harmony 鸿蒙Data Transfer and Processing

2022-08-09
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Data Transfer and Processing

props

You can use props to declare attributes of a custom component and pass the attributes to the child component. The supported types of props include String, Number, Boolean, Array, Object, and Function. Note that a camel case attribute name (compProp) should be converted to the kebab case format (comp-prop) when you reference the attribute in an external parent component. The following is sample for adding props to a custom component and passing the attribute to the child component.

<!-- comp.hml -->
<div class="item"> 
   <text class="title-style">{{compProp}}</text> 
</div>

// comp.js 
export default { 
  props: ['compProp'],
}

<!-- xxx.hml -->
<element name='comp' src='../common/component/comp/comp.hml'></element>
<div class="container"> 
   <comp comp-prop="{{title}}"></comp> 
</div>

NOTE

The name of a custom attribute cannot start with reserved keywords such as on, @, on:, and grab:.

Default Value

You can set the default value for a child component attribute via default. The default value is used if the parent component does not have default set. In this case, the props attribute must be in object format instead of an array.

<!-- comp.hml -->
<div class="item"> 
   <text class="title-style">{{title}}</text> 
</div>

// comp.js
export default { 
  props: {
    title: {
      default: 'title',
    },
  },
}

In this example, a <text> component is added to display the title. The title content is a custom attribute, which displays the text specified by a user. If the user has not set a title, the default text will be displayed. Add the attribute when referencing the custom component.

<!-- xxx.hml -->
<element name='comp' src='../common/component/comp/comp.hml'></element>
<div class="container"> 
   <comp title="Custom component"></comp>
</div>

Unidirectional Value Transfer

Data can be transferred only from the parent component to child components. You are not allowed to change the value passed to the child component. However, you can receive the value passed by props as a default value in data, and then change the data value.

// comp.js
export default { 
  props: ['defaultCount'],
  data() {
    return {
      count: this.defaultCount,
    };
  },
  onClick() {
    this.count = this.count + 1;
  },
}

Monitoring Data Changes by $watch

To listen for attribute changes in a component, you can use the $watch method to add a callback. The following code snippet shows how to use $watch:

// comp.js
export default { 
  props: ['title'],
  onInit() {
    this.$watch('title', 'onPropertyChange');
  },
  onPropertyChange(newV, oldV) {
    console.info('title attribute changed'+ newV + ' ' + oldV)
  },
}

computed

To improve the development efficiency, you can use a computed property to pre-process an attribute in a custom component before reading or setting the attribute. In the computed field, you can set a getter or setter to be triggered when the attribute is read or written, respectively. The following is an example:

// comp.js
export default { 
  props: ['title'],
  data() {
    return {
      objTitle: this.title,
      time: 'Today',
    };
  },
  computed: {
    message() {
      return this.time + ' ' + this.objTitle;
    },
    notice: {
      get() {
        return this.time;
      },
      set(newValue) {
        this.time = newValue;
      },
    },
  },
  onClick() {
    console.info('get click event ' + this.message);
    this.notice = 'Tomorrow';
  },
}

The first computed property message has only a getter. The value of message changes depending on the objTitle value. The getter can only read but cannot set the value (such as time defined during initialization in data). You need a setter (such as notice in the sample code) to set the value of the computed property.