I'm trying to return a value from an API function with dataTask. When editing the code, I get the error:

Type of expression is ambiguous without more context.

I added the code below. Does anyone know how to fix this? I'm new to Swift.

func getNonce() {
    let headers = [
      "accept": "application/json",
      "content-type": "application/json"
    ]
    let parameters = [
      "id": 1,
      "jsonrpc": "2.0",
      "params": [addressETH, "pending"],
      "method": "eth_getTransactionCount"
    ] as [String : Any]
    let postData = try? JSONSerialization.data(withJSONObject: parameters, options: [])
    let request = NSMutableURLRequest(url: NSURL(string: "https://eth-goerli.g.alchemy.com/v2/myapikeygoeshere")! as URL, cachePolicy: .useProtocolCachePolicy, timeoutInterval: 10.0)
    request.httpMethod = "POST"
    request.allHTTPHeaderFields = headers
    request.httpBody = postData! as Data
    let session = URLSession.shared
    let dataTask = session.dataTask(with: request as URLRequest, completionHandler: { (data, response, error) -> String in
        if (error != nil) {
            print(error as Any)
        } else {
            let dataString = String(data: data!, encoding: .utf8)!
            let start = dataString.index(dataString.startIndex, offsetBy: 36)
            let end = dataString.index(dataString.endIndex, offsetBy: -2)
            let range = start..<end
            let user_Nonce = String(dataString[range])
            return user_Nonce?
        }
    })
    dataTask.resume()
}

The error is on this line:

let dataTask = session.dataTask(with: request as URLRequest, completionHandler: { (data, response, error) -> String in

I tried to return the value but the error popped up.

CodePudding user response：

You should read up on how completion handlers or async await work in swift. Here are some examples on how you could structure this function to work - and you'll have to account for the completion handler or async await at the call site. There are also a fair number of force unwraps (!) in here, which you should avoid, same with try?. Catch those conditions and report the errors to the caller.

// Example using closures

func getNonce(completion: @escaping (Result<String, Error>) -> Void) {
    let headers = [
      "accept": "application/json",
      "content-type": "application/json"
    ]
    
    let parameters = [
      "id": 1,
      "jsonrpc": "2.0",
      "params": ["addressETH", "pending"],
      "method": "eth_getTransactionCount"
    ] as [String : Any]
    
    let postData = try? JSONSerialization.data(withJSONObject: parameters, options: [])
    var request = URLRequest(url: NSURL(string: "https://eth-goerli.g.alchemy.com/v2/myapikeygoeshere")! as URL, cachePolicy: .useProtocolCachePolicy, timeoutInterval: 10.0)
    request.httpMethod = "POST"
    request.allHTTPHeaderFields = headers
    request.httpBody = postData!
    
    let task = URLSession.shared.dataTask(with: request) { data, response, error in
        if let error = error {
            completion(.failure(error))
        } else {
            let dataString = String(data: data!, encoding: .utf8)!
            let start = dataString.index(dataString.startIndex, offsetBy: 36)
            let end = dataString.index(dataString.endIndex, offsetBy: -2)
            let range = start..<end
            let user_Nonce = String(dataString[range])
            completion(.success(user_Nonce))
        }
    }
    task.resume()
}

// Example using async await

func getNonce() async throws -> String {
    let headers = [
      "accept": "application/json",
      "content-type": "application/json"
    ]
    
    let parameters = [
      "id": 1,
      "jsonrpc": "2.0",
      "params": ["addressETH", "pending"],
      "method": "eth_getTransactionCount"
    ] as [String : Any]
    
    let postData = try? JSONSerialization.data(withJSONObject: parameters, options: [])
    var request = URLRequest(url: NSURL(string: "https://eth-goerli.g.alchemy.com/v2/myapikeygoeshere")! as URL, cachePolicy: .useProtocolCachePolicy, timeoutInterval: 10.0)
    request.httpMethod = "POST"
    request.allHTTPHeaderFields = headers
    request.httpBody = postData!
    
    let (data, _) = try await URLSession.shared.data(for: request)
    let dataString = String(data: data, encoding: .utf8)!
    let start = dataString.index(dataString.startIndex, offsetBy: 36)
    let end = dataString.index(dataString.endIndex, offsetBy: -2)
    let range = start..<end
    let user_Nonce = String(dataString[range])

    return user_Nonce
}

// Assuming called from viewDidLoad in a view controller

override func viewDidLoad() {
    super.viewDidLoad()

    // How to call using closure example
    
    getNonce { result in
        switch result {
        case .success(let nonce):
            print("got nonce: \(nonce)")
            
        case .failure(let error):
            // handle error
            break
        }
    }
    
    // How to call using async await example
    Task {
        do {
            let nonce = try await getNonce()
            print("got nonce: \(nonce)")
        } catch {
            // handle error
        }
    }
}