Why is the observable sending old values - angular6

Question

I have created a service which sends a request to the backend and makes the result available to the component via an observable.
export class QuestionManagementService {
questionsArray$: Observable<Result>;
private questionsArraySubject: BehaviorSubject<Result>; //to send list of questions
...
constructor(private http: HttpClient, private helper:HelperService, private bs:WebToBackendInterfaceService, private loaderService:LoaderService) {
this.questionsArraySubject = new BehaviorSubject<Result>(new Result('initial',{})); //A Subject can act both as an Observable and an Observer
this.questionsArray$ = this.questionsArraySubject.asObservable(); //create Observable. Other components can subcribe to it now to get notifications/values
...
}
//this method sends the request to network via another `bs` service. The request is sent using `http.post`
getQuestions(questionFilter:GetQuestionsfilter){
console.log("In QuestionManagementService: getQuestions");
let observable:Observable<HttpEvent<any>> = this.bs.getQuestions(questionFilter);
let subscription:Subscription = observable.subscribe((ev:HttpEvent<any>)=>{
if(ev.type === HttpEventType.Response) { //null means that the response wasn't an HttpResponse but probably some internal Rxjs event (eg type 0)
let response= <HttpResponse<any>>ev;
console.log("http response received: ",response);
//should remove the token from storage
console.log('response body from server: ',ev.body);
let isResponseStructureOK: boolean = this.helper.validateServerResponseStructure(ev.body);
if (isResponseStructureOK) {
console.log("response structure is OK");
let response: ServerResponseAPI = ev.body;
let result:string = response.result;
console.log("result is : " + result);
/*if result could be success or error*/
/*additionalInformation is a string and the string contains a valid json which has array of questions
in format {"questions-list":[{"tag":"some tag1","description":"some description1"},{{"tag":"some tag2","description":"some description2"},...]}
*/
let message:string = response['additional-info'];
console.log("message is "+message);
if(result === "success") {
let jsonQuestionList: string = response['additional-info'];
console.log("jsonQuestionList response as string: ", jsonQuestionList);
//let jsonQuestions: PracticeQuestionsListAPI = JSON.parse(jsonQuestionList);
//console.log("jsonQuestion array:", jsonQuestions);
//this.questionsArraySubject.next(jsonQuestions['questions-list']);
this.questionsArraySubject.next(new Result('success', response["additional-info"]));
} else {
this.questionsArraySubject.next(new Result('error', response["additional-info"]));
}
}
else {
/**
* If something goes wrong, send error rather than send next with result="error"
*/
console.log("received incorrect response structure from server: ", ev.body);
//TODOM - need to change hard coded responses and pick them from a config or global variable.
this.questionsArraySubject.error(new Result('error',"Invalid response structure from server"));
}
}
else {
console.log("not response. ignoring");
}
},
(error:ServerResponseAPI)=>{/*web to backend service will send error in ServerResponseAPI format. This is what handleError throws*/
console.log("got error from the Observable: ",error);
this.questionsArraySubject.error(new Result('error',error['additional-info']));
},
()=>{ //observable complete
console.log("observable completed")
});
}
}
The following component subscribes to this service.
export class PraticeQuestionListComponent implements OnInit, OnDestroy {
questions: PracticeQuestionsListAPI; //the result from observable will be stored here.
questionListSubscription:Subscription; //reference of the subscription
ngOnDestroy(): void {
console.log("destroying component. unsubscribing");
this.questionListSubscription.unsubscribe()
}
//on initialisation, I subscribe to the observable
ngOnInit(){
console.log("in question list on init. question is ",this.questions);
...
this.questions= new PracticeQuestionsListAPI(new AdditionalPagingInfo("",new PartitionInfo(0,0)),
[]);
let tagSubscription = this.questionManagementService.getSupportedTags(new TagId("coding"));
console.log("subscribing to question mgmt service");
this.questionListSubscription = this.questionManagementService.questionsArray$.subscribe((result:Result)=>{
console.log('received result from question mgmgt service - array observable',result);
if(result.result === "success") { //received response from server
let questionList = JSON.parse(result.additionalInfo) as PracticeQuestionsListAPI;
console.log("got list of questions value ", questionList);
this.questions['pagination-info'] = questionList['pagination-info'];
this.questions['questions-list'] = questionList['questions-list'];
/*
0 length of questions-list means no questions.
this could be response from the server indicating that there are no more questions
*/
/*
* the server indicates that there are no more questions by either sending empty question list or by sending
* 0 values for pagination state and partition info
*/
if (questionList["questions-list"].length !== 0) { //server has send list of questions
this.questions['pagination-info']['page-state'] = questionList['pagination-info']['page-state'];
this.questions['pagination-info']['partition-info'] = questionList['pagination-info']['partition-info'];
this.questions['questions-list'] = questionList['questions-list'];
console.log("previous question filter is ",this.questionsFilter);
this.questionsFilter["pagination-info"]["page-state"]=questionList["pagination-info"]["page-state"];
this.questionsFilter["pagination-info"]["partition-info"].month=questionList["pagination-info"]["partition-info"].month;
this.questionsFilter["pagination-info"]["partition-info"].year=questionList["pagination-info"]["partition-info"].year;
console.log("new question filter is ",this.questionsFilter);
//TODOM - maybe this assignment below was causing memory leak. So changed this as above
//this.questionsFilter['pagination-info'] = questionList['pagination-info'];
this.lastPage = false; //the server indicates that there are no more questions by sending these values (no paging state and no partition info)
if (this.questions['pagination-info']['page-state'].length == 0 &&
this.questions['pagination-info']['partition-info'].year == 0 &&
this.questions['pagination-info']['partition-info'].month == 0) {
this.lastPage = true;
} else {//if the list is empty then there are no (more) questions for the selected tag
this.lastPage = false;
}
} else {
this.lastPage = true; //Don't show next button if there are no questions.
this.showDialog(new PracticeQuestionListContext("Reached end of the search. No more results available", new PracticeQuestionListAdditionalInfo()));
}
} else {
//TODOM - I should probably display the error in case there is an error from the server
console.log("ignoring value");
}
},
(err:Result)=>{
console.log("received error from QuestionArray observable",err);
//TODOM - probably should change the name of DialogContext to Component specific additional context
this.showDialog(new PracticeQuestionListContext(err.additionalInfo,new PracticeQuestionListAdditionalInfo()));
},
()=>{
console.log("question mgmt service, questionarray observable completed.");
});
}
}
The issue I am facing is that if I visit the component for the first time, I get the values from the observable after making some selections in the UI (expected behavior). Then i visit the home page of the application, the component gets destroyed (again, expected behavior). Then if I visit the component again, the observable sends the old values (from the first visit) even when I have not made any UI selections.
Why is the observable sending old values and how can I stop it from doing that? I have created another question in SO with pictures which might explain the scenario better
angular component is retaining old value maybe because the observable is resending past data