Angular 2，设置默认值以选择选项 [英] Angular 2, set default value to select option

问题描述

我尝试为选项添加默认值.它会像一种占位符，我使用 这个方法来做到这一点.在纯 HTML 中它可以工作，但是如果我尝试使用 angular 2 属性的 *ngFor，它不会选择任何内容.

I try to add a default value to an option. It will be like a kind of placeholder and I use this method to do it. In pure HTML it work, but if I try to use *ngFor of angular 2 attribute, it doesn't select anything.

这是我在纯 HTML 中使用的代码:

Here my code that I use in pure HTML:

  <select name="select-html" id="select-html">
    <option value="0" selected disabled>Select Language</option>
    <option value="1">HTML</option>
    <option value="2">PHP</option>
    <option value="3">Javascript</option>
  </select>

并使用 Angular 模板:

And using Angular template:

 <select name="select" id="select" [(ngModel)]="selectLanguage">
    <option *ngFor="let item of selectOption" 
      [selected]="item.value==0" 
      [disabled]="item.value==0">{{item.label}}</option>
  </select>

班级是

import {Component} from '@angular/core';

@Component({
  moduleId: module.id,
  selector: 'app-form-select',
  templateUrl: 'default.component.html'
})
export class DefaultComponent {
  private selectOption: any;
  constructor() {
    this.selectOption = [
      {
        id: 1,
        label: "Select Language",
        value: 0
      }, {
        id: 2,
        label: "HTML 5",
        value: 1
      }, {
        id: 3,
        label: "PHP",
        value: 2
      }, {
        id: 4,
        label: "Javascript",
        value: 3
      }
    ]
  }
}

我希望 Select Language 被选为默认值.它已禁用但未选中.

I want Select Languageto be selected as a default value. It's disabled but not selected.

如何选择它作为默认值?

How can I select it as a default value?

实例

推荐答案

更新

4.0.0-beta.6 添加了 compareWith

<select [compareWith]="compareFn"  [(ngModel)]="selectedCountries">
    <option *ngFor="let country of countries" [ngValue]="country">
        {{country.name}}
    </option>
</select>

compareFn(c1: Country, c2: Country): boolean {
   return c1 && c2 ? c1.id === c2.id : c1 === c2;
} 

这样分配给 selectedCountries 的实例不需要是相同的对象，但可以传递自定义比较函数，例如能够匹配具有相同属性的不同对象实例值.

this way the instance assigned to selectedCountries doesn't need to be the identical object, but a custom comparison function can be passed, for example to be able to match an different object instance with identical property values.

原创

如果你想使用一个对象作为值，你需要在选项元素上使用 [ngValue].

If you want to use an object as value you need to use [ngValue] on the option element.

  <select name="select" id="select" [(ngModel)]="selectLanguage">
    <option *ngFor="let item of selectOption" [ngValue]="item"
      [disabled]="item.value==0">{{item.label}}</option>
  </select>

当 selectLanguage 有一个选项值分配给 [(ngModel)]="..." 时，这个选项是否会被默认选中:

When selectLanguage has an options value assigned [(ngModel)]="..." will this one make the one selected by default:

import {Component} from '@angular/core';

@Component({
  moduleId: module.id,
  selector: 'app-form-select',
  templateUrl: 'default.component.html'
})
export class DefaultComponent {
  private selectOption: any;
  constructor() {
    this.selectOption = [
      {
        id: 1,
        label: "Select Language",
        value: 0
      }, {
        id: 2,
        label: "HTML 5",
        value: 1
      }, {
        id: 3,
        label: "PHP",
        value: 2
      }, {
        id: 4,
        label: "Javascript",
        value: 3
      }
    ];
    this.selectLanguage = this.selectOption[0];
  }
}

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