I have a table that needs to be refreshed each time a user delete a row, or edit data with a designate function called reloadUsersData();
I've created a BehaviorSubject that gets the table's data from a service - once the table is loaded - but i'm not sure that my implementation of reloadUsersData is good since i repeat the same code and resubscribe to the same observable ....
export class TableBasicExample implements OnDestroy, OnInit{
dataSource$ = new BehaviorSubject<any>([]);
private subs: Subscription[] = [];
constructor(private apiService: ApiService) {}
ngOnInit() {
this.subs.push(
this.apiService.getUsers().subscribe((res: any) => {
this.dataSource$.next(res);
}));
}
reloadUsersData(){
// how can i subscribe to the same api more effectively ?
this.subs.push(
this.apiService.getUsers().subscribe((res: any) => {
this.dataSource$.next(res);
}));
}
ngOnDestroy(): void {
this.subs.forEach((us) => us.unsubscribe());
}
}
Fortunately, this can be simplified! I'm assuming that .getUsers() emits one array of users and completes. With that in mind, you have the right idea with creating a subject to handle reload events.
One strategy you can use in RxJS is to create an observable that includes a subject in its .pipe(). This is because a Subject inherits all capabilities of an Observable.
Here's the code.
export class TableBasicExample {
private fetchEvent = new BehaviorSubject<'fetch'>('fetch');
public users$: Observable<User[]>;
constructor(private apiService: ApiService) {
this.users$ = this.fetchEvent.pipe(
switchMapTo(this.apiService.getUsers())
);
}
reloadUsersData() {
this.fetchEvent.next('fetch');
}
}
We create a BehaviorSubject (which is an extension of Subject) to handle our fetch event. We have it emit the string 'fetch', but it could honestly be any value.
Next, we declare the main Observable we use for our table rendering. It will subscribe to fetchEvent and will switchMap to an inner observable (.getUsers()) and emit its value.
Last, we have our public method that will emit a new value in our private BehaviorSubject.
In your component's template file, you can subscribe to this observable using the async pipe.
<table *ngIf="users$ | async as users">
<tr *ngFor="user of users">
<!-- table row data -->
</tr>
</table>
<button (click)="reloadUsersData()">Reload</button>
The async pipe handles the subscribe/unsubscribe events for you, so you no longer need onInit and onDestroy in your component's TS file.
And that's it! When reloadUsersData() is invoked, it causes fetchEvent to emit a new value. Because users$ has an active subscription, it will receive that new value, and again subscribe to the inner switchMap observable.
Note: The switchMapTo(obs$) operator is similar to switchMap((value)=>$obs) operator. It's just a little less code because we don't actually need the value from our BehaviorSubject.
Using React-Redux
I have a select list that when the user chooses one of the options, a item is created and placed in the database (if it matters, the reason its a select box is that there are multiple variations of the same item and what variation is important).
This is working correctly.
My problem is that I am not sure how I can get the id of the new item out of the redux store.
And just for chuckles, the prior developer set all this up with sagas. So I am still coming up to speed on how it all works together.
So when the select box is checked, the function checkFunction is called that calls the function createItem in the saga file. These functions are below:
in Repositories.jsx
checkFunction = (data) => {
const {createItem} = this.props;
// data holds the info that we need to send to the action
const created = createItem(data);
// once data comes back, redirect the user ot the details of the created item
// need id of created item
// navigateTo(`/item-details/${created.id}`);
}
in Repositories.saga.js
export function* createItem(action) {
try {
const {payload: newItem} = action;
// call api to create item
const created = yield call(requestAPI.post, ITEMS_URL, newItem);
// send message that its been done
yield put(actions.repositories.createItem.ok(created));
// send message to refresh item list
yield put(actions.inventories.fetchItems.start());
} catch (e) {
yield put(actions.repositories.createItem.fail(e));
}
}
I don't understand how to return the id of the created item once its created. I feel like I am missing something basic here. Can anyone point me in the right direction?
Actually getting data from saga back to react component is not trivial. There are multiple approaches to do what you need although each has its downside.
