harmony 鸿蒙\@Watch Decorator: Getting Notified of State Variable Changes

  • 2023-06-24
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\@Watch Decorator: Getting Notified of State Variable Changes

\@Watch is used to listen for state variables. If your application needs watch for value changes of a state variable, you can decorate the variable with \@Watch.

NOTE

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

Overview

An application can request to be notified whenever the value of the \@Watch decorated variable changes. The \@Watch callback is called when the value change has occurred. \@Watch uses strict equality (===) to determine whether a value is updated in the ArkUI framework. If false is returned, the \@Watch callback is triggered.

Decorator Description

\@Watch Decorator Description
Decorator parameters Mandatory. Constant string, which is quoted. Reference to a (string) => void custom component member function.
Custom component variables that can be decorated All decorated state variables. Regular variables cannot be watched.
Order of decorators It is recommended that the \@State, \@Prop, \@Link, or other decorators precede the \@Watch decorator.

Syntax

Type Description
(changedPropertyName? : string) => void This function is a member function of the custom component. changedPropertyName indicates the name of the watched attribute.
It is useful when you use the same function as a callback to several watched attributes.
It takes the attribute name as a string input parameter and returns nothing.

Observed Changes and Behavior

  1. When a state variable change (including the change of the named attribute in AppStorage or LocalStorage) is observed, the corresponding \@Watch callback is triggered.

  2. \@The Watch callback is executed synchronously after the variable change in the custom component.

  3. If the \@Watch callback mutates other watched variables, their variable @Watch callbacks in the same and other custom components as well as state updates are triggered.

  4. A \@Watch function is not called upon custom component variable initialization, because initialization is not considered as variable mutation. A \@Watch function is called upon updating of the custom component variable.

Restrictions

  • Pay attention to the risk of infinite loops. Loops can be caused by the \@Watch callback directly or indirectly mutating the same variable. To avoid loops, avoid mutating the \@Watch decorated state variable inside the callback handler.

  • Pay attention to performance. The attribute value update function delays component re-render (see the preceding behavior description). The callback should only perform quick computations.

  • Calling async await from an \@Watch function is not recommended, because asynchronous behavior may cause performance issues of re-rendering.

Application Scenarios

\@Watch and Custom Component Update

This example is used to clarify the processing steps of custom component updates and \@Watch. count is decorated by \@State in CountModifier and \@Prop in TotalView.

@Component
struct TotalView {
  @Prop @Watch('onCountUpdated') count: number = 0;
  @State total: number = 0;
  // @Watch cb
  onCountUpdated(propName: string): void {
    this.total += this.count;
  }

  build() {
    Text(`Total: ${this.total}`)
  }
}

@Entry
@Component
struct CountModifier {
  @State count: number = 0;

  build() {
    Column() {
      Button('add to basket')
        .onClick(() => {
          this.count++
        })
      TotalView({ count: this.count })
    }
  }
}

Processing steps:

  1. The click event Button.onClick of the CountModifier custom component increases the value of count.

  2. In response to the change of the @State decorated variable count, \@Prop in the child component TotalView is updated, and its \@Watch(‘onCountUpdated’) callback is triggered, which updates the total variable in TotalView.

  3. The Text component in the child component TotalView is re-rendered.

Combination of \@Watch and \@Link

This example illustrates how to watch an \@Link decorated variable in a child component.

class PurchaseItem {
  static NextId: number = 0;
  public id: number;
  public price: number;

  constructor(price: number) {
    this.id = PurchaseItem.NextId++;
    this.price = price;
  }
}

@Component
struct BasketViewer {
  @Link @Watch('onBasketUpdated') shopBasket: PurchaseItem[];
  @State totalPurchase: number = 0;

  updateTotal(): number {
    let total = this.shopBasket.reduce((sum, i) => sum + i.price, 0);
    // A discount is provided when the amount exceeds 100 euros.
    if (total >= 100) {
      total = 0.9 * total;
    }
    return total;
  }
  // @Watch callback
  onBasketUpdated(propName: string): void {
    this.totalPurchase = this.updateTotal();
  }

  build() {
    Column() {
      ForEach(this.shopBasket,
        (item:PurchaseItem) => {
          Text(`Price: ${item.price.toFixed(2)} €`)
        },
        (item:PurchaseItem) => item.id.toString()
      )
      Text(`Total: ${this.totalPurchase.toFixed(2)} €`)
    }
  }
}

@Entry
@Component
struct BasketModifier {
  @State shopBasket: PurchaseItem[] = [];

  build() {
    Column() {
      Button('Add to basket')
        .onClick(() => {
          this.shopBasket.push(new PurchaseItem(Math.round(100 * Math.random())))
        })
      BasketViewer({ shopBasket: $shopBasket })
    }
  }
}

The processing procedure is as follows:

  1. Button.onClick of the BasketModifier component adds an item to BasketModifier shopBasket.

  2. The value of the \@Link decorated variable BasketViewer shopBasket changes.

  3. The state management framework calls the \@Watch callback BasketViewer onBasketUpdated to update the value of BasketViewer TotalPurchase.

  4. Because \@Link decorated shopBasket changes (a new item is added), the ForEach component executes the item Builder to render and build the new item. Because the @State decorated totalPurchase variables changes, the Text component is also re-rendered. Re-rendering happens asynchronously.

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