Mat-select：让它选择第一个mat-option

Question

I have many angular material selects dropdowns and their respective mat-options are updated in aa reactive way depending on the other values (in other words there is filtering of the options). It can look like that

<mat-select #selects (selectionChange)='formChanges()' [placeholder]='element.label' [disabled]='false' required>
  <ng-container *ngFor="let opt of item.options; index as index">
  <mat-option *ngIf="!videoService.filterStore[item.id] || videoService.filterStore[item.id].filter.includes(index)" [value]="opt">
    {{opt.label}}
  </mat-option>
</ng-container>
</mat-select>

I am unhappy with one behaviour: I don't want to have the select unassigned. I want them to always pick the first option that passes the filter.

Answer 1

You merely need to set the value of the select whenever its list is updated. For example:

export class SelectExample {

  @ViewChild(MatSelect) select: MatSelect;

  updateSelectOptions() {
    // update the options
    ...

    // update the select value to the first item
    // might need to use a timeout to wait until the select has reloaded the options
    setTimeout(() => this.select.value = this.select.options[0].value);
  }
}