the value of **persistedMapString** changes, and the UI is re-rendered. In addition, the value of **persistedMapString** is persisted.

```ts
PersistentStorage.persistProp("persistedMapString", new Map<number, string>([]));

@Entry
@Component
struct PersistedMap {
  @StorageLink("persistedMapString") persistedMapString: Map<number, string> = new Map<number, string>([]);

  persistMapString() {
    this.persistedMapString = new Map<number, string>([[3, "one"], [6, "two"], [9, "three"]]);
  }

  build() {
    List() {
      ListItem() {
        Column() {
          Text(`Persisted Map String is `)
            .margin(20)
          ForEach(Array.from(this.persistedMapString.entries()), (item: [number, string]) => {
            Text(`${item[0]} ${item[1]}`)
          })

          Button() {
            Text('Persist Map String')
              .fontSize(25)
              .fontWeight(FontWeight.Bold)
              .fontColor(Color.White)
          }
          .type(ButtonType.Capsule)
          .margin({
            top: 20
          })
          .backgroundColor('#0D9FFB')
          .width('60%')
          .height('5%')
          .onClick(() => {
            this.persistMapString();
          })

        }.width('100%')
      }
    }
  }
}
```

### Decorating Variables of the Set Type

In this example, the **persistedSet** variable decorated by @StorageLink is of the Set\<number\> type. After the button is clicked, the value of **persistedSet** changes, and the UI is re-rendered. In addition, the value of **persistedSet** is persisted.

```ts
PersistentStorage.persistProp("persistedSet", new Set<number>([]));

@Entry
@Component
struct PersistedSet {
  @StorageLink("persistedSet") persistedSet: Set<number> = new Set<number>([]);

  persistSet() {
    this.persistedSet = new Set<number>([33, 1, 3]);
  }

  clearSet() {
    this.persistedSet.clear();
  }

  build() {
    List() {
      ListItem() {
        Column() {
          Text(`Persisted Set is `)
            .margin(20)
          ForEach(Array.from(this.persistedSet.entries()), (item: [number, string]) => {
            Text(`${item[1]}`)
          })

          Button() {
            Text('Persist Set')
              .fontSize(25)
              .fontWeight(FontWeight.Bold)
              .fontColor(Color.White)
          }
          .type(ButtonType.Capsule)
          .margin({
            top: 20
          })
          .backgroundColor('#0D9FFB')
          .width('60%')
          .height('5%')
          .onClick(() => {
            this.persistSet();
          })

          Button() {
            Text('Persist Clear')
              .fontSize(25)
              .fontWeight(FontWeight.Bold)
              .fontColor(Color.White)
          }
          .type(ButtonType.Capsule)
          .margin({
            top: 20
          })
          .backgroundColor('#0D9FFB')
          .width('60%')
          .height('5%')
          .onClick(() => {
            this.clearSet();
          })

        }
        .width('100%')
      }
    }
  }
}
```