How to Custom Decode each element of an array using Swift Decodable

Question

I am decoding a weather JSON response, and I want to pass information to a child decoder which will be decoding an array. How can I pass in this information for every JSON array element while decoding with Swift Decodable?

I am trying to do the above in the latest Swift version (4.2), and I've already implemented the necessary code to successfully custom decode the same model as is being used in the array. I have been unable to decode the entire array.

struct DataBlock: Decodable {
    /..
    let weather: [DataPoint]?

    init(from decoder: Decoder) throws {
        try self.init(from: decoder, units: .unit)
    }

    init(from decoder: Decoder, units: Units) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        /..
        let nestedDecoder = try values.superDecoder(forKey: .weather)
        self.weather = try [DataPoint(from: nestedDecoder, units: units)]
    }

    enum CodingKeys: String, CodingKey {
        /..
        case weather
    }
}

And in the DataPoint model:

init(from decoder: Decoder, units: Units) throws {
    let values = try decoder.container(keyedBy: CodingKeys.self)
    /...
    if let value = try values.decodeIfPresent(Double.self, forKey: .value) {
        self.value = Example(value: value, units: units)
    } else {
        self.value = nil
    }
    /..
}

The relevant part of the JSON structure I am trying to decode:

"datablock": {
    "weather": [
        {
            /..
            "value": 22.72
            /..
        }
    ]
}

I expect the decoder to pass in

units

and decode each array element manually.

However I get a debug error:

"Expected to decode Dictionary<String, Any> but found an array instead."

The error gets triggered here:

self.weather = try [DataPoint(from: nestedDecoder, units: units)]

Can you post the url you are using to fetch the JSON response? or a link to a file with the json data received? — Leo Dabus
– Leo Dabus, Commented Jan 3, 2019 at 2:01

JackFrost · Accepted Answer · 2019-01-03 21:55:59Z

3

This solves my issue by letting me 'walkthrough' each JSON array element and decode each separately:

struct DataBlock: Decodable {
    /..    
    init(from decoder: Decoder, units: Units) throws {
        /..
        var data = [DataPoint]()
        var dataContainer = try values.nestedUnkeyedContainer(forKey: .data)

        while !dataContainer.isAtEnd {
            let nestedDecoder = try dataContainer.superDecoder()
            let dataPoint = try DataPoint(from: nestedDecoder, units: units)
            data.append(dataPoint)
        }

        self.data = data
        /..
    }
    /..
}

answered Jan 3, 2019 at 21:55

JackFrost

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