harmony(鸿蒙)@Builder

2022-08-09
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@Builder

The @Builder decorated method is used to define the declarative UI description of a component and quickly generate multiple layouts in a custom component. If a custom component is used in the @Builder decorated method, this component will be re-created each time the method is invoked. The functionality and syntax of the @Builder decorator are the same as those of the build function.

// xxx.ets

@Component
struct CompB {
  @State CompValue: string = '';

  aboutToAppear() {
    console.info('CompB aboutToAppear.');
  }

  aboutToDisappear() {
    console.info('CompB aboutToDisappear.');
  }

  build() {
    Column() {
      Button(this.CompValue);
    }
  }
}

@Entry
@Component
struct CompA {
  size1: number = 100;
  @State CompValue1: string = "Hello,CompValue1";
  @State CompValue2: string = "Hello,CompValue2";
  @State CompValue3: string = "Hello,CompValue3";

  // Use a custom component in the @Builder decorated method.
  @Builder CompC(value: string) {
    CompB({ CompValue: value });
  }

  @Builder SquareText(label: string) {
    Text(label)
      .width(1 * this.size1)
      .height(1 * this.size1)
  }

  @Builder RowOfSquareTexts(label1: string, label2: string) {
    Row() {
      this.SquareText(label1)
      this.SquareText(label2)
    }
    .width(2 * this.size1)
    .height(1 * this.size1)
  }

  build() {
    Column() {
      Row() {
        this.SquareText("A")
        this.SquareText("B")
        // or as long as tsc is used
      }
      .width(2 * this.size1)
      .height(1 * this.size1)

      this.RowOfSquareTexts("C", "D")
      Column() {
        // Use the custom component in the @Builder decorated method three times.
        this.CompC(this.CompValue1);
        this.CompC(this.CompValue2);
        this.CompC(this.CompValue3);
      }
      .width(2 * this.size1)
      .height(2 * this.size1)
    }
    .width(2 * this.size1)
    .height(2 * this.size1)
  }
}

@BuilderParam⁸⁺

The @BuilderParam decorator is used to modify the function type attributes (for example, @BuilderParam content: () => any;) in a custom component. When the custom component is initialized, the attributes modified by the @BuilderParam decorator must be assigned values.

Background

In certain circumstances, you may need to add a specific function, such as a click-to-jump action, to a custom component. However, embedding an event method directly inside of the component will add the function to all places where the component is initialized. This is where the @BuilderParam decorator comes into the picture. When initializing a custom component, you can assign a @Builder decorated method to the @BuilderParam decorated attribute, thereby adding the specific function to the custom component.

Component Initialization Through Parameters

When initializing a custom component through parameters, assign a @Builder decorated method to the @BuilderParam decorated attribute — content, and call the value of content in the custom component. If no parameter is passed when assigning a value to the @BuilderParam decorated attribute (for example, content: this.specificParam), define the type of the attribute as a function without a return value (for example, @BuilderParam content: () => void). If any parameter is passed when assigning a value to the @BuilderParam decorated attribute (for example, callContent: this.specificParam1("111")), define the type of the attribute as any (for example,@BuilderParam callContent: any;).

// xxx.ets
@Component
struct CustomContainer {
  header: string = "";
  @BuilderParam noParam: () => void;
  @BuilderParam withParam: any;
  footer: string = "";
  build() {
    Column() {
      Text(this.header)
        .fontSize(50)
      this.noParam()
      this.withParam()
      Text(this.footer)
        .fontSize(50)
    }
  }
}

@Entry
@Component
struct CustomContainerUser {
  @Builder specificNoParam() {
    Column() {
      Text("noParam").fontSize(50)
    }
  }
  @Builder SpecificWithParam(label: string) {
    Column() {
      Text(label).fontSize(50)
    }
  }

  build() {
    Column() {
      CustomContainer({
        header: "Header",
        noParam: this.specificNoParam,
        withParam: this.SpecificWithParam("WithParam"),
        footer: "Footer",
      })
    }
  }
}

Component Initialization Through Trailing Closure

In a custom component, use the @BuilderParam decorated attribute to receive a trailing closure. When the custom component is initialized, the component name is followed by a pair of braces ({}) to form a trailing closure (CustomComponent(){}). You can consider a trailing closure as a container and add content to it. For example, you can add a component ({Column(){Text("content")}) to a trailing closure. The syntax of the closure is the same as that of the build function. In this scenario, the custom component has one and only one @BuilderParam decorated attribute.

Example: Add a <Column> component and a click event to the closure, and call the specificParam method decorated by @Builder in the new <Column> component. After the <Column> component is clicked, the value of the component’s header attribute will change to changeHeader. In addition, when the component is initialized, the content of the trailing closure will be assigned to the closer attribute decorated by @BuilderParam.

// xxx.ets
@Component
struct CustomContainer {
  header: string = "";
  @BuilderParam closer: () => void;
  build() {
    Column() {
      Text(this.header)
        .fontSize(50)
      this.closer()
    }
  }
}
@Builder function specificParam(label1: string, label2: string) {
  Column() {
    Text(label1)
      .fontSize(50)
    Text(label2)
      .fontSize(50)
  }
}
@Entry
@Component
struct CustomContainerUser {
  @State text: string = "header"
  build() {
    Column() {
      CustomContainer({
        header: this.text,
      }){
        Column(){
          specificParam("111", "22")
        }.onClick(()=>{
          this.text = "changeHeader"
        })
      }
    }
  }
}