Question

我正在尝试使用来自远程API服务的值填充材料选择下拉列表。我一直在组件上获取空值或未定义的值。

这是针对使用MEAN堆栈和Angular Material的Angular 7。我尝试了compareWith函数并记录了值。我注意到该组件始终首先加载其值为null，然后再加载具有值的API服务。

<mat-form-field>
    <mat-select formControlName="company"  placeholder="Select 
Company" [(value)]="selectedCompany" [compareWith]="compareFn">
        <mat-option *ngFor="let lC of loadedCompanies" 
 [value]="lC.id">
          {{lC.name}}
        </mat-option>
    </mat-select>
</mat-form-field>

ngOnInit() {
  this.authStatusSub = this.authService
  .getAuthStatusListener()
  .subscribe(authStatus => {
      this.isLoading = false;
  });

  this.form = new FormGroup({
      id: new FormControl(null),
      company: new FormControl(this.loadedCompanies)
  });

  this.companyService.getCompanyList();
  this.companySub = this.companyService
  .getcompanyUpdateListener()
  .subscribe((companyData: {companies: Company[]}) => {
    this.isLoading = false;
    this.loadedCompanies =  companyData.companies;
  });
}

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

我希望值会从可观察对象中加载，并在加载组件之前填充选择输入。

Answer 1

您可以将*ngIf添加到父容器中，以便在订阅了Observable并分配了loadedCompanies之前不会加载mat-select组件。

 <mat-form-field *ngIf = "loadedCompanies">
   <mat-select formControlName="company"  placeholder="Select 
Company" [(value)]="selectedCompany" [compareWith]="compareFn">
     <mat-option *ngFor="let lC of loadedCompanies" 
 [value]="lC.id">
       {{lC.name}}
     </mat-option>
  </mat-select>
</mat-form-field>

FormControl公司的初始值应为null，而不是整个loadedCompanies数组。

this.form = new FormGroup({
  id: new FormControl(null),
  company: new FormControl(null)
});

这是填充loadedCompanies数组的方式。

loadedCompanies: Company[];

ngOnInit() {
  .
  .
  // other lines of code before this
  this.companyService.getCompanyList().subscribe(companyData => {
    this.loadedCompanies = companyData.companies;
  });
}

如何从远程API服务填充Material select下拉列表？

1 个答案: