使用HTTP GET请求,Swift动态转换失败

Question

在使用RubyMotion创建应用程序之后,这是我第一次使用xCode在Swift中开发应用程序.

我使用以下代码从Rails API获取所有帖子,请求成功但解析JSON会出错.

代码:

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()
}

错误:

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)

提前致谢!

Answer 1

从您的URL http://localhost:3000/api/posts我可以假设您收到一个对象数组,因此您应该将结果转换为NSArray.

代码应该是:

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

但是正如Yatheesha所提到的,你可以收到nil所以最好转换为可以为null的数组并检入if let...:

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