my_userO : userO = undefined;

constructor (private my_userS:userS){
    my_userS.userChanged.suscribe(temp => this.updateNav());

updateNav(){
    this.my_userO = this.my_userS.getUser();
}

logOut(){
    this.my_userS.setUser(undefined);
}

<li><a *ngIf="!my_userO" [routerLink]="['LoginPage']">Login</a></li>
<li><a *ngIf="my_userO" [routerLink]="['HomePage']"(click)="logOut()">Logout</a></li>

public userChanged: EventEmitter<{}>
currentUser : userO = undefined;

constructor(private http:Http){
    this.userChanged = new EventEmitter();
}

getLogin(username: string, password: string): Promise<userO>{
    return this.http.get(this.getLoginUrl+username).toPromise().then(response => response.json().data).catch(this.handleError);
}

getUser(){
    return this.currentUser;
}

setUser(x_userO: userO){
    this.currentUser = x_userO;
    this.userChanged.emit(undefined);
}