Angular 6 无法自动 select/bind 提供对象的下拉列表值

Question

我从服务中获取国家对象并将其绑定到具有国家下拉列表的表单。从服务中检索后，国家/地区未显示 selected，但所有其他数据都显示正常，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 select 国家。如何自动将国家/地区显示为 selected？

个人-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>

个人-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

Answer 1

Angular 将对象标识用于 select 选项，还提供特殊输入属性来自定义默认选项比较算法：compareWith:

html

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

ts

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

Ng-run Example

Answer 2

要么将 resp.personalDetails.countryBorn 设置为列表中的实际对象，然后再将其分配给表单（因为对象比较 {...}==={...}始终为 false，Angular 将无法匹配并匹配 select）

resp.personalDetails.countryBorn = this.countries.find(country=> resp.personalDetails.countryBorn.id);

或单独设置 id 作为值

<mat-option *ngFor="let country of countries" [value]="country.id">{{country.name}}</mat-option>

数据将只有国家 ID 而不是整个对象。

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

否则您可能需要使用 compareWith 定义自己的比较函数：https://angular.io/api/forms/SelectControlValueAccessor#caveat-option-selection

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