1. Call navigateTo in the saga.
export function* createItem(action) {
...
const created = yield call(requestAPI.post, ITEMS_URL, newItem);
yield call(navigateTo, `/item-details/${created.id}`)
}
This would be my recommended solution if you can get the navigateTo function into the saga. Navigation is a side effect and sagas are there to deal with side effects. Make sure to use the call effect, changing the url by directly calling the function can lead to some issues.
2. Store the latest created item id in redux store
In your reducer, when action actions.repositories.createItem.ok(created) is dispatched, store the created item info and then send the latest created item to the component. Finally you can use componentDidUpdate or useEffect to call navigateTo when the prop changes.
render() {
const created = Redux.useSelector(state => state.created);
useEffect(() => navigateTo(`/item-details/${created.id}`), [created])
...
}
This has the disadvantage that the component will rerender becuase of the changed created value.
3. Send callback in the createItem action
You can put a function into your action and then call it from the saga and essentially using it as a callback.
Component:
checkFunction = (data) => {
const {createItem} = this.props;
// data holds the info that we need to send to the action
const created = createItem(data, (created) => {
navigateTo(`/item-details/${created.id}`);
});
}
Saga:
export function* createItem(action) {
...
const created = yield call(requestAPI.post, ITEMS_URL, newItem);
action.callback(created)
...
}
The problem with this approach is that functions are not serializable and so you ideally should avoid them in your actions. Also, technically, there could be multiple sagas handling the same action and then it gets kind of confusing who should call the callback.
I have an Angular 2 service:
import {Storage} from './storage';
import {Injectable} from 'angular2/core';
import {Subject} from 'rxjs/Subject';
#Injectable()
export class SessionStorage extends Storage {
private _isLoggedInSource = new Subject<boolean>();
isLoggedIn = this._isLoggedInSource.asObservable();
constructor() {
super('session');
}
setIsLoggedIn(value: boolean) {
this.setItem('_isLoggedIn', value, () => {
this._isLoggedInSource.next(value);
});
}
}
Everything works great. But I have another component which doesn't need to subscribe, it just needs to get the current value of isLoggedIn at a certain point in time. How can I do this?
A Subject or Observable doesn't have a current value. When a value is emitted, it is passed to subscribers and the Observable is done with it.
If you want to have a current value, use BehaviorSubject which is designed for exactly that purpose. BehaviorSubject keeps the last emitted value and emits it immediately to new subscribers.
It also has a method getValue() to get the current value.
The only way you should be getting values "out of" an Observable/Subject is with subscribe!
If you're using getValue() you're doing something imperative in declarative paradigm. It's there as an escape hatch, but 99.9% of the time you should NOT use getValue(). There are a few interesting things that getValue() will do: It will throw an error if the subject has been unsubscribed, it will prevent you from getting a value if the subject is dead because it's errored, etc. But, again, it's there as an escape hatch for rare circumstances.
There are several ways of getting the latest value from a Subject or Observable in a "Rx-y" way:
Using BehaviorSubject: But actually subscribing to it. When you first subscribe to BehaviorSubject it will synchronously send the previous value it received or was initialized with.
Using a ReplaySubject(N): This will cache N values and replay them to new subscribers.
A.withLatestFrom(B): Use this operator to get the most recent value from observable B when observable A emits. Will give you both values in an array [a, b].
A.combineLatest(B): Use this operator to get the most recent values from A and B every time either A or B emits. Will give you both values in an array.
shareReplay(): Makes an Observable multicast through a ReplaySubject, but allows you to retry the observable on error. (Basically it gives you that promise-y caching behavior).
publishReplay(), publishBehavior(initialValue), multicast(subject: BehaviorSubject | ReplaySubject), etc: Other operators that leverage BehaviorSubject and ReplaySubject. Different flavors of the same thing, they basically multicast the source observable by funneling all notifications through a subject. You need to call connect() to subscribe to the source with the subject.
I had similar situation where late subscribers subscribe to the Subject after its value arrived.
I found ReplaySubject which is similar to BehaviorSubject works like a charm in this case.
