# AppStorage: Application-wide UI State Storage


AppStorage provides the central storage for mutable application UI state attributes. It is bound to the application process and is created by the UI framework at application startup.


Unlike LocalStorage, which is usually used for page-level state sharing, AppStorage enables application-wide UI state sharing. AppStorage is equivalent to the hub of the entire application. [PersistentStorage](arkts-persiststorage.md) and [Environment](arkts-environment.md) data is passed first to AppStorage and then from AppStorage to the UI component.


This topic describes only the AppStorage application scenarios and related decorators: \@StorageProp and \@StorageLink.


## Overview

AppStorage is a singleton LocalStorage object that is created by the UI framework at application startup. Its purpose is to provide the central storage for mutable application UI state attributes. AppStorage retains all those attributes and their values as long as the application remains running. Attributes are accessed using a unique key string value.

UI components synchronize application state attributes with the AppStorage. Implementation of application business logic can access AppStorage as well.

Selected state attributes of AppStorage can be synched with different data sources or data sinks. Those data sources and sinks can be on a local or remote device, and have different capabilities, such as data persistence (see [PersistentStorage](arkts-persiststorage.md)). These data sources and sinks are implemented in the business logic, separate from the UI. Link those AppStorage attributes to [@StorageProp](#storageprop) and [@StorageLink](#storagelink) whose values should be kept until application re-start.


## \@StorageProp

As mentioned above, if you want to establish a binding between AppStorage and a custom component, you need to use the \@StorageProp and \@StorageLink decorators. Use \@StorageProp(key) or \@StorageLink(key) to decorate variables in the component. **key** identifies the attribute in AppStorage.

When a custom component is initialized, the \@StorageProp(key)/\@StorageLink(key) decorated variable is initialized with the value of the attribute with the given key in AppStorage. Local initialization is mandatory. If an attribute with the given key is missing from AppStorage, it will be added with the stated initializing value. (Whether the attribute with the given key exists in AppStorage depends on the application logic.)


By decorating a variable with \@StorageProp(key), a one-way data synchronization is established with the attribute with the given key in AppStorage. A local change can be made, but it will not be synchronized to AppStorage. An update to the attribute with the given key in AppStorage will overwrite local changes.


### Rules of Use

| \@StorageProp Decorator| Description                                      |
| ------------------ | ---------------------------------------- |
| Decorator parameters             | **key**: constant string, mandatory (note, the string is quoted)                 |
| Allowed variable types         | Object, class, string, number, Boolean, enum, and array of these types. For details about the scenarios of nested objects, see [Observed Changes and Behavior](#observed-changes-and-behavior).<br>The type must be specified and must be the same as the corresponding attribute in LocalStorage. **any** is not supported. The **undefined** and **null** values are not allowed.|
| Synchronization type              | One-way: from the attribute in AppStorage to the component variable.<br>The component variable can be changed locally, but an update from AppStorage will overwrite local changes.|
| Initial value for the decorated variable         | Mandatory. It is used as the default value for initialization if the attribute does not exist in AppStorage.|


### Variable Transfer/Access Rules

| Transfer/Access     | Description                                      |
| ---------- | ---------------------------------------- |
| Initialization and update from the parent component| Forbidden.|
| Subnode initialization    | Supported; can be used to initialize a n \@State, \@Link, \@Prop, or \@Provide decorated variable in the child component.|
| Access | None.                                      |


  **Figure 1** \@StorageProp initialization rule 


![en-us_image_0000001552978157](figures/en-us_image_0000001552978157.png)


### Observed Changes and Behavior

**Observed Changes**


- When the decorated variable is of the Boolean, string, or number type, its value change can be observed.

- When the decorated variable is of the class or Object type, its value change and value changes of all its attributes, that is, the attributes that **Object.keys(observedObject)** returns.

- When the decorated variable is of the array type, the addition, deletion, and updates of array items can be observed.


**Framework Behavior**


- When the value change of the \@StorageProp(key) decorated variable is observed, the change is not synchronized to the attribute with the give key value in AppStorage.

- The value change of the \@StorageProp(key) decorated variable only applies to the private member variables of the current component, but not other variables bound to the key.

- When the data decorated by \@StorageProp(key) is a state variable, the change of the data is not synchronized to AppStorage, but the owning custom component is re-rendered.

- When the attribute with the given key in AppStorage is updated, the change is synchronized to all the \@StorageProp(key) decorated data, and the local changes of the data are overwritten.


## \@StorageLink

\@StorageLink(key) creates a two-way data synchronization with the attribute with the given key in AppStorage.

1. If a local change occurs, it is synchronized to AppStorage.

2. Changes in AppStorage are synchronized to all attributes with the given key, including one-way bound variables (\@StorageProp decorated variables and one-way bound variables created through \@Prop), two-way bound variables (\@StorageLink decorated variables and two-way bound variables created through \@Link), and other instances (such as PersistentStorage).


