我无法从ion-select中清除项目

Question

当我单击清除按钮时，我想从离子选择中删除所有项目。 但是我无法删除。

Township.ts

private cdr: ChangeDetectorRef;
townships: string[] = ['Ygn','Myd'];
clear(){
    this.cdr.detectChanges();
    this.townships = [];
    console.log(this.townships);
  }

Township.html

<ion-select name="sample">
   <ion-label position="floating">Township</ion-label>
   <ion-select-option value="{{township}}" *ngFor="let township of townships">{{township}}</ion-select-option>
</ion-select>
<ion-button expand="block" (click)="clear()">Clear</ion-button>