SelectMultipleControlValueAccessor

The accessor for writing a value and listening to changes on a select element.

Caveat: Options selection

Angular uses object identity to select options. It's possible for the identities of items to change while the data does not. This can happen, for example, if the items are produced from an RPC to the server, and that RPC is re-run. Even if the data hasn't changed, the second response will produce objects with different identities.

To customize the default option comparison algorithm, <select multiple> supports compareWith input. compareWith takes a function which has two arguments: option1 and option2. If compareWith is given, Angular selects options by the return value of the function.

Syntax

<select multiple [compareWith]="compareFn" [(ngModel)]="selectedCountries"> <option *ngFor="let country of countries" [ngValue]="country"> {{country.name}} </option> </select> compareFn(c1: Country, c2: Country): boolean { return c1 && c2 ? c1.id === c2.id : c1 === c2; }