Angular2 reactive forms select multiple attribute?

问题

I use the following code in my app with reactive forms.

If I uncomment the [multiple] line, the Choose ... option does not set the dformControl form control object back to status INVALID.

dformControl.multiple by the way returns false. Even if I change the commented line to [multiple]="false", still switching back to the Choose ... option does NOT set the form control status to INVALID.

<select class="form-control"
        [id]="dformControl.key"
        [formControlName]="dformControl.key"
        /*[multiple]="dformControl.multiple"*/>

  <option *ngIf="!dformControl.value"
          value="">
    Choose ...
  </option>

  <option *ngFor="let opt of dformControl.options"
          [value]="opt.value"
          [selected]="dformControl.value == opt.value">
    {{opt.label}}
  </option>

</select>

回答1:

Bind to the multiple property at the select level to a boolean isMultiple. Then you can change it and the select will change as well. Take a look at the this, I change it using a button. plnkr

  <select formControlName="cars" [multiple]="isMultiple">
      <option></option>
      <option *ngFor="let car of cars" >{{car}}</option>
  </select>

It seems when adding the multiple property it is affecting the required validator. I was able to just add an extra validator and it worked as expected.

Validators.compose([Validators.required,Validators.pattern('.+')]

来源：https://stackoverflow.com/questions/43080198/angular2-reactive-forms-select-multiple-attribute