swift - 如何编写处理可选值的字典扩展

Question

我正在尝试实现一个Dictionary 扩展，我想处理可选值。但无论我做什么，如果我在 [String: String?] 字典上使用我的方法，它都无法选择性地绑定(bind)值。如何编写字典扩展以优雅地处理可选值？

---

考虑以下扩展:

extension Dictionary {
    func someMethod() {
        for (key, value) in self {
            if let valueString = value as? String {
                println("  \(key) = \(valueString)")
            } else {
                println("  \(key) = \(value) cannot be cast to `String`")
            }
        }
    }
}

因此请考虑以下代码:

let dictionary: [String: AnyObject?] = ["foo": "bar"]
dictionary.someMethod()

它奇怪地报告

foo = Optional(bar) cannot be cast to `String`

我可以编写一个非扩展方法来处理具有可选值的字典参数，但不知道如何将其作为 Dictionary 的扩展。

Answer 1

你可以通过反射(reflection)来做到这一点。不需要比您已有的更多的代码:

extension Dictionary
{
    func someMethod()
    {
        for (key, value) in self
        {
            var valueRef = _reflect(value)

            while valueRef.disposition == .Optional && valueRef.count > 0 && valueRef[0].0 == "Some"
            {
                valueRef = valueRef[0].1
            }

            if let valueString: String = valueRef.value as? String
            {
                print("  \(key) = \(valueString)")
            }
            else
            {
                print("  \(key) = \(value) cannot be cast to `String`")
            }
        }
    }
}

---

let dictionary: [String : AnyObject?] = ["foo" : "bar"]
dictionary.someMethod()

返回

foo = bar

---

let dictionary: [String : AnyObject?] = ["foo" : nil]
dictionary.someMethod()

返回

foo = nil cannot be cast to `String`

---

let dictionary: [String : AnyObject?] = ["foo" : UIViewController()]
dictionary.someMethod()

返回

foo = Optional(<UIViewController: 0x7fee7e819870>) cannot be cast to `String`