Combine, retrieve array of images from array of URLs and assign them to corresponding object

Question

I am quite new using Combine and frankly I don't know whether this question is silly or not, anyway I have a publisher which I then return to the caller with an array of objects retrieved from a RESTful operation in the following way:

       let publisher = URLSession.shared.dataTaskPublisher(for: URL)
        .handleEvents(
            receiveSubscription: { _ in
                activityIndicatorPublisher.send(true)
            }, receiveCompletion: { _ in
                activityIndicatorPublisher.send(false)
            }, receiveCancel: {
                activityIndicatorPublisher.send(false)
            })
        .tryMap { data, response -> Data in
            guard let httpResponse = response as? HTTPURLResponse,
                  httpResponse.statusCode == 200 else {
                      throw NetworkError.httpError
                  }
            print(String(data: data, encoding: .utf8) ?? "")
            return data
        }
        .decode(type: [ShowElement].self, decoder: JSONDecoder())
        .flatMap { $0.publisher }
        .map(\.show?.image?.medium)
        .flatMap(maxPublishers:.max(1)) { url in
            URLSession.shared.dataTaskPublisher(for: url)
                .map(\.data)
                .replaceError(with: Data())
        }
        .map{ imageData, item -> ShowElement in
            var mutableItem = item
            mutableItem.imageData = imageData
            return mutableItem
        }
        .collect()
        .catch { error -> Just<[ShowElement]> in
            print(error)
            return Just([])
        }
        .eraseToAnyPublisher()
    return publisher
}

When I do the dataTaskPublisher I receive the error: "No exact match in call to instance methode dataTaskPublisher, and when I do the mapping of the imageData I receive another error: "Contextual closure type '(Publishers.FlatMap<Publishers.SetFailureType<Publishers.ReplaceError<Publishers.MapKeyPath<URLSession.DataTaskPublisher, Data>>, Error>, Publishers.MapKeyPath<Publishers.FlatMap<Publishers.SetFailureType<Publishers.Sequence<[ShowElement], Never>, Error>, Publishers.Decode<Publishers.TryMap<Publishers.HandleEvents<URLSession.DataTaskPublisher>, JSONDecoder.Input>, [ShowElement], JSONDecoder>>, String?>>.Output) -> ShowElement' (aka '(Data) -> ShowElement') expects 1 argument, but 2 were used in closure body"

vadian · Accepted Answer · 2022-01-15 11:02:34Z

3

Every array can be used as a publisher.

Let's assume that showArray represents the [ShowElement] object.

Just call publisher

showArray.publisher

then map each item to its URL

.map(\.url) // replace `url` with the real property name

then flatMap the URL to load each image sequentially

.flatMap(maxPublishers:.max(1)) { url in
            URLSession.shared.dataTaskPublisher(for: url)
               .map(\.data)
               .replaceError(with: Data())
}

then zip the result with the corresponding ShowElement item

.zip(showArray.publisher)

then map the result and assign the value

.map{ (imageData, item) -> ShowElement in
       var mutableItem = item
       if let image = UIImage(data: imageData) {
          mutableItem.image = image // replace `image` with the real property name
       }
       return mutableItem
}

Finally collect the items

.collect()

The result is an array of ShowElement including the images.

edited Jan 15, 2022 at 11:02

answered Jan 15, 2022 at 10:51

vadian

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1 Comment

Fabrizio Prosperi Over a year ago

Hi @vadian, thank you for your answer, I am sure it is perfect, but I am finding hard to understand where to start putting your lines of code into mine. If I put them at the end I receive the error that 'Value of type '[ShowElement]' has not my property, because it is taking it as an array, i's not going into the single element of the array.

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