Swift dynamic migration failed with HTTP GET request

Question

After building the application with RubyMotion, this is my first experience developing applications in Swift with xCode.

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

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() } 

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 are getting an array of objects, so you should cast the result to NSArray.

The code should be:

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

However, as Yatheesha mentioned, you can get null, so it's best to drop to a null array and check if let... :

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

Answer 2

In your case, the data may be nil , after which you will get nil from NSJSONSerialization.JSONObjectWithData() , so type casting will fail, try

  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() }