I'm somewhat familiar with basic RxJS concepts like Observables, Observers and Subjects but RxJS Notifications concept is completely new to me.
What is it for? When should I use it?
The documentation you quoted mentions :
This class is particularly useful for operators that manage notifications, like materialize, dematerialize, observeOn, and others. Besides wrapping the actual delivered value, it also annotates it with metadata of, for instance, what type of push message it is (next, error, or complete).
So the question turns out to be about use cases for materialize and the like.
Basically, you use materialize to get meta-information about the dataflow without incurring into the associated side-effects (an error incurring in a stream for example propagates, a stream which completes can lead to the completion of other streams etc.). dematerialize allows to restore the side-effects.
Here are uses case from former SO questions :
Receiving done notifications from observables built using switch
RxJs - parse file, group lines by topics, but I miss the end
A use case: as errors or completions are propagated immediately, you can't for example delay them. To do so, you can try this approach:
// sample stream
interval(500).pipe(
mapTo('normal value'),
// sometimes value, sometimes throw
map(v => {
if (randomInt() > 50) {
throw new Error('boom!')
} else return v;
}),
materialize(),
// turns Observable<T> into Notification<Observable<T>>
// so we can delay or what you want
delay(500),
// and we need to do some magic and change Notification of error into
// Notification of value (error message)
map(n => n.hasValue? n : new Notification('N', n.error.message, null)),
// back to normal
dematerialize()
)
// now it never throw so in console we will have
// `normal value` or `boom!` but all as... normal values (next() emmision)
// and delay() works as expected
.subscribe(v => console.log(v))
Related
I have tried to unsubscribe within the subscribe method. It seems like it works, I haven't found an example on the internet that you can do it this way.
I know that there are many other possibilities to unsubscribe the method or to limit it with pipes. Please do not suggest any other solution, but answer why you shouldn't do that or is it a possible way ?
example:
let localSubscription = someObservable.subscribe(result => {
this.result = result;
if (localSubscription && someStatement) {
localSubscription.unsubscribe();
}
});
The problem
Sometimes the pattern you used above will work and sometimes it won't. Here are two examples, you can try to run them yourself. One will throw an error and the other will not.
const subscription = of(1,2,3,4,5).pipe(
tap(console.log)
).subscribe(v => {
if(v === 4) subscription.unsubscribe();
});
The output:
1
2
3
4
Error: Cannot access 'subscription' before initialization
Something similar:
const subscription = of(1,2,3,4,5).pipe(
tap(console.log),
delay(0)
).subscribe(v => {
if (v === 4) subscription.unsubscribe();
});
The output:
1
2
3
4
This time you don't get an error, but you also unsubscribed before the 5 was emitted from the source observable of(1,2,3,4,5)
Hidden Constraints
If you're familiar with Schedulers in RxJS, you might immediately be able to spot the extra hidden information that allows one example to work while the other doesn't.
delay (Even a delay of 0 milliseconds) returns an Observable that uses an asynchronous scheduler. This means, in effect, that the current block of code will finish execution before the delayed observable has a chance to emit.
This guarantees that in a single-threaded environment (like the Javascript runtime found in browsers currently) your subscription has been initialized.
The Solutions
1. Keep a fragile codebase
One possible solution is to just ignore common wisdom and continue to use this pattern for unsubscribing. To do so, you and anyone on your team that might use your code for reference or might someday need to maintain your code must take on the extra cognitive load of remembering which observable use the correct scheduler.
Changing how an observable transforms data in one part of your application may cause unexpected errors in every part of the application that relies on this data being supplied by an asynchronous scheduler.
For example: code that runs fine when querying a server may break when synchronously returned a cashed result. What seems like an optimization, now wreaks havoc in your codebase. When this sort of error appears, the source can be rather difficult to track down.
Finally, if ever browsers (or you're running code in Node.js) start to support multi-threaded environments, your code will either have to make do without that enhancement or be re-written.
2. Making "unsubscribe inside subscription callback" a safe pattern
Idiomatic RxJS code tries to be schedular agnostic wherever possible.
