角度将空垫子选项添加到我的应用程序中所有不需要的垫子选择

如何动态添加空mat-option给所有不需要的mat-select在我的应用程序中。

like :

 <mat-form-field appearance="outline">
         <mat-label>HMO</mat-label>
         <mat-select>
              <--how can I add this dynamically-->
              <mat-option>--</mat-option>
              <mat-option *ngFor="let hmo of HMOList"
                              [value]="hmo.Code">
                    {{ hmo.Desc }}
              </mat-option>
         </mat-select>
  </mat-form-field>

任何想法？

我使用了 Prince 的回复，但我的模型没有更新。我在OptionalMatSelectDirective中添加了以下内容

import { Directive, HostListener, OnInit, Renderer2, ElementRef, AfterViewInit } from '@angular/core';
import { MatSelect } from '@angular/material/select';

@Directive({
  selector: 'mat-select'
})
export class OptionalMatSelectDirective {

  constructor(private matSelect: MatSelect, private elementRef: ElementRef,
              private renderer: Renderer2) { }

  @HostListener('openedChange', ['$event'])
  onOpenedChange( isOpened : boolean)
  {

    if(!isOpened || this.matSelect.required)
    {
      return;
    }
    const matOption = this.renderer.createElement('mat-option');
    this.renderer.setAttribute(matOption, 'class', 'mat-option');
    this.renderer.listen(matOption,'click', ()=>
    {
      this.matSelect.value = '';
      **this.matSelect.ngControl.viewToModelUpdate(undefined);**
      this.matSelect.close();
    });

    const panel= document.querySelector('.mat-select-panel');
    if(!panel)
    {
      throw new Error('Cannot find mat select panel');
    }
    this.renderer.insertBefore(panel,matOption, panel.firstChild);


  }
}

而且，它对我有用！