无法实例化函数无法转换类型为'__NSArrayI'的值

Question

I have made the following function in Swift 3:

    func parseJSON() {
    var JsonResult: NSMutableArray = NSMutableArray()

    do {
        JsonResult = try JSONSerialization.jsonObject(with: self.data as Data, options:JSONSerialization.ReadingOptions.allowFragments) as! NSMutableArray
    } catch let error as NSError {
        print(error)
    }
    var jsonElement:NSDictionary=NSDictionary()
    let locations: NSMutableArray = NSMutableArray()

    for i in 0 ..< JsonResult.count
    {
        jsonElement = JsonResult[i] as! NSDictionary
        let location = Parsexml()

        if let title = jsonElement["Title"] as? String,
            let body = jsonElement["Body"] as? String,
            let userId = jsonElement["UserId"] as? Int,
            let Id = jsonElement["Id"] as? Int
        {

            location.title = title
            location.body = body
            location.userId = userId
            location.id = Id

        }

        locations.add(location)
    }
    DispatchQueue.main.async { () -> Void in

        self.delegate.itemsDownloaded(items: locations)

    }

When i call this function from another method, i get the following error:

Could not cast value of type '__NSArrayI' (0x105d4fc08) to 'NSMutableArray' (0x105d4fcd0).

It points me towards the element here:

  JsonResult = try JSONSerialization.jsonObject(with: self.data as Data, options:JSONSerialization.ReadingOptions.allowFragments) as! NSMutableArray

Where it exits with a SIGBRT..

What have i missed here?

Answer 1

You are trying to convert an NSArray into an NSMutable array which is what the warning is complaining about.

Take the array it provides you, and then convert it into a mutable one.

let jsonArray = try JSONSerialization.jsonObject(with: self.data as Data, options:JSONSerialization.ReadingOptions.allowFragments) as! NSArray
jsonResult = jsonArray.mutableCopy() as! NSMutableArray

Unrelated, but you may also want to user a lower case value for the JsonResult to fit with normal iOS style guidelines. It should instead be jsonResult.

Answer 2

Another way to improve your code:

You are not mutating your JsonResult , so you have no need to declare it as NSMutableArray :

    var JsonResult = NSArray()

    do {
        JsonResult = try JSONSerialization.jsonObject(with: self.data as Data, options:JSONSerialization.ReadingOptions.allowFragments) as! NSArray
    } catch let error as NSError {
        print(error)
    }

---

And some steps to improve your code...

enum MyError: Error {
    case NotArrayOfDict
}
func parseJSON() {
    do {
        guard let jsonResult = try JSONSerialization.jsonObject(with: self.data as Data) as? [[String: Any]] else {
            throw MyError.NotArrayOfDict
        }
        let locations: NSMutableArray = NSMutableArray()

        for jsonElement in jsonResult {
            let location = Parsexml()

            if let title = jsonElement["Title"] as? String,
                let body = jsonElement["Body"] as? String,
                let userId = jsonElement["UserId"] as? Int,
                let Id = jsonElement["Id"] as? Int
            {
                location.title = title
                location.body = body
                location.userId = userId
                location.id = Id

            }

            locations.add(location)
        }
        DispatchQueue.main.async { () -> Void in
            self.delegate.itemsDownloaded(items: locations)
        }
    } catch let error {
        print(error)
    }
}

• as! casting sometimes crashes your app, use it only when you are 100%-sure that the result is safely converted to the type. If you are not, using guard-let with as? is safer.

• Use Swift types rather than NSSomething as far as you can.

• Specifying .allowFragments is not needed, as you expect the result as an Array.

---

And if you can modify some other parts of your code, you can write your code as:

func parseJSON() {
    do {
        //If `self.data` was declared as `Data`, you would have no need to use `as Data`.
        guard let jsonResult = try JSONSerialization.jsonObject(with: self.data) as? [[String: Any]] else {
            throw MyError.NotArrayOfDict
        }
        var locations: [Parsexml] = [] //<-Use Swift Array

        for jsonElement in jsonResult {
            let location = Parsexml()

            if let title = jsonElement["Title"] as? String,
                let body = jsonElement["Body"] as? String,
                let userId = jsonElement["UserId"] as? Int,
                let Id = jsonElement["Id"] as? Int
            {

                location.title = title
                location.body = body
                location.userId = userId
                location.id = Id

            }

            locations.append(location)
        }
        DispatchQueue.main.async { () -> Void in
            self.delegate.itemsDownloaded(items: locations)
        }
    } catch let error {
        print(error)
    }
}