swift - 当涉及 Optional(_SwiftValue) 时，如何检查 nil/NSNull/optional

Question

从 swift3 开始，我无法可靠地检测 nil/NSNull。在此代码段中，我得到了一个我不知道如何检测的 Optional(_SwiftValue) 值，而不是 nil。

我试过了checking classForCoder没有运气。

其他地方有talk about id-as-Any causing this kind of problem

func testVotingMachine() {
    let vote: String? = nil // Can be a string or nil
    //let vote: String? = "president" // Can be a string or nil
    var dict = [String: AnyObject?]()
    dict["vote"] = vote as AnyObject?
    let obj: AnyObject? = dict as AnyObject?

    guard let dict2 = obj as? Dictionary<String, AnyObject?> else {
        fatalError()
    }

    let value = dict2["vote"] as AnyObject?
    let didVote = notNil(objectOrNil: value)
    XCTAssertFalse(didVote)   // BOOM! notNil() fails to detect nil
}

func notNil(objectOrNil: AnyObject?) -> Bool {
    guard let object: AnyObject = objectOrNil else {
        return false
    }
    if object is NSNull {
        return false
    }
    //if object.classForCoder == NSNull.classForCoder() {
    //  return false
    //}
    // TODO: how to check for? Optional(_SwiftValue)
    print("class for coder: \(object.classForCoder)")
    return true
}

在将数据传递给 JSONSerialization.data() 之前，我需要对其进行清理。因此我需要检测 Optional(_SwiftValue)

Answer 1

安String?是 Optional<String> 的简写.当它为零时，它实际上表示为 Optional<String>.none ，符合条件的是非零 AnyObject所以你的测试没有成功。

你可以这样修复它:

func notNil(objectOrNil: AnyObject?) -> Bool {
    guard let object: AnyObject = objectOrNil,
        object as! _OptionalNilComparisonType != AnyObject?.none else {
        return false
    }
    if object is NSNull {
        return false
    }
    //if object.classForCoder == NSNull.classForCoder() {
    //  return false
    //}
    // TODO: how to check for? Optional(_SwiftValue)
    print("class for coder: \(object.classForCoder)")
    return true
}