harmony(鸿蒙)Initialization of Custom Components’ Member Variables

2022-08-09
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Initialization of Custom Components’ Member Variables

The member variables of a component can be initialized in either of the following ways:

Local initialization. For example:

  @State counter: Counter = new Counter()

Initialization using constructor parameters. For example:

  MyComponent({counter: $myCounter})

The allowed method depends on the decorator of the state variable, as shown in the following table.

Decorator Type	Local Initialization	Initialization Using Constructor Parameters
@State	Mandatory	Optional
@Prop	Forbidden	Mandatory
@Link	Forbidden	Mandatory
@StorageLink	Mandatory	Forbidden
@StorageProp	Mandatory	Forbidden
@Provide	Mandatory	Optional
@Consume	Forbidden	Forbidden
@ObjectLink	Forbidden	Mandatory
Normal member variable	Recommended	Optional

As indicated by the preceding table:

The @State decorated variables need to be initialized locally. The initial value can be overwritten by the constructor parameter.
The @Prop and @Link decorated variables must be initialized only by constructor parameters.

Comply with the following rules when using constructors to initialize member variables:

From the Variable in the Parent Component (Below) to the Variable in the Child Component (Right)	@State	@Link	@Prop	Normal Variable
@State	Not allowed	Allowed	Allowed	Allowed
@Link	Not allowed	Allowed	Not recommended	Allowed
@Prop	Not allowed	Not allowed	Allowed	Allowed
@StorageLink	Not allowed	Allowed	Not allowed	Allowed
@StorageProp	Not allowed	Not allowed	Not allowed	Allowed
Normal variable	Allowed	Not allowed	Not allowed	Allowed

As indicated by the preceding table:

The normal variables of the parent component can be used to initialize the @State decorated variables of the child component, but not the @Link or @Prop decorated variables.
The @State decorated variable of the parent component can be used to initialize the @Prop, @Link (using $), or normal variables of the child component, but not the @State decorated variables of the child component.
The @Link decorated variables of the parent component can be used to initialize the @Link decorated or normal variables of the child component. However, initializing the @State decorated members of the child component can result in a syntax error. In addition, initializing the @Prop decorated variables is not recommended.
The @Prop decorated variables of the parent component can be used to initialize the normal variables or @Prop decorated variables of the child component, but not the @State or @Link decorated variables.
Passing @StorageLink and @StorageProp from the parent component to the child component is prohibited.
In addition to the preceding rules, the TypeScript strong type rules need to be followed.

Example

// xxx.ets
class ClassA {
  public a:number
  constructor(a: number) {
    this.a = a
  }
}
@Entry
@Component
struct Parent {
  @State parentState: ClassA = new ClassA(1)
  
  build() {
    Column() {
      Flex({ justifyContent: FlexAlign.Start, alignItems: ItemAlign.Center }) {
        CompA({ aState: new ClassA(2), aLink: $parentState })
      }
      Flex({ justifyContent: FlexAlign.Start, alignItems: ItemAlign.Center }) {
        CompA({ aLink: $parentState })
      }
      Flex({ justifyContent: FlexAlign.Start, alignItems: ItemAlign.Center }) {
        CompA({ aState: new ClassA(3), aLink: $parentState })
      }
    }
  }
}

@Component
struct CompA {
  @State aState: any = false
  @Link aLink: ClassA
  
  build() {
    Column() {
      CompB({ bLink: $aLink, bProp: this.aState })
      CompB({ bLink: $aState, bProp: false })
    }
  }
}

@Component
struct CompB {
  @Link bLink: ClassA
  @Prop bProp: boolean
  
  build() {
    Flex({ justifyContent: FlexAlign.Start, alignItems: ItemAlign.Center }) {
      Text(JSON.stringify(this.bLink.a)).fontSize(30)
      Text(JSON.stringify(this.bProp)).fontSize(30).fontColor(Color.Red)
     }.margin(10)
  }
}