问题
I have a situation like the following:
myObservable1.pipe(
switchMap((result1: MyObservable1) => {
if (condition) {
return myObservable2;
} else {
return of(null);
}
})
).subscribe(myObservable1 | myObservable2) => {
So if condition is false I only have to perform one request, if the condition is true, I have to chain the first request to the following one, they are essentially two requests to the server api.
Is there a better solution without returning that null
?
Is there any conditional operator in RxJs?
Thanks!
回答1:
Another possibility is to use the operator iif in a SwitchMap.
https://www.learnrxjs.io/learn-rxjs/operators/conditional/iif
https://rxjs-dev.firebaseapp.com/api/index/function/iif
but it can restrain the possibility to control specfic conditions on your Observable :
myObservable1
.pipe(
switchMap(result1 =>
iif(() => condition
, myObservable2
, myObservable1
)
)
.subscribe(result => console.log(result));
Where you can replace 'condition' by a function returning a boolean. With a function :
myObservable1
.pipe(
switchMap(result1 =>
iif(() => test(result1)
, myObservable2
, myObservable1
)
)
.subscribe(result => console.log(result));
test(result1) : boolean {
if(result1){
// the iif() will return myObservable2
return true;
}else{
/: the iif() will return myObservable1
return false ;
}
}
Like @amjuhire said , you can write with a filter and a switchMap :
myObservable1.pipe(
filter((result1) => condition)
switchMap((result1: MyObservable1) => {
return myObservable2;
})
).subscribe(result2 => { ... })
Another possibility given your example :
myObservable1.pipe(
switchMap((result1: MyObservable1) => {
if (condition) {
return myObservable2;
} else {
return of(result1);
}
})
subscribe(result => console.log(result));
Or using EMPTY in the switchMap :
myObservable1.pipe(
switchMap((result1: MyObservable1) => {
if (condition) {
return myObservable2;
} else {
// Observable that immediately completes
return EMPTY;
}
})
subscribe(result2 => console.log(result2));
The problem here is that we don't know the type in your observable.
So i can't judge wich way is the better.
It also depends on how you want to handle the success/error between the different calls of your observable.
回答2:
No need to return of(null), you should use the RxJs filter operator
myObservable1.pipe(
filter((result1) => condition)
switchMap((result1: MyObservable1) => {
return myObservable2;
})
).subscribe((result2) => { ... })
回答3:
In my understanding, you shouldn't use filter
operator because in that case you wouldn't get any value.
myObservable1.pipe(
switchMap((value) => {
return iif(() => condition, myObservable2, of(value));
})
).subscribe((value) => console.log(value) // either from myObservable1 or from myObservable2)
回答4:
I don't know your real code, this is the pseudo code, and I don't know which rxjs version are you using now, but you can do something like below:
assuming you are using rxjs 6:
iif(condition, obervable1, concatMap(observable1, observable2))
.subscribe()
来源:https://stackoverflow.com/questions/50452947/rxjs-conditional-switchmap-based-on-a-condition