how to filter *ngFor result based on a selected dropdown value

Choosing a category:

 <ion-select name="categories">
    <ion-option *ngFor="let category of categories;">
      {{category}}
    </ion-option>
  </ion-select>

Listing the items:

 <ion-item-sliding *ngFor="let item of items; let idx = index;">
    <ion-item>
        <h2>{{item.title}}</h2>
    </ion-item>
...
</ion-item-sliding>

How can I use the value from (select) to choose which category gets listed? I tried adding a ngModule in and passing it as a condition into the sliding ngFor, but it wouldn't work.