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.

PersistentStorage provides capability for persisting the state variables. However, the persistence and UI reading capabilities depend on AppStorage. Before reading this topic, you are advised to read AppStorage and PersistentStorage API reference.

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.

The data storage path of PersistentStorage is at the module level. That is, the data copy is stored in the persistent file of the corresponding module when the module calls PersistentStorage. If multiple modules use the same key, the data is copied from and stored in the module that uses PersistentStorage first.

The storage path of PersistentStorage, determined when the first ability of the application is started, is the module to which the ability belongs. If an ability calls PersistentStorage and can be started by different modules, the number of data copies is the same as the number of startup modes of the ability.

PersistentStorage is coupled with AppStorage in terms of functions, and errors may occur when using data in different modules. Therefore, you are advised to use the globalConnect API of PersistenceV2 to replace the persistProp API of PersistentStorage. For details about how to migrate data from PersistentStorage to PersistenceV2, see PersistentStorage->PersistenceV2. For details about PersistenceV2, see PersistenceV2: Persisting Application State.

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.
• 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.
• 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.
• undefined and null since API version 12.
• 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 is called).

// 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.

   PersistentStorage.persistProp('aProp', 47);

1. Obtain the corresponding attribute from AppStorage.

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

Alternatively, apply local definition within the component:

   @StorageLink('aProp') aProp: number = 48;

The complete code is as follows:

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

• 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:

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.

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.

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.

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 year is ${this.persistedDate.getFullYear()}`)
            .margin(20)

          Text(`Persisted Date hours is ${this.persistedDate.getHours()}`)
            .margin(20)

          Text(`Persisted Date minutes is ${this.persistedDate.getMinutes()}`)
            .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.

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.

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%')
      }
    }
  }
}

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