Q:

writeValue function in ControlValueAccessor class called once in angular 2 custom component

there is a custom angular2 component like below:
template
<input class="form-control" id="searchbox" placeholder="شهر" (keyup)="search()" [(ngModel)]="name" autocomplete="off"/>
<div id="searchresult" *ngFor="let city of cities" (click)="cityselected(city)">
    {{city.name}}
</div>

component
        @Component({
        moduleId: module.id,
        selector: 'city-search',
        templateUrl: './city-search.component.html',
        styleUrls: ['./city-search.component.css'],
        providers: [
            {
                provide: NG_VALUE_ACCESSOR,
                useExisting: forwardRef(() => CitySearchComponent),
                multi: true
            }
        ]
    })
    export class CitySearchComponent implements OnInit, ControlValueAccessor{

        cities: Array<City>;
        name: string;
        displaysearchbox: boolean;

        errorMessage: string;

        constructor(private cityService: CityService 
, private helper : Helper) { }

        ngOnInit() {
            this.cities = new Array<City>();
            this.displaysearchbox = false;
        }

        search(): void {
            if (this.name) {              
 this.cityService.
search(this.helper.arabictopersian(this.name))
.subscribe(
                    data => {
                        this.cities = data as Array<City>
                    }
                    , error => this.errorMessage = <any>error);
            }       
        }

        cityselected(city: City): void {
            this.name = city.name;
        }
        writeValue(value: any) {   //just fire once to get the inital value    
            if (value !== undefined) {
                this.name = value; 
            }
            this.propagateChange(this.name);
        }
        propagateChange = (_: any) => { };

        registerOnChange(fn) {
            this.propagateChange = fn;
        }
        registerOnTouched() { }
    }

as it is shown in the code component class implements ControlValueAccessor
and here is how i use this:
<form class="form-horizontal">
    <city-search id="bc" name="city" [(ngModel)]="city"></city-search>
    city:{{city}}<!--nothing happens -->
</form>

but custom component has 1-way binding not 2-way binding .. this component gets initial value but does not change based on input changes at all..when I debug it on browser the writeValue function just get fired at loading and after that does not get fired at all.

A:

I suppose you should call propagateChange method when you're choosing new value
cityselected(city: any): void {
  this.name = city.name;
  this.propagateChange(this.name); 
}

Plunker Example
So whenever you want to update value for ngControl from ControlValueAccessor you should call function that you set inside registerOnChange method
registerOnChange(fn) {
  this.propagateChange = fn;
}

Usually such method is called onChange function https://github.com/angular/angular/blob/4.2.0-rc.1/packages/forms/src/directives/default_value_accessor.ts#L79

