harmony 鸿蒙\@Builder Decorator: Custom Builder Function

2023-06-24
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\@Builder Decorator: Custom Builder Function

As previously described, you can reuse UI elements by creating a custom component, which comes with a fixed internal UI structure and allows for data transfer only with its caller. ArkUI also provides a more lightweight mechanism for reusing UI elements: \@Builder. An \@Builder decorated function is a special function that serves similar purpose as the build function. The \@Builder function body follows the same syntax rules as the build function. You can abstract reusable UI elements into a method and call the method in build.

To simplify language, here we refer to an \@Builder decorated function also as a custom builder function.

NOTE

Since API version 9, this decorator is supported in ArkTS widgets.

Rules of Use

Custom Builder Function

Syntax:

@Builder MyBuilderFunction() { ... }

Usage:

this.MyBuilderFunction() { ... }

Defining one or more custom builder (\@Builder decorated) functions inside a custom component is allowed. Such a custom builder function can be considered as a private, special type of member functions of that component.
The custom builder function can be called from the owning component’s build or another custom builder (within that custom component) function only.
Inside the custom builder function body, this refers to the owning component. Component state variables are accessible from within the custom builder function implementation. Using the custom components’ state variables is recommended over parameter passing.

Global Custom Builder Function

Syntax:

@Builder function MyGlobalBuilderFunction() { ... }

Usage:

MyGlobalBuilderFunction()

A global custom builder function is accessible from the entire application. this and the bind method are not allowed.
Use of a global custom builder function is recommended if no own state is required.

Parameter Passing Rules

There are two types of parameter passing for custom builder functions: by-value parameter passing and by-reference parameter passing. Both of them must comply with the following rules:

The parameter type must be the same as the declared parameter type. The undefined or null constants as well as expressions evaluating to these values are not allowed.
All parameters are immutable inside the @Builder decorated function.
The \@Builder function body follows the same syntax rules as the build function.

By-Reference Parameter Passing

In by-reference parameter passing, the passed parameters can be state variables, and the change of these state variables causes the UI re-rendering in the \@Builder method. ArkUI provides $$ as a paradigm for by-reference parameter passing.

class ABuilderParam {
  paramA1: string = ''
  paramB1: string = ''
}

@Builder function ABuilder($$ : ABuilderParam) {...}

class ABuilderParam {
  paramA1: string = ''
}

@Builder function ABuilder($$: ABuilderParam) {
  Row() {
    Text(`UseStateVarByReference: ${$$.paramA1} `)
  }
}
@Entry
@Component
struct Parent {
  @State label: string = 'Hello';
  build() {
    Column() {
      // Pass the this.label reference to the ABuilder component when the ABuilder component is called in the Parent component.
      ABuilder({ paramA1: this.label })
      Button('Click me').onClick(() => {
        // After Click me is clicked, the UI text changes from Hello to ArkUI.
        this.label = 'ArkUI';
      })
    }
  }
}

By-Value Parameter Passing

By default, parameters in the \@Builder decorated functions are passed by value. When the passed parameter is a state variable, the change of the state variable does not cause the UI re-rendering in the \@Builder method. Therefore, when using state variables, you are advised to use by-reference parameter passing.

@Builder function ABuilder(paramA1: string) {
  Row() {
    Text(`UseStateVarByValue: ${paramA1} `)
  }
}
@Entry
@Component
struct Parent {
  label: string = 'Hello';
  build() {
    Column() {
      ABuilder(this.label)
    }
  }
}