Unselect event for ion-option

I’m trying to implement the select All in ion-select,
So I need to capture the select and unselect event for ion-option ,

I implemented the select all by using ionSelect event , but i’m still trying to implement unselect all:

<form  [formGroup]="addserviceForm"> 
<ion-select   formControlName='areacover' multiple="true">
<ion-option value="All" (ionSelect)="selectAll()" >select All </ion-option>
   <ion-option     *ngFor='let district of districtlist'   [value]="district.nameen">{{district.namear}} </ion-option>

Also , I need to load the select in option in full screen, I tried to set width as 100% but it doesn’t work.