解析 JSON 数据 Swift “线程 4：致命错误：在展开可选值时意外发现 nil”

Question

I am trying to parse some JSON data returned from an API call. The path I want to navigate is media > faces > points. I can navigate to faces but getting points is the issue.

here is code

let dictionary = try JSONSerialization.jsonObject(with: data!, options:.mutableContainers) as? [String:Any]

let media = dictionary!["media"] as? [String: Any]
print(media!["faces"]) // Works see the returned data below
let faces = media!["faces"] as? [String: Any]
print(faces!["points"]) // Thread 4: Fatal error: Unexpectedly found nil while unwrapping an Optional value

API returned data for print(media!["faces"])

{
    angle = "1.2222";
    "appearance_id" = 0;
    "detection_score" = 1;
    duration = "00:00:00";
    "face_uuid" = "78338d20-9ced-11ea-b153-0cc47a6c4dbd";
    height = "74.31999999999999";
    "media_uuid" = "fec83ac3-de00-44f0-ad5b-e1e990a29a8c";
    "person_id" = "";
    points =     (
                {
            name = "basic eye left";
            type = 512;
            x = "85.16";
            y = "86.62";
        },
                {
            name = "basic eye right";
            type = 768;
            x = "110.92";
            y = "86.66";
        }

Answer 1

Since you are a beginner I'm helping you out. These questions have been asked many times (There is a high chance you get duplicate flag). Here is how you avoid these kinds of errors, try not to use force-unwraps (this symbol ! ) anywhere without any exception to avoid these errors. Instead of if let you can use guard let , it's a personal choice.

if let data = data,
    let dictionary = try JSONSerialization.jsonObject(with: data, options:.mutableContainers) as? [String: Any],
    let media = dictionary["media"] as? [String: Any],
    let faces = media["faces"] as? [[String: Any]],
    let points = faces[0]["points"] {
    print(points)
}

Edit: Since "faces" parameter is an array you need to parse them as an Array and not as Dictionary . The above example shows how to get to the first set of "points" .

Update: Better approach would be to use JSONDecoder . Here's how: Create this struct outside the class:

struct Response: Codable { var media: Media }
struct Media: Codable { var faces: [Face] }
struct Face: Codable { var points: [Point] }
struct Point: Codable { }

And then decode like this:

if let data = data,
    let response = try JSONDecoder().decode(Response.self, from: data) {
    print(response.media.faces[0].points)
}

Answer 2

The main problem here is that you are force unwrapping optionals that you can't guarantee have a value - and when they don't have one that's causing a fatal error.

A better approach is to unwrap the variables safely and throw an error if you need to handle it. I've tried to show a few different ways to do that below.

guard let data = data else { return }
do {
    let dictionary = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as? [String:Any]
    guard let media = dictionary?["media"] as? [String: Any] else { throw MyError.mediaNotFoundError }

    if let faces = media["faces"] as? [String: Any] {
        print(faces)

        guard let points = faces["points"] else { throw MyError.pointsNotFound}
        print(points)
    } else {
        throw MyError.facesNotFound
    }
} catch {
    // Handle error here
}

Note, I have used a custom error enum. I would also advise adding a localised description to it in order to add more details for debugging.

enum MyError: Error {
    case mediaNotFoundError
    case facesNotFound
    case pointsNotFound
}

I would also recommend reading this answer here for a more in-depth explanation on optionals.