harmony 鸿蒙Arkui Subsystem State Management Changelog

2023-10-30
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Arkui Subsystem State Management Changelog

Support by @Prop for Object Behavior Changes

cl. Change from Shallow Copy to Deep Copy of Objects by @Prop

Change Impact

In API version 9, @Prop supports shallow copy of objects. The specific behavior is as follows:

Object type: shallow copy of all properties returned by Object.keys(propObj). That is, only the outermost object is copied, and the property in the object points to the data source of the original parent component.
Array type: shallow copy of all array items. That is, only the outermost array is copied. Other behaviors are the same as those of the Object type.
The prototype of the object and array is copied.

Since API version 10, @Prop supports deep copy of objects. The specific behavior is as follows: - Object type: deep copy of all properties returned by Object.keys(propObj). - Array type: deep copy of all array items. - The prototype of the object and array is copied.

Adaptation Guide

The impact of the change on UI rendering mainly lies in @ObjectLink-related scenarios, because @ObjectLink functions as a proxy for its decorated object properties.

The following shows an example:

API version 9: If @Prop objArray in the PropClassArray component changes the property of ClassA or sets a new array item, @ObjectLink obj in ObjectLinkClassA created by the parent component StateClassAArray changes.

The update is caused by the shallow copy from @State stateClassAArray in the parent component StateClassAArray to @Prop objArray in the child component PropClassAArray. The shallow copy copies only the reference of array items, and therefore the data source is changed. As @ObjectLink functions as a proxy for the properties of the data source, the update of @ObjectLink obj (constructed from the StateClassAArray component) in the ObjectLinkClassA component is triggered.

API version 10: In the preceding scenario, the copy from @State stateClassAArray in the parent component StateClassAArray to @Prop objArray in the child component PropClassAArray is a deep copy. As the object of ClassA is completely copied, the data source is not changed, and the update of @ObjectLink obj (constructed from the StateClassAArray component) in the ObjectLinkClassA component is not triggered.

en-us_image_0000001588291546

let nextId = 0;

@Observed
class ClassA  {
  id : number;
  a : number;
  constructor(a : number = 0) {
    this.id = nextId++;
    this.a = a;
  }
}

@Component
struct ObjectLinkClassA {
  @ObjectLink obj : ClassA;

  build() {
    Row() {
      Text(`ObjectLink: obj: ${this.obj.a}`)
        .height(100)
        .onClick(() => {
          this.obj.a += 1;
          console.info(`ObjectLink onClick ClassA property changed to  ${this.obj.a}`)
        })
    }.border({width: 3, color: Color.Red})
  }
}

@Component
struct PropClassAArray {
  @Prop objArray : Array<ClassA> = [];

  build() {
    Column() {
      Text(`green box: @Prop : Array<ObjectClassA> item [0] + [1]`)
      Row() {
        ObjectLinkClassA({ obj:  this.objArray[0] })
        Text("[0] Assign new ClassA")
          .height(100)
          .onClick(() => {
            this.objArray[0] = new ClassA();
            console.info(`PropClassAArray[0] onClick ClassA object assign ${this.objArray[0].a}`)
          })
        Text("Change ClassA property")
          .height(100)
          .onClick(() => {
            this.objArray[0].a += 1;
            console.info(`PropClassAArray[1] onClick ClassA property change  ${this.objArray[1].a}`)
          })
      }
    }.border({width: 3, color: Color.Green})
  }
}

@Entry
@Component
struct StateClassAArray {
  @State stateClassAArray : Array<ClassA> = [ new ClassA(), new ClassA() ];

  build() {
    Column() {
      Column() {
        Text("Red box: @ObjectLink from @State array item[0]")
        Row() {
          ObjectLinkClassA({obj : this.stateClassAArray[0] })
          Text("Assign new ClassA")
            .height(100)
            .onClick(() => {
              this.stateClassAArray[0] = new ClassA();
              console.info(`StateClassAArray[0] onClick ClassA object assign ${this.stateClassAArray[0].a}`)
            })
          Text("Change ClassA property")
            .height(100)
            .onClick(() => {
              this.stateClassAArray[0].a += 1;
              console.info(`StateClassAArray onClick stateClassAArray[0] changed to  ${this.stateClassAArray[0].a}`)
            })
        }

      }.border({width: 3, color: Color.Blue})

      Divider().height(5)

      // Shallow copy in API version 9: Only the reference pointing to the source array item is copied, and the ClassA instance itself is not copied.
      // Deep copy in API version 10: The this.stateClassAArray instance is completely copied, including its array items.
      PropClassAArray({ objArray: this.stateClassAArray })
    }
  }
}