And here is a link to better explanation: http://reactivex.io/rxjs/manual/overview.html#replaysubject
const observable = of('response')
function hasValue(value: any) {
return value !== null && value !== undefined;
}
function getValue<T>(observable: Observable<T>): Promise<T> {
return observable
.pipe(
filter(hasValue),
first()
)
.toPromise();
}
const result = await getValue(observable)
// Do the logic with the result
// .................
// .................
// .................
You can check the full article on how to implement it from here.
https://www.imkrish.com/blog/development/simple-way-get-value-from-observable
I encountered the same problem in child components where initially it would have to have the current value of the Subject, then subscribe to the Subject to listen to changes. I just maintain the current value in the Service so it is available for components to access, e.g. :
import {Storage} from './storage';
import {Injectable} from 'angular2/core';
import {Subject} from 'rxjs/Subject';
#Injectable()
export class SessionStorage extends Storage {
isLoggedIn: boolean;
private _isLoggedInSource = new Subject<boolean>();
isLoggedIn = this._isLoggedInSource.asObservable();
constructor() {
super('session');
this.currIsLoggedIn = false;
}
setIsLoggedIn(value: boolean) {
this.setItem('_isLoggedIn', value, () => {
this._isLoggedInSource.next(value);
});
this.isLoggedIn = value;
}
}
A component that needs the current value could just then access it from the service, i.e,:
sessionStorage.isLoggedIn
Not sure if this is the right practice :)
A similar looking answer was downvoted. But I think I can justify what I'm suggesting here for limited cases.
While it's true that an observable doesn't have a current value, very often it will have an immediately available value. For example with redux / flux / akita stores you may request data from a central store, based on a number of observables and that value will generally be immediately available.
If this is the case then when you subscribe, the value will come back immediately.
So let's say you had a call to a service, and on completion you want to get the latest value of something from your store, that potentially might not emit:
You might try to do this (and you should as much as possible keep things 'inside pipes'):
serviceCallResponse$.pipe(withLatestFrom(store$.select(x => x.customer)))
.subscribe(([ serviceCallResponse, customer] => {
// we have serviceCallResponse and customer
});
The problem with this is that it will block until the secondary observable emits a value, which potentially could be never.
I found myself recently needing to evaluate an observable only if a value was immediately available, and more importantly I needed to be able to detect if it wasn't. I ended up doing this:
serviceCallResponse$.pipe()
.subscribe(serviceCallResponse => {
// immediately try to subscribe to get the 'available' value
// note: immediately unsubscribe afterward to 'cancel' if needed
let customer = undefined;
// whatever the secondary observable is
const secondary$ = store$.select(x => x.customer);
// subscribe to it, and assign to closure scope
sub = secondary$.pipe(take(1)).subscribe(_customer => customer = _customer);
sub.unsubscribe();
// if there's a delay or customer isn't available the value won't have been set before we get here
if (customer === undefined)
{
// handle, or ignore as needed
return throwError('Customer was not immediately available');
}
});
Note that for all of the above I'm using subscribe to get the value (as #Ben discusses). Not using a .value property, even if I had a BehaviorSubject.
Although it may sound overkill, this is just another "possible" solution to keep Observable type and reduce boilerplate...
You could always create an extension getter to get the current value of an Observable.
To do this you would need to extend the Observable<T> interface in a global.d.ts typings declaration file. Then implement the extension getter in a observable.extension.ts file and finally include both typings and extension file to your application.
You can refer to this StackOverflow Answer to know how to include the extensions into your Angular application.
// global.d.ts
declare module 'rxjs' {
interface Observable<T> {
/**
* _Extension Method_ - Returns current value of an Observable.
* Value is retrieved using _first()_ operator to avoid the need to unsubscribe.
*/
value: Observable<T>;
}
}
// observable.extension.ts
Object.defineProperty(Observable.prototype, 'value', {
get <T>(this: Observable<T>): Observable<T> {
return this.pipe(
filter(value => value !== null && value !== undefined),
first());
},
});
// using the extension getter example
this.myObservable$.value
.subscribe(value => {
// whatever code you need...
});
There are two ways you can achieve this.
BehaviorSubject has a method getValue() which you can get the value in a specific point of time.