Here is how you might use the pattern above without worrying about which scheduler an observable is using. This is effectively scheduler agnostic, though it likely complicates a rather simple task much more than it needs to.
const stream = publish()(of(1,2,3,4,5));
const subscription = stream.pipe(
tap(console.log)
).subscribe(x => {
if(x === 4) subscription.unsubscribe();
});
stream.connect();
This lets you use a "unsubscribe inside a subscription" pattern safely. This will always work regardless of the scheduler and would continue to work if (for example) you put your code in a multi-threaded environment (The delay example above may break, but this will not).
3. RxJS Operators
The best solutions will be those that use operators that handle subscription/unsubscription on your behalf. They require no extra cognitive load in the best circumstances and manage to contain/manage errors relatively well (less spooky action at a distance) in the more exotic circumstances.
Most higher-order operators do this (concat, merge, concatMap, switchMap, mergeMap, ect). Other operators like take, takeUntil, takeWhile, ect let you use a more declarative style to manage subscriptions.
Where possible, these are preferable as they're all less likely to cause strange errors or confusion within a team that is using them.
The examples above re-written:
of(1,2,3,4,5).pipe(
tap(console.log)
first(v => v === 4)
).subscribe();
It's working method, but RxJS mainly recommend use async pipe in Angular. That's the perfect solution. In your example you assign result to the object property and that's not a good practice.
If you use your variable in the template, then just use async pipe. If you don't, just make it observable in that way:
private readonly result$ = someObservable.pipe(/...get exactly what you need here.../)
And then you can use your result$ in cases when you need it: in other observable or template.
Also you can use pipe(take(1)) or pipe(first()) for unsubscribing. There are also some other pipe methods allowing you unsubscribe without additional code.
There are various ways of unsubscribing data:
Method 1: Unsubscribe after subscription; (Not preferred)
let localSubscription = someObservable.subscribe(result => {
this.result = result;
}).unsubscribe();
---------------------
Method 2: If you want only first one or 2 values, use take operator or first operator
a) let localSubscription =
someObservable.pipe(take(1)).subscribe(result => {
this.result = result;
});
b) let localSubscription =
someObservable.pipe(first()).subscribe(result => {
this.result = result;
});
---------------------
Method 3: Use Subscription and unsubscribe in your ngOnDestroy();
let localSubscription =
someObservable.subscribe(result => {
this.result = result;
});
ngOnDestroy() { this.localSubscription.unsubscribe() }
----------------------
Method 4: Use Subject and takeUntil Operator and destroy in ngOnDestroy
let destroySubject: Subject<any> = new Subject();
let localSubscription =
someObservable.pipe(takeUntil(this.destroySubject)).subscribe(result => {
this.result = result;
});
ngOnDestroy() {
this.destroySubject.next();
this.destroySubject.complete();
}
I would personally prefer method 4, because you can use the same destroy subject for multiple subscriptions if you have in a single page.
What's the best way to handle asynchronous updates in the middle of an Observable stream.
Let's say there are 3 observables:
Obs1 (gets data from API) -> pipes to Obs2
Obs2 (transforms data) -> pipes to Obs3
Obs3 (sends transformed data)
(The actual application is more complex, and there's reasons it's not done in a single Observable, this is just a simple example).
That all works well and good if it's a linear synchronous path.
But we also have async messages that will change the output of Obs2.
3 scenarios I'm asking about are:
- we fetch data, and go through Obs1, Obs2 & Obs3
- we get a message to make a change, go through Obs2 & Obs3
- we get a different message to make a change which also needs to apply the change from the previous message, through Obs2 & Obs3
The main problem here is that there are different types of asynchronous messages that will change the outcome of Obs2, but they all need to still know what the previous outcome of Obs2 was (so the any other changes from messages that happened before is still applied)
I have tried using switchMap in Obs2 with a scan in Obs1 like this:
obs1
const obs1$ = obs1$.pipe(
// this returns a function used in the reducer.
map((data) => (prevData) => 'modifiedData',
scan((data, reducer) => reducer(betsMap), {})
)
obs2
const obs2$ = obs1$.pipe(
switchMap(data =>
someChange$.pipe(map(reducer => reducer(data)))
)
)
where someChange$ is a BehaviorSubject applying a change using another reducer function.
