harmony 鸿蒙Basic Concepts

2022-12-05
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Basic Concepts

In the multi-dimensional state management mechanism for ArkUI, UI-related data can be used not only within a component, but also be transferred between different component levels (for example, between parent and child components, between grandparent and grandchild components, or globally). In addition, data transfer can be classified as one-way (read-only) or two-way (mutable). You can use these capabilities at your disposal to implement linkage between data and the UI.

State Management with Page-level Variables

Decorator	Decorates…	Description
@State	Primitive data types, classes, and arrays	If the decorated state data is modified, the build method of the component will be called to update the UI.
@Prop	Primitive data types	This decorator is used to establish one-way data binding between the parent and child components. When the data associated with the parent component is modified, the UI of the current component is re-rendered.
@Link	Primitive data types, classes, and arrays	This decorator is used to establish two-way data binding between the parent and child components. The internal state data of the parent component is used as the data source. Any changes made to one component will be reflected to the other.
@Observed	Class	This decorator is used to indicate that the data changes in the class will be managed by the UI page.
@ObjectLink	Objects of @Observed decorated classes	When the decorated state variable is modified, the parent and sibling components that have the state variable will be notified for UI re-rendering.
@Provide	Primitive data types, classes, and arrays	As the data provider, @Provide can update the data of child nodes and trigger page re-rendering.
@Consume	Primitive data types, classes, and arrays	When the @Consume decorated variable detects the update of the @Provide decorated variable, the re-rendering of the current custom component is triggered.

State Management with Application-level Variables

AppStorage is the central store of the application states in the entire UI. ArkUI creates a singleton AppStorage object for the application and provides the corresponding decorators and APIs for the application.

@StorageLink: works in a way similar to that of @Consume. The difference is that the target object is obtained from the AppStorage based on the given name. @StorageLink establishes two-way binding between the decorated UI component and AppStorage to synchronize data.
@StorageProp: synchronizes UI component attributes with the AppStorage unidirectionally. That is, the value change in the AppStorage will trigger an update of the corresponding UI component, but the change of the UI component will not cause an update of the attribute value in the AppStorage.
Service logic implementation API: adds, reads, modifies, or deletes the state data of applications. The changes made by this API will be synchronized to the UI component for UI update.
LocalStorage: provides ability-specific storage.
@LocalStorageLink: establishes two-way data binding between a component and the LocalStorage. Specifically, this is achieved by decorating the component’s state variable with @LocalStorageLink(key). Wherein, key is the attribute key value in the LocalStorage.
@LocalStorageProp: establishes one-way data binding between a component and the LocalStorage. Specifically, this is achieved by decorating the component’s state variable with @LocalStorageProp(key). Wherein, key is the attribute key value in the LocalStorage.
PersistentStorage: provides a set of static methods for managing persistent data of applications. Persistent data with specific tags can be linked to the AppStorage, and then the persistent data can be accessed through the AppStorage APIs. Alternatively, the @StorageLink decorator can be used to access the variable that matches the specific key.
Environment: provides the AppStorage with an array of environment state attributes that are required by the application and describe the device environment where the application runs. It is a singleton object created by the framework when the application is started.