You can subscribe directly with the BehaviorSubject and you may pass the subscribed value to a class member, field or property.
I wouldn't recommend both approaches.
In the first approach, it's a convenient method you can get the value anytime, you may refer to this as the current snapshot at that point of time. Problem with this is you can introduce race conditions in your code, you may invoke this method in many different places and in different timing which is hard to debug.
The second approach is what most developers employ when they want a raw value upon subscription, you can track the subscription and when do you exactly unsubscribe to avoid further memory leak, you may use this if you're really desperate to bind it to a variable and there's no other ways to interface it.
I would recommend, looking again at your use cases, where do you use it? For example you want to determine if the user is logged in or not when you call any API, you can combine it other observables:
const data$ = apiRequestCall$().pipe(
// Latest snapshot from BehaviorSubject.
withLatestFrom(isLoggedIn),
// Allow call only if logged in.
filter(([request, loggedIn]) => loggedIn)
// Do something else..
);
With this, you may use it directly to the UI by piping data$ | async in case of angular.
A subscription can be created, then after taking the first emitted item, destroyed. In the example below, pipe() is a function that uses an Observable as its input and returns another Observable as its output, while not modifying the first observable.
Sample created with Angular 8.1.0 packages "rxjs": "6.5.3", "rxjs-observable": "0.0.7"
ngOnInit() {
...
// If loading with previously saved value
if (this.controlValue) {
// Take says once you have 1, then close the subscription
this.selectList.pipe(take(1)).subscribe(x => {
let opt = x.find(y => y.value === this.controlValue);
this.updateValue(opt);
});
}
}
You could store the last emitted value separately from the Observable. Then read it when needed.
let lastValue: number;
const subscription = new Service().start();
subscription
.subscribe((data) => {
lastValue = data;
}
);
The best way to do this is using Behaviur Subject, here is an example:
var sub = new rxjs.BehaviorSubject([0, 1])
sub.next([2, 3])
setTimeout(() => {sub.next([4, 5])}, 1500)
sub.subscribe(a => console.log(a)) //2, 3 (current value) -> wait 2 sec -> 4, 5
Another approach, If you want / can to use async await (has to be inside of an async functions) you can do this with modern Rxjs:
async myFunction () {
const currentValue = await firstValueFrom(
of(0).pipe(
withLatestFrom(this.yourObservable$),
map((tuple) => tuple[1]),
take(1)
)
);
// do stuff with current value
}
This will emit a value "Right away" because of withLatestFrom, and then will resolve the promise.
The goal is to iterate through a collection of IDs, making an HTTP call for each ID. For each ID, I'm using a service with a get() method that returns an Observable. Each time the get() method is called, I'm subscribing to the returning Observable and trying to push the result into an array, which will eventually get passed on to a different method for a new operation.
Relevant service method:
public get(departmentId: number): Observable<IDepartmentModel> {
return super.get<IDepartmentModel>(departmentId);
}
note: the super class is leveraging Angular Http, which is well tested and confirmed to be working correctly. The problem with the logic isn't here...
Relevant component methods:
note the departmentService.get() call that's being called several times within the forEach.
setInitialDepartmentsAssignedGridData(): void {
this.settingsForDropdownSelectedCompanyId = this.userForm.get('defaultCompany').get('defaultCompanyId').value;
let departments: IDepartmentModel[] = [];
this.userService.user.getValue() //confirmed: valid user is being pulled back from the userService (logic is fine here..)
.userCompanies.find(comp => comp.companyId === this.settingsForDropdownSelectedCompanyId) // getting a valid match here (logic is fine here..)
.departmentIds.forEach(deptId => this.departmentService.get(deptId).first().subscribe(dept => { // getting a valid department back here (logic is fine here...)
departments.push(dept); // HERE LIES THE PROBLEM
}));
this.setDepartmentsAssignedRowData(departments);
}
setDepartmentsAssignedRowData(departments: IDepartmentModel[]): void {
console.log('setDeptAssignedRowData called'); // confirmed: method is getting called...
console.log(departments); // confirmed: fully-composed collection of departments logged to the console...
departments.forEach(dept => {
console.log(dept);
}); // Y U NO WORK!?
departments.map((department) => {
console.log(department); // Y U NO WORK?
this.departmentAssignedRowData.push({
departmentId: department.id,
departmentName: department.name
});
});
this.departmentAssignedGridOptions.api.setRowData(this.departmentAssignedRowData);
}
The problem is, although what's getting logged to the console is a fully-composed department-objects array, it's not TRULY "there"; what's getting passed to setDepartmentsAssignedRowData is an empty array.
