arrays - 无法下标类型 '[UInt32]' 的值

Question

我有一个随机生成的数字数组，然后我尝试在 Firebase 中查询一个问题，该问题等于所选数字数组中索引 [0] 处的值。目前的问题是我收到错误 Cannot subscript a value of type '[UInt32]'。附言我在 swift 方面不是很有经验，所以非常感谢精确的代码解决方案!我还附上了我的 firebase 结构......

import UIKit
import Firebase

class QuestionViewController: UIViewController {


let ref = Firebase(url: "https://123test123.firebaseio.com/questions")

override func viewDidLoad() {
    super.viewDidLoad()

    // An empty array to hold the selected numbers
    var selectedNumbers: [UInt32] = []

    // A range of acceptable numbers
    let randomNumberRange = 1...10

    // How many numbers are needed?
    let randomNumbersToChoose = 10

    // Crash if asking for more numbers than are available in the range
    assert(randomNumberRange.count >= randomNumbersToChoose, "Must have enough numbers to choose from!")

    // Repeat this loop until all enough numbers have been selected
    while selectedNumbers.count < randomNumbersToChoose {

        // Pick a random number within the allowed range
        let selectedNumber = arc4random_uniform(UInt32(randomNumberRange.endIndex - randomNumberRange.startIndex)) + UInt32(randomNumberRange.startIndex)

        // If it's not already in the selected array, add it
        if (selectedNumbers.indexOf(selectedNumber) == nil) {
            selectedNumbers.append(selectedNumber)
        }
    }

    // Print the result
    print(selectedNumbers)


        print(selectedNumbers)
        let selectedNumberIndex: UInt32 = 2
        ref.queryOrderedByChild("value").queryEqualToValue(selectedNumbers[0])
            .observeEventType(.ChildAdded, withBlock: {
                snapshot in
                //Do something with the question
                print(snapshot.key)
                print(snapshot.value.valueForKey("question"))
            })
    }

@IBAction func truepressed(sender: AnyObject) {
}

@IBAction func falsePressed(sender: AnyObject) {
}

}

JSON 数据:

{
  "question1" : {
    "answer" : "Nohghpe",
    "question" : "Do you know swift",
    "value" : 1
  },
  "question10" : {
    "answer" : "A fdsbit",
    "question" : "Do you kndfggow firebase",
    "value" : 10
  },
  "question2" : {
    "answer" : "A bfhit",
    "question" : "Dodhfg you know firebase",
    "value" : 2
  },
  "question3" : {
    "answer" : "A bsdit",
    "question" : "Do you know firebsgdfase",
    "value" : 3
  },
  "question4" : {
    "answer" : "A vcxbit",
    "question" : "Do yosgfdu know firebase",
    "value" : 4
  },
  "question5" : {
    "answer" : "A bivcxt",
    "question" : "Do you kfghnow firebase",
    "value" : 5
  },
  "question6" : {
    "answer" : "A bxcvit",
    "question" : "Do you know fnhirebase",
    "value" : 6
  },
  "question7" : {
    "answer" : "A bivxct",
    "question" : "Do you sgdfknow firebase",
    "value" : 7
  },
  "question8" : {
    "answer" : "A bivcxt",
    "question" : "Do you knsfdow firebase",
    "value" : 8
  },
  "question9" : {
    "answer" : "A bdsfit",
    "question" : "Do you kdfgnow ffsdirebase",
    "value" : 9
  }
}

Answer 1

这是 Swift 编译器掩盖另一个编译器背后真正错误的典型情况。 .queryEqualToValue(..) method需要一个 AnyObject 类型的参数；它只能包含引用(类)类型，而 UInt32 是一个值类型。

另一个混淆的主题可能是我们通常习惯于 AnyObject 类型可以，看似，持有 Int 类型，实际上，此类赋值会将 Swift 原生 Int 值类型隐式转换为 Foundation __NSCFNumber 引用类型。但是，此隐式转换不适用于 UInt32 类型。

var a : AnyObject?
let foo : [Int] = [1, 2, 3]
let bar : [UInt32] = [1, 2, 3]

/* OK: Int -> [implicitly] -> __NSCFNumber */
a = foo[0]
print(a!.dynamicType) // __NSCFNumber

/* Not OK */
a = bar[0]
/* error: cannot subscript a value of type '[UInt32]' a = bar[0] */

因此，您可以通过以下方式解决此问题:

1. 让 selectedNumbers 成为数组 0f Int，而不是 UInt32(并相应地修改代码中受影响的部分)。

2. 在对 .queryEqualToValue(..) 的调用中执行从 UInt32 到 Int 的类型转换。例如，在上面的示例中:

   a = Int(bar[0])
   

   请注意，对于 32 位系统(例如 iPhone 5)，UInt32 类型可表示的数字上限的一半不能用 Int 类型表示，因为在 32 位系统上，Int 对应于 Int32。

   INT32_MAX   // 2147483647
   UINT32_MAX  // 4294967295
   

   但是，快速浏览一下您的代码，这似乎不是这里的问题，因为 selectedNumbers 的元素将不包含 可表示的较大数字UInt32(对于 64 位系统:OK，因为 Int 对应于 Int64)。但是，如果选择此选项，为了良好实践，您应该在转换之前断言 UInt32 值可以由旨在运行您的系统的 Int 类型表示申请。