Swift 4 JSON解析报错求助：法语区Swift新手未找到解决办法

Hey there! Let's get that JSON parsing working for you. I took a look at the JSON file you're trying to decode, and I see the main issue right away—your model structure doesn't match the actual JSON hierarchy.

The Root Problem #

Your JSON isn't a single Station object—it's a top-level object that contains an array of stations under the key stations. Here's a snippet of what the JSON looks like:

{
"stations": [
{
"ID": 1,
"name": "Saint-Lô - Gare Routière",
"lines": "A, C"
},
// More stations here...
]
}

Trying to decode directly to Station.self will throw an error because the decoder is expecting a single station, not an object wrapping an array of stations.

Fixed Code #

First, update your model structures to match the JSON:

import UIKit

// This top-level struct matches the root of your JSON
struct StationResponse: Decodable {
    let stations: [Station]
}

// Your original Station struct works perfectly for each item in the array
struct Station: Decodable {
    let ID: Int
    let name: String
    let lines: String
}

Then, complete your viewDidLoad with proper parsing logic (I filled in the missing URLSession code):

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let urlString = "https://celestinlebehot.com/saintlobus-export-2.json"
        guard let url = URL(string: urlString) else { 
            print("Invalid URL—double-check the link!")
            return 
        }
        
        URLSession.shared.dataTask(with: url) { data, response, error in
            // Handle network errors first
            if let error = error {
                print("Network error: \(error.localizedDescription)")
                return
            }
            
            // Make sure we actually got data back
            guard let data = data else {
                print("No data received from the server")
                return
            }
            
            do {
                // Decode the top-level response first, then grab the stations array
                let response = try JSONDecoder().decode(StationResponse.self, from: data)
                let stations = response.stations
                
                // Test it out by printing the first station
                if let firstStation = stations.first {
                    print("Success! First station: \(firstStation.name) (ID: \(firstStation.ID))")
                }
                
                // Important: If you're updating UI with this data, switch back to the main thread
                DispatchQueue.main.async {
                    // Add your UI update code here (e.g. populate a table view)
                }
            } catch {
                // Print detailed parsing errors to debug
                print("Parsing failed: \(error)")
                print("Detailed error info: \(error.localizedDescription)")
            }
        }.resume() // Don't forget to resume the data task!
    }
}

Quick Tips to Avoid Future Issues #

• Always double-check that your Swift structs match the exact structure and key names of your JSON (case sensitivity matters—ID vs id would break things!)
• Never update UI from a background thread—always use DispatchQueue.main.async for UI changes
• Print errors when decoding fails; they usually tell you exactly what's wrong (e.g. "expected dictionary but got array")

内容的提问来源于stack exchange，提问作者itinous