Swift 4.1(Xcode9.3)中NSCocoaErrorDomain Code=3840错误排查求助

Hey there, let's break down that annoying Error Domain=NSCocoaErrorDomain Code=3840 you're hitting. In short, this error almost always boils down to JSON-related issues—either the data you're sending to your MySQL backend is malformed, or the response coming back from your createteam.php isn't valid JSON. Let's walk through the fixes step by step.

1. First, Check What Your Server Is Actually Returning #

This is the most common culprit. Your app expects a clean JSON response, but if your PHP script is spitting out HTML error pages, debug echoes, or malformed JSON, you'll get this error every time.

• Use a tool like Postman or run a curl command to hit your API directly:

  curl -X POST http://my_url/ios_api/api/createteam.php -d "name=TestTeam&member=5"
  

• Look at the output:
  • If you see something like {"status":"success","message":"Team created"}—great, that's valid JSON.
  • If you get HTML (like an internal server error page) or text with syntax mistakes (e.g., missing quotes, extra commas), that's the problem.

2. Fix Your Swift Request Code #

Your buttonSave method is cut off, but odds are you're not formatting the POST request correctly for JSON. Here's the proper way to send data to your API and handle the response (with error checking):

@IBAction func buttonSave(_ sender: UIButton) {
    // Validate input fields first
    guard let teamName = textFieldName.text, !teamName.trimmingCharacters(in: .whitespaces).isEmpty,
          let memberCountText = textFieldMember.text, !memberCountText.trimmingCharacters(in: .whitespaces).isEmpty,
          let memberCount = Int(memberCountText) else {
        // Show alert for empty/invalid input
        let alert = UIAlertController(title: "Oops", message: "Please fill in all fields correctly", preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: "OK", style: .default))
        present(alert, animated: true)
        return
    }
    
    // Build parameters to send
    let requestParams: [String: Any] = [
        "name": teamName,
        "member": memberCount
    ]
    
    guard let apiURL = URL_SAVE_TEAM else {
        print("Invalid API URL")
        return
    }
    
    var request = URLRequest(url: apiURL)
    request.httpMethod = "POST"
    // Critical: Set content type to JSON so your PHP knows how to parse it
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    
    // Serialize parameters to JSON data
    do {
        request.httpBody = try JSONSerialization.data(withJSONObject: requestParams, options: [])
    } catch {
        print("Failed to serialize JSON: \(error.localizedDescription)")
        return
    }
    
    // Send the request
    let task = URLSession.shared.dataTask(with: request) { data, response, error in
        // Handle network errors first
        if let error = error {
            print("Network Error: \(error.localizedDescription)")
            return
        }
        
        // Make sure we got data back
        guard let responseData = data else {
            print("No data received from server")
            return
        }
        
        // Debug step: Print the raw response to see what's coming back
        if let rawResponse = String(data: responseData, encoding: .utf8) {
            print("Raw Server Response: \(rawResponse)")
        }
        
        // Try parsing the JSON
        do {
            if let jsonResponse = try JSONSerialization.jsonObject(with: responseData, options: []) as? [String: Any] {
                DispatchQueue.main.async {
                    // Update UI on main thread (e.g., show success alert)
                    let alert = UIAlertController(title: "Success", message: "Team saved!", preferredStyle: .alert)
                    alert.addAction(UIAlertAction(title: "OK", style: .default))
                    self.present(alert, animated: true)
                }
            }
        } catch {
            print("JSON Parsing Error: \(error.localizedDescription)")
            // This is where you'll hit Code=3840 if the response isn't valid JSON
            DispatchQueue.main.async {
                let alert = UIAlertController(title: "Error", message: "Failed to process server response", preferredStyle: .alert)
                alert.addAction(UIAlertAction(title: "OK", style: .default))
                self.present(alert, animated: true)
            }
        }
    }
    task.resume()
}

3. Fix Your PHP Backend (createteam.php) #

Your PHP script needs to return valid JSON and nothing else. Even a stray echo "Debug info"; or a PHP error message will break the JSON parsing.

Here's a clean example of how your PHP should look:

<?php
// Set content type to JSON (critical!)
header("Content-Type: application/json");

// Initialize response array
$response = array("status" => "error", "message" => "Unknown error");

// Database connection (replace with your credentials)
$conn = mysqli_connect("your_host", "your_user", "your_password", "your_db");

if (!$conn) {
    $response["message"] = "Database connection failed: " . mysqli_connect_error();
    echo json_encode($response);
    exit; // Stop execution here to avoid extra output
}

// Get JSON data from request
$input = json_decode(file_get_contents("php://input"), true);

// Validate input
if (!isset($input["name"]) || !isset($input["member"])) {
    $response["message"] = "Missing required fields";
    echo json_encode($response);
    exit;
}

$teamName = mysqli_real_escape_string($conn, $input["name"]);
$memberCount = intval($input["member"]);

// Insert into database
$query = "INSERT INTO teams (name, member_count) VALUES ('$teamName', $memberCount)";

if (mysqli_query($conn, $query)) {
    $response["status"] = "success";
    $response["message"] = "Team created successfully";
} else {
    $response["message"] = "Database error: " . mysqli_error($conn);
}

// Return JSON response
echo json_encode($response);
mysqli_close($conn);
exit;
?>

Key notes for PHP:

• Always set header("Content-Type: application/json"); at the top.
• Use exit; after echoing JSON to prevent any extra output (like whitespace from closing PHP tags).
• Never echo debug messages directly—include them in the JSON response instead.

4. Bonus: Check App Transport Security (ATS) #

If your API uses http:// instead of https://, iOS 9+ will block the request by default. To fix this, add an exception in your Info.plist:

<key>NSAppTransportSecurity</key>
<dict>
    <key>NSAllowsArbitraryLoads</key>
    <true/>
</dict>

(For production, it's better to use NSExceptionDomains to allow only your specific domain instead of disabling ATS entirely.)

Start with step 1—printing the raw server response will immediately tell you if the issue is on the backend. From there, you can adjust either your Swift code or PHP script to ensure valid JSON is sent and received.

内容的提问来源于stack exchange，提问作者Kawazoe Kazuke