Swift dynamic cast failed with HTTP GET request

Question

After creating an app with RubyMotion, this is my first experience with developing apps in Swift with xCode.

I use the following code to get all the posts from an Rails API, the request is successful but parsing the JSON gives an error.

The code:

func loadData() {
    let url = NSURL(string: "http://localhost:3000/api/posts")
    let task = NSURLSession.sharedSession().dataTaskWithURL(url, completionHandler: {(data, response, error) in
        var err: NSError?
        var json =  NSJSONSerialization.JSONObjectWithData(data, options: .MutableLeaves, error: &err) as NSDictionary
    })
    task.resume()
}

The error:

libswift_stdlib_core.dylib`swift_dynamicCastObjCClassUnconditional:
0x103dbae20:  pushq  %rbp
0x103dbae21:  movq   %rsp, %rbp
0x103dbae24:  pushq  %rbx
0x103dbae25:  pushq  %rax
0x103dbae26:  movq   %rsi, %rcx
0x103dbae29:  movq   %rdi, %rbx
0x103dbae2c:  xorl   %eax, %eax
0x103dbae2e:  testq  %rbx, %rbx
0x103dbae31:  je     0x103dbae4d               ;     swift_dynamicCastObjCClassUnconditional + 45
0x103dbae33:  movq   0x72b66(%rip), %rsi       ; "isKindOfClass:"
0x103dbae3a:  movq   %rbx, %rdi
0x103dbae3d:  movq   %rcx, %rdx
0x103dbae40:  callq  *0x5e212(%rip)            ; (void *)0x0000000105b6f000: objc_msgSend
0x103dbae46:  testb  %al, %al
0x103dbae48:  movq   %rbx, %rax
0x103dbae4b:  je     0x103dbae54               ;     swift_dynamicCastObjCClassUnconditional +     52
0x103dbae4d:  addq   $0x8, %rsp
0x103dbae51:  popq   %rbx
0x103dbae52:  popq   %rbp
0x103dbae53:  retq   
0x103dbae54:  leaq   0xe427(%rip), %rax        ; "Swift dynamic cast failed"
0x103dbae5b:  movq   %rax, 0x7974e(%rip)       ; gCRAnnotations + 8
0x103dbae62:  int3   
0x103dbae63:  nopw   %cs:(%rax,%rax)

Thanks in advance!

Answer 1

From your URL http://localhost:3000/api/posts I can assume that you receive an array of objects, so you should be casting the result to NSArray.

Code should be:

var json =  NSJSONSerialization.JSONObjectWithData(data, options: .MutableLeaves, error: &err) as NSArray

However as mentioned by Yatheesha, you can receive nil so it is better to cast to nullable array and check in if let...:

if let json =  NSJSONSerialization.JSONObjectWithData(data, options: .MutableLeaves, error: &err) as? NSArray { 
    ... 
}

Answer 2

In your case data may be nil then you will get nil from NSJSONSerialization.JSONObjectWithData() so type casting leads to crash, Try this

 func loadData() {
     let url = NSURL(string: "http://localhost:3000/api/posts")
     let task = NSURLSession.sharedSession().dataTaskWithURL(url, completionHandler: {(data, response, error) in
         var err: NSError?
         if let json = NSJSONSerialization.JSONObjectWithData(data, options: .MutableLeaves, error: &err) as? NSArray {

               //Valid NSArray object
          }
         else {
            //NSArray is nil
         }
           })
        task.resume()
        }