CODEWITHSUNDEEP
iosswiftcombine
MegaManX
MegaManX

Reputation: 8948

iOS Combine Start new request only if previous has finished

I have network request that triggers every last cell in switui appearas. Sometimes if user scrolls fast enough down -> up -> request will trigger before first one finishes. Without combine or reactive approach I have completion block and bool value to handle this:

public func load() {
        guard !isLoadingPosts else { return }
        isLoadingPosts = true
        postsDataProvider.loadMorePosts { _ in
            self.isLoadingPosts = false
        }
    }

I was wondering if with combine this can be resolved more elegantly, without the need to use bool value. For example execute request only if previous has finished?

Upvotes: 1

Views: 1095

Answers (1)

New Dev
New Dev

Reputation: 49620

It looks like you want to skip making the call if it's already in progress.

Since you didn't share any of the Combine code you might have, I'll assume that you have a publisher-returning function like this:

func loadMorePosts() -> AnyPublisher<[Post], Error> {
   //...
}

Then you can use a subject to initiate a load call, a flatMap(maxPublishers:_:) downstream, with a number of publishers limited to 1:

let loadSubject = PassthroughSubject<Void, Never>()

loadSubject
   .flatMap(maxPublishers: .max(1)) { 
       loadMorePosts()
   }
   .sink(
       receiveCompletion: { _ in },
       receiveValue: { posts in
          // update posts
       })
   .store(in: &cancellables)

The above pipeline subscribes to the subject, but if another value arrives before flatMap is ready to receive it, it would simply be dropped.

Then the load function becomes:

func load() {
   loadSubject.send(())
}

Upvotes: 6

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