I'm sure what's happening is that the async operations are not complete before the departments array gets passed to the second method. Some of what I've read online says to use forkJoin, but I can't see how that will look in this context. I've also read concatMap may work, but again, in this context, I'm not sure how to make that work...
In this context, how do I leverage RxJS to make sure the intended, fully-composed departments array is truly ready to be passed?
thanks for any insight you can provide. help is much appreciated!
You are correct, you need forkJoin
let observableArray = this.userService.user.getValue()
.userCompanies.find(comp => comp.companyId === this.settingsForDropdownSelectedCompanyId)
.departmentIds.map(deptId => this.departmentService.get(deptId)) // map is building out an array of observables
This will be an array of http request observables that you want to make in parallel. Now you can pass this array to forkJoin.
Observable.forkJoin(...observableArray)
The return of forkJoin will be an array of results from observableArray. forkJoin will not emit to the next operator in the sequence until all of the observables in observableArray have completed (so when all of the http requests have finished)
So altogether the code will be
let observableArray = this.userService.user.getValue()
.userCompanies.find(comp => comp.companyId === this.settingsForDropdownSelectedCompanyId)
.departmentIds.map(deptId => this.departmentService.get(deptId));
Observable.forkJoin(...observableArray).subscribe(res => {
// res = [resId0, resId1, resId2, ..., resIdX];
});
You mentioned passing the result to another operator. If that operator is another http request where you pass an array of data (from forkJoin), then you can use the flatMap operator.
Observable.forkJoin(...observableArray)
.flatMap(res => {
return this.otherApi(res);
})
.subscribe(res => {
// res is the result of the otherApi call
});
flatMap will chain your api requests together. So altogether what is happening is
run array of observables in parallel
once complete, run second api (otherApi)
I'm just getting started with RXJS to see if it can replace my currently manual data streams. One thing I'm trying to port is a situation whereby the last value in the stream is remembered, so future observers will always get the 'current' value as well as subsequent ones. This seems to be fulfilled by BehaviorSubject.
However, I need to do this for a group of entities. For example, I might have data that represents a message from a user:
{ userId: 1, message: "Hello!" }
And I want a BehaviorSubject-like object that'll store the last message for all users. Is this something I can do with RXJS out-of-the-box, or would I need to do it myself? (If so, any pointers would be appreciated).
EDIT: After some more thought, it perhaps seems logical to having an 'incoming' subject, an observer that updates a Map, and then a function which I can call which initialises an Observable from the map values, and merges with the incoming stream...?
I use RxJS with a redux-like state setup. I have a BehaviorSubject that holds the current state, and every time an event/action is fired that current state gets passed through functions that produce a new state, which the subject is subscribed to.
Here's a simplified version of what I use:
export default class Flux {
constructor(state) {
//all resources are saved here for disposal (for example, when hot loading)
//this is just a flux dispatcher
this.dispatcher = new rx.Subject(),
// BehaviorSuject constructed with initial state
this.state = new Rx.BehaviorSubject(state),
}
addStore(store, initialState, feature = store.feature) {
this.dispatcher
.share()
// this is where the "reduction" happens. store is a reducer that
// takes an existing state and returns the new state
.flatMap(({action, payload}) =>
store(this.state.getValue(), action, payload))
.startWith(initialState || {})
.subscribe(this.state)
);
return this;
}
addActions(actions: rx.Subject) {
// actions are fed to the dispatcher
this.resources.push(actions.share().subscribe(this.dispatcher));
return this;
}
}
I create a global Flux object with manages state. Then, for every "feature" or "page" or whatever I wish I add actions and stores. It makes managing state very easy, and things like time-travel are something that are a given with Rx.