AngularLearn/src/app/app.component.html

<form #form="ngForm">
  <div>
    單價：<input
      type="text"
      name="price"
      #price="ngModel"
      ngModel
      appDecimalValidator
      [digitLength]="2"
      [scaleLength]="1"
    />
    <ng-container *ngIf="price.touched">
      <span *ngIf="price.hasError('decimal')">
        單價應為 {{ price.errors?.decimal.digitLength }} 位整數位與
        {{ price.errors?.decimal.scaleLength }} 位整數位的數值
      </span>
    </ng-container>
  </div>
</form>