Combine and nested JSON objects

Question

I have the following model:

struct Book: Codable, Identifiable {
    var id: Int
    var title: String
    var author: String
}

struct BookWrapper: Codable {
    var books: [Book]

}

and JSON:

{
  "books": [
    {
        "id":     1,
        "title":  "Nineteen Eighty-Four: A Novel",
        "author": "George Orwell"
    }, {
        "id":     2,
        "title":  "Animal Farm",
        "author": "George Orwell"
    }
  ],
  "errMsg": null
}

I'm trying to grab data using Combine, but cannot find a way how to go around that books array. In case of flat data I would use following:

func fetchBooks() {
        URLSession.shared.dataTaskPublisher(for: url)
            .map{ $0.data }
            .decode(type: [Book].self, decoder: JSONDecoder())
            .replaceError(with: [])
            .eraseToAnyPublisher()
            .receive(on: DispatchQueue.main)
            .assign(to: &$books)
    }

I tried to use BookWrapper.self, but it doesn't make sense. Is there any elegant way how to solve it?

Unrelated but eraseToAnyPublisher() makes no sense in this context (no return value) — vadian
– vadian, Commented Aug 30, 2021 at 17:57

jnpdx · Accepted Answer · 2021-08-30 18:00:22Z

1

You can just map the books property of BooksWrapper before it gets to your assign:

func fetchBooks() {
    URLSession.shared.dataTaskPublisher(for: url)
        .map{ $0.data }
        .decode(type: BookWrapper.self, decoder: JSONDecoder())
        .replaceError(with: BookWrapper(books: [])) //<-- Here
        .map { $0.books }  //<-- Here
        .receive(on: DispatchQueue.main)
        .assign(to: &$books)
}

edited Aug 30, 2021 at 18:00

answered Aug 30, 2021 at 17:48

jnpdx

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2 Comments

BobC Over a year ago

It certainly works, but I have no idea what's going on there. Could you point me to any doc which can explain what you did?

jnpdx Over a year ago

The only significant thing added is the map function — you could look at the documentation for it. It’s a good thing to get familiar with if using combine. It takes an input and transforms it to a different output. In this case the input is the BooksWrapper and the output is just the books property.

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