Angular 6 无法从提供的对象中自动选择/绑定下拉列表值 [英] Angular 6 cannot automatically select/bind dropdown list value from supplied object

问题描述

我从服务中获取一个国家/地区对象并将其绑定到一个包含国家/地区下拉列表的表单.从服务中检索该国家/地区后，该国家/地区并未显示为选中状态，但所有其他数据均显示正常，包括绑定到下拉列表的字符串性别字段.不过，我可以从列表中手动选择国家/地区.如何自动显示所选国家/地区?

I get a country object from a service and bind it to a form that has a dropdown list of countries. The country doesn't appear selected after retrieving it from the service but all the other data is shown OK including a string gender field which is bound to a dropdown list. I can manually select the country from the list though. How can I automatically show the country as selected?

personal-details.component.html

personal-details.component.html

    <form [formGroup]="form" (submit)="doSave(form.values)">
        <mat-card class="main-card">

            <mat-card-content>
                <p>Personal Details</p>
                <mat-form-field class="full-width-input">
                  <input matInput placeholder="Given name" formControlName="givenName" >
                </mat-form-field>
                <mat-form-field class="full-width-input">
                  <input matInput placeholder="Surname" formControlName="surname" required>
                </mat-form-field>
                <mat-form-field class="full-width-input">
                    <mat-select placeholder="Select gender" formControlName="gender" required>
                        <mat-option *ngFor="let g of genders" [value]="g">{{g}}</mat-option>
                    </mat-select>      
                </mat-form-field>
                <mat-form-field class="full-width-input">
                    <mat-select placeholder="Select country of birth" formControlName="countryBorn" required>
                      <mat-option *ngFor="let country of countries" [value]="country">{{country.name}}</mat-option>
                    </mat-select>      
                </mat-form-field>
            </mat-card-content>

        </mat-card>
    </form>

personal-details.component.ts

personal-details.component.ts

  countries = [
    new Country('1100', 'Australia', '1'),
    new Country('1201', 'New Zealand', '160'),
    new Country('3105', 'Malta', '140')
  ];
  genders = ['F', 'M'];

  ngOnInit() {
    this.form = this.fb.group({
      givenName: ['', Validators.required],
      surname: ['', Validators.required],
      gender: ['', Validators.required],
      countryBorn: ''
    });

    this.response = this.applicantService.getDetails()
      .pipe(take(1), tap(resp => {
        this.form.patchValue(resp.personalDetails);
      }));

  }

域:

export class PersonalDetails {
  givenName: string;
  surname: string;
  gender: string;
  countryBorn: Country;

  constructor(givenName?: string, surname?: string, gender?: string, countryBorn?: string) {
    this.id = id;
    this.surname = surname;
    this.gender = gender;
    this.countryBorn = countryBorn;
  }
}  

export class Country {
  id: string;
  name: string;
  displayOrder: string;

  constructor(id?: string, name?: string, displayOrder?: string) {
    this.id = id;
    this.name = name;
    this.displayOrder = displayOrder;
  }
}

数据:

incoming value from service: { "surname": "Oliver", "givenName": "Bert", "gender": "M", "countryBorn": { "id": "1201", "name": "New Zealand", "displayOrder": "160" }, "birthdate": "1990-04-21" }

Form value on load ({{form.value | json }}) =  { "givenName": "Bert", "surname": "Oliver", "gender": "M", "countryBorn": { "id": "1201", "name": "New Zealand", "displayOrder": "160" }

谢谢

推荐答案

Angular 使用对象标识来选择选项，并且还提供了特殊的输入属性来自定义默认的选项比较算法:compareWith:

Angular uses object identity to select option and also provides special input property to customize the default option comparison algorithm: compareWith:

html

<mat-select ... formControlName="countryBorn" [compareWith]="compareFn">
                                              ^^^^^^^^^^^^^^^^^^^^^^^^^   

ts

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

Ng-run 示例

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