### Rules of Use

| \@StorageLink Decorator| Description                                      |
| ------------------ | ---------------------------------------- |
| Decorator parameters             | **key**: constant string, mandatory (note, the string is quoted)                 |
| Allowed variable types         | Object, class, string, number, Boolean, enum, and array of these types. For details about the scenarios of nested objects, see [Observed Changes and Behavior](#observed-changes-and-behavior).<br>The type must be specified and must be the same as the corresponding attribute in AppStorage. **any** is not supported. The **undefined** and **null** values are not allowed.|
| Synchronization type              | Two-way: from the attribute in AppStorage to the custom component variable and back|
| Initial value for the decorated variable         | Mandatory. It is used as the default value for initialization if the attribute does not exist in AppStorage.|


### Variable Transfer/Access Rules

| Transfer/Access     | Description                                      |
| ---------- | ---------------------------------------- |
| Initialization and update from the parent component| Forbidden.                                     |
| Subnode initialization    | Supported; can be used to initialize a regular variable or \@State, \@Link, \@Prop, or \@Provide decorated variable in the child component.|
| Access | None.                                      |


  **Figure 2** \@StorageLink initialization rule 


![en-us_image_0000001501938718](figures/en-us_image_0000001501938718.png)


### Observed Changes and Behavior

**Observed Changes**


- When the decorated variable is of the Boolean, string, or number type, its value change can be observed.

- When the decorated variable is of the class or Object type, its value change and value changes of all its attributes, that is, the attributes that **Object.keys(observedObject)** returns.

- When the decorated variable is of the array type, the addition, deletion, and updates of array items can be observed.


**Framework Behavior**


1. When the value change of the \@StorageLink(key) decorated variable is observed, the change is synchronized to the attribute with the give key value in AppStorage.

2. Once the attribute with the given key in AppStorage is updated, all the data (including \@StorageLink and \@StorageProp decorated variables) bound to the attribute key is changed synchronously.

3. When the data decorated by \@StorageLink(key) is a state variable, the change of the data is synchronized to AppStorage, and the owning custom component is re-rendered.


## Application Scenarios


### Example of Using AppStorage and LocalStorage from Application Logic

Since AppStorage is a singleton, its APIs are all static ones. How these APIs work resembles the non-static APIs of LocalStorage.


```ts
AppStorage.SetOrCreate('PropA', 47);

let storage: LocalStorage = new LocalStorage({ 'PropA': 17 });
let propA: number = AppStorage.Get('PropA') // propA in AppStorage == 47, propA in LocalStorage == 17
var link1: SubscribedAbstractProperty<number> = AppStorage.Link('PropA'); // link1.get() == 47
var link2: SubscribedAbstractProperty<number> = AppStorage.Link('PropA'); // link2.get() == 47
var prop: SubscribedAbstractProperty<number> = AppStorage.Prop('PropA'); // prop.get() = 47

link1.set(48); // two-way sync: link1.get() == link2.get() == prop.get() == 48
prop.set(1); // one-way sync: prop.get()=1; but link1.get() == link2.get() == 48
link1.set(49); // two-way sync: link1.get() == link2.get() == prop.get() == 49

storage.get('PropA') // == 17 
storage.set('PropA', 101);
storage.get('PropA') // == 101

AppStorage.Get('PropA') // == 49
link1.get() // == 49
link2.get() // == 49
prop.get() // == 49
```


### Example of Using AppStorage and LocalStorage from Inside the UI

\@StorageLink works together with the AppStorage in the same way as \@LocalStorageLink works together with LocalStorage. It creates two-way data synchronization with an attribute in AppStorage.


```ts
AppStorage.SetOrCreate('PropA', 47);
let storage = new LocalStorage({ 'PropA': 48 });

@Entry(storage)
@Component
struct CompA {
  @StorageLink('PropA') storLink: number = 1;
  @LocalStorageLink('PropA') localStorLink: number = 1;

  build() {
    Column({ space: 20 }) {
      Text(`From AppStorage ${this.storLink}`)
        .onClick(() => this.storLink += 1)

      Text(`From LocalStorage ${this.localStorLink}`)
        .onClick(() => this.localStorLink += 1)
    }
  }
}
```


## Restrictions

When using AppStorage together with [PersistentStorage](arkts-persiststorage.md) and [Environment](arkts-environment.md), pay attention to the following:

- A call to **PersistentStorage.PersistProp()** after creating the attribute in AppStorage uses the type and value in AppStorage and overwrites any attribute with the same name in PersistentStorage. In light of this, the opposite order of calls is recommended. For an example of incorrect usage, see [Accessing Attribute in AppStorage Before PersistentStorage](arkts-persiststorage.md#accessing-attribute-in-appstorage-before-persistentstorage).

- A call to **Environment.EnvProp()** after creating the attribute in AppStorage will fail. This is because AppStorage already has an attribute with the same name, and the environment variable will not be written into AppStorage. Therefore, you are advised not to use the preset environment variable name in AppStorage.