This works fine for async message #1 that makes some change.
But when message #2 comes in and a different change is needed, the first change is lost.
the changes that should be in "prevData" in obs1$ is always undefined because it happens before the message is applied.
How can I take the output from obs2$ and apply asynchronous updates to it that remembers what all of the past updates was? (in a way where I can clear all changes if needed)
So if i got the question right, there are two problems that this question tackles:
First: How to cache the last 2 emitted values from stream.
scan definitely is the right way, if this cache logic is needed in more than one place/file, I would go for a custom pipe operator, like the following one
function cachePipe() {
return sourceObservable =>
sourceObservable.pipe(
scan((acc, cur) => {
return acc.length === 2 ? [...acc.slice(1), cur] : [...acc, cur];
}, [])
);
}
cachePipe will always return the latest 2 values passed trough the stream.
...
.pipe(
cachePipe()
)
Second: How to access data from multiple streams at the same time, upon stream event
Here rxjs's combineLatest creation operator might do the trick for you,
combineLatest(API$, async1$ ,async2$,async3$)
.pipe(
// here I have access to an array of the last emitted value of all streams
// and the data can be passed to `Obs2` in your case
)
In the pipe I can chain whatever number of observables, which resolves the second problem.
Note:
combineLatest needs for all streams, inside of it, to emit once, before the operator strats to emit their combined value, one workaround is to use startWith operator with your input streams, another way to do it is by passing the data trough BehaviorSubject-s.
Here is a demo at CodeSandbox , that uses the cachePipe() and startWith strategy to combine the source (Obs1) with the async observables that will change the data.
I'm new in RxSwift, I don't understand what is difference between do(onNext:) and subscribe(onNext:).
I google it but did't found good resources to explain the difference.
At the beginning of a cold Observable chain there is a function that generates events, for e.g. the function that initiates a network request.
That generator function will not be called unless the Observable is subscribed to (and by default, it will be called each time the observable is subscribed to.) So if you add a do(onNext:) to your observable chain, the function will not be called and the action that generates events will not be initiated. You have to add a subscribe(onNext:) for that to happen.
(The actual internals are a bit more complex than the above description, but close enough for this explanation.)
The do operator allows you to insert side effects; that is, handlers to do things that will not change the emitted event in any way. do will just pass the event through to the next operator in the chain.
The method for using the do operator is here.
And you can provide handlers for any or all of these events.
Let's say We have an observable that never emits anything. Even though it emits nothing, it is still an observable and we can subscribe to it. do operator allows us to do something when a subscription was made to it.
So below example will print "Subscribed" when a subscription was made to that observable.
Feel free to include any of the other handlers if you’d like; they work just like subscribe’s handlers do
let observable = Observable<Any>.never()
let disposeBag = DisposeBag()
observable
.do(onSubscribe: {
print("Subscribed")
})
.subscribe(
onNext: { element in
print(element)
},
onCompleted: {
print("Completed")
},
onDisposed: {
print("Disposed")
}
)
.disposed(by: disposeBag)
I have the following epic I use in my application to handle api requests:
action$ => {
return action$.ofType(actions.requestType)
.do(() => console.log('handled epic ' + actions.requestType))
.switchMap((action) => (
Observable.create((obs) => {
obs.next({ type: type, value: action.value, form: action.form });
})
.debounceTime(250)
.switchMap((iea) => (
Observable.ajax(ajaxPost(url(iea.value), body ? body(iea.value) : action.form))
.mergeMap(payload => {
return Observable.merge(
Observable.of(actions.success(payload)),
/* some other stuff */
);
})
.catch(payload => {
return [actions.failure(payload)];
})
))
))
.takeUntil(action$.filter((a) => (a.type === masterCancelAction))
.repeat();
};
Basically, any time I perform an api request, I dispatch a request action. If I dispatch another request quickly, the previous one is ignored using debounceTime. Additionally, the request can be cancelled using the masterCancelAction and when cancelled repeat() restarts the epic. This epic works as intended in all cases expect one.
