setListFM.swift

import Foundation

/*
  WHY YOURS WASN'T WORKING

  Line 320: Your URL is incorrect, it's missing the search in /1.0[/search/]setlists
  Line 332: Setlist is not what is returned by the response (more below)
    You can see that the response is the root dict and *setlist* is actually a key/value within.
    - response
      - setlist
        ...
      ...
*/


enum SetListFMError : Error {
  case invalidURL
}

// SetList and its internal data is or could be incomplete. You'll need to add the tour, set, etc. data.
// This is a very basic implementation based on the endpoint provided from Reddit (countryCode=US&p=1)
struct SetList : Codable {
  struct Artist : Codable {
    let mbid: String
    
    let name: String
    let sortName: String
    
    let disambiguation: String
    
    let url: String
  }
  
  struct Venue : Codable {
    struct City : Codable {
      struct Coodrinates : Codable {
        let lat: Double
        let long: Double
      }
      
      struct Country : Codable {
        let code: String
        let name: String
      }
      
      let id: String
      
      let name: String
      let state: String
      let stateCode: String
      
      let coords: Coordinates
      
      let country: Country
    }
    
    let id: String
    
    let name: String
    
    let city: City

    let url: String
  }

  struct Set : Codable {
    // Incomplete
  }
  
  struct Tour : Codable {
    // Incomplete
  }
  
  let id: String
  let versionId: String
  
  let eventDate: String
  let lastUpdated: String
  
  let artist: Artist

  let venue: Venue

  let sets: [Set]

  let url: String
  
  // Incomplete
}

struct Response : Codable {
  let setlist: [SetList]
}


// CLASS
class SetListFM {
  static let shared = SetListFM()
  let endpoint = "https://api.setlist.fm/rest/1.0"
  
  func search(_ countryCode:  String, _ page: Int) async throws -> [SetList] {
    guard let url = URL(string: "\(endpoint)/search/setlists?countryCode?\(countryCode)&p=\(page)") else {
      throw SetListFMError.invalidURL
    }
    
    var request = URLRequest(url: url)
    request.set("APIKEY", forHTTPHeaderField: "x-api-key") // Add your API key
    request.set("application/json", forHTTPHeaderField: "Accept")
    request.set("application/json", forHTTPHeaderField: "Content-Type")
    
    let (data, _) = try await URLSession.shared.data(for: request) // (data, response)
    let decodedData = try JSONDecoder().decode(Response.self, from: data)
    return decodedData.setlist
  }
}


// USAGE
Task {
    let setlists = try await SetListFM.shared.search(countryCode: "US", page: 1)
    // OR
    let setlists = try await SetListFM.shared.search("US", 1)
}

Revision 3: Added a small comment explaining some issues.