The failure case occurs when a user uses the browser back during a request. In this case I fire the masterCancelAction to the request. However, on the same execution context as a result from the masterCancelAction, another request action dispatches to perform a new request on the same epic, but the api request does not occur (the console.log does occur though) as if there was no repeat(). In other cases where cancels occur, the next request is not invoked from the same execution context and it works fine, so it seems in this case my code does not give repeat a chance to restart the epic?
A dirty workaround I found was to use setTimeout(dispatch(action), 0) on the request that dispatches after the cancellation. This seems to allow repeat() to execute. I tried passing different schedulers into repeat, but that didn't seem to help. Also, attaching takeUntil and repeat into my inner switchMap solves the problem, but then other cases where my next request does not execute in the same call stack fail.
Is there a way I can solve this problem without using setTimeout? Maybe it is not a repeat related problem, but it seems to be the case.
Using rxjs 5.0.3 and redux-observable 0.14.1.
The issue is not 100% clear without something like a jsbin to see what you mean, but I do see some general issues that might help:
Anonymous Observable never completes
When creating a custom anonymous Observable it's important to call observer.complete() if you do indeed want it to complete. In most cases, not doing so will cause the subscription to be a memory leak and might also other strange behaviors
Observable.create((observer) => {
observer.next({ type: type, value: action.value, form: action.form });
observer.complete();
})
Observable.of would have been equivalent:
Observable.of({ type: type, value: action.value, form: action.form })
However, it's not clear why this was done as the values it emits are in captured in scope.
debounceTime in this case does not debounce, it delays
Since the anonymous observable it's applied to only ever emits a single item, debounceTime will act just as a regular .delay(250). I'm betting you intended instead to debounce actions.requestType actions, in which case you'd need to apply your debouncing outside the switchMap, after the action$.ofType(actions.requestType).
Observable.of accepts any number of arguments to emit
This is more of a "did you know?" rather than an issue, but I noticed you're merging your of and /* some other actions */ I assume would be other of observables merged in. Instead, you can just return a single of and pass the actions as arguments.
Observable.of(
actions.success(payload),
/* some other actions */
actions.someOtherOne(),
actions.etc()
);
Also, when you find yourself emitting multiple actions synchronously like this, consider whether your reducers should be listening for the same, single action instead of having two or more. Sometimes this wouldn't make sense as you want them to have completely unrelated actions, just something to keep in mind that people often forget--that all reducers receive all actions and so multiple reducers can change their state from the same action.
.takeUntil will stop the epic from listening for future actions
Placing the takeUntil on the top-level observable chain causes the epic to stop listening for action$.ofType(actions.requestType), which is why you added the .repeat() after. This might work in some cases, but it's inefficient and can cause other hard to realize bugs. Epics should be thought of instead as sort of like sidecar processes that usually "start up" with the app and then continue listening for a particular action until the app "shuts down" aka the user leaves the app. They aren't actually processes, it's just helpful to conceptually think of them this way as an abstraction.
So each time it matches its particular action it then most often will switchMap, mergeMap, concatMap, or exhaustMap into some side effect, like an ajax call. That inner observable chain is what you want to make cancellable. So you'd place your .takeUntil on it, at the appropriate place in the chain.
Summary
As mentioned, it's not clear what you intended to do and what the issue is, without a more complete example like a jsbin. But strictly based on the code provided, this is my guesstimate:
const someRequestEpic = action$ => {
return action$.ofType(actions.requestType)
.debounceTime(250)
.do(() => console.log('handled epic ' + actions.requestType))
.switchMap((action) =>
Observable.ajax(ajaxPost(url(action.value), body ? body(action.value) : action.form))
.takeUntil(action$.ofType(masterCancelAction))
.mergeMap(payload => {
return Observable.of(
actions.success(payload),
/* some other actions */
...etc
);
})
.catch(payload => Observable.of(
actions.failure(payload)
))
);
};
Check out the Cancellation page in the redux-observable docs.
If this is a bit confusing, I'd recommend digging a bit deeper into what Observables are and what an "operator" is and does so that it doesn't feel magical and where you should place an operator makes more sense.
Ben's post on Learning Observable by Building Observable is a good start.
I'm working on something that is recording data coming from a queue. It was easy enough to process the queue into an Observable so that I can have multiple endpoints in my code receiving the information in the queue.
Furthermore, I can be sure that the information arrives in order. That bit works nicely as well since the Observables ensure that. But, one tricky bit is that I don't want the Observer to be notified of the next thing until it has completed processing the previous thing. But the processing done by the Observer is asynchronous.
As a more concrete example that is probably simple enough to follow. Imagine my queue contains URLs. I'm exposing those as an Observable in my code. The I subscribe an Observer whose job is to fetch the URLs and write the content to disk (this is a contrived example, so don't take issue with these specifics). The important point is that fetching and saving are async. My problem is that I don't want the observer to be given the "next" URL from the Observable until they have completed the previous processing.
But the call to next on the Observer interface returns void. So there is no way for the Observer to communicate back to me that has actually completed the async task.
Any suggestions? I suspect there is probably some kind of operator that could be coded up that would basically withhold future values (queue them up in memory?) until it somehow knew the Observer was ready for it. But I was hoping something like that already existed following some established pattern.
similar use case i ran into before
window.document.onkeydown=(e)=>{
return false
}
let count=0;
let asyncTask=(name,time)=>{
time=time || 2000
return Rx.Observable.create(function(obs) {
setTimeout(function() {
count++
obs.next('task:'+name+count);
console.log('Task:',count ,' ', time, 'task complete')
obs.complete();
}, time);
});
}
let subject=new Rx.Subject()
let queueExec$=new Rx.Subject()
Rx.Observable.fromEvent(btnA, 'click').subscribe(()=>{
queueExec$.next(asyncTask('A',4000))
})
Rx.Observable.fromEvent(btnB, 'click').subscribe(()=>{
queueExec$.next(asyncTask('B',4000))
})
Rx.Observable.fromEvent(btnC, 'click').subscribe(()=>{
queueExec$.next(asyncTask('C',4000))
})
queueExec$.concatMap(value=>value)
.subscribe(function(data) {
console.log('onNext', data);
},
function(error) {
console.log('onError', error);
},function(){
console.log('completed')
});
What you describe sounds like "backpressure". You can read about it in RxJS 4 documentation https://github.com/Reactive-Extensions/RxJS/blob/master/doc/gettingstarted/backpressure.md. However this is mentioning operators that don't exist in RxJS 5. For example have a look at "Controlled Observables" that should refer to what you need.
I think you could achieve the same with concatMap and an instance of Subject:
const asyncOperationEnd = new Subject();
source.concatMap(val => asyncOperationEnd
.mapTo(void 0)
.startWith(val)
.take(2) // that's `val` and the `void 0` that ends this inner Observable
)
.filter(Boolean) // Always ignore `void 0`
.subscribe(val => {
// do some async operation...
// call `asyncOperationEnd.next()` and let `concatMap` process another value
});
Fro your description it actually seems like the "observer" you're mentioning works like Subject so it would make maybe more sense to make a custom Subject class that you could use in any Observable chain.
Isn't this just concatMap?
// Requests are coming in a stream, with small intervals or without any.
const requests=Rx.Observable.of(2,1,16,8,16)
.concatMap(v=>Rx.Observable.timer(1000).mapTo(v));
// Fetch, it takes some time.
function fetch(query){
return Rx.Observable.timer(100*query)
.mapTo('!'+query).startWith('?'+query);
}
requests.concatMap(q=>fetch(q));
https://rxviz.com/v/Mog1rmGJ
If you want to allow multiple fetches simultaneously, use mergeMap with concurrency parameter.