ios - Swift 中的可选项，为什么这个简单的代码不打印可选项？

Question

var shoppingList = ["item1": "bread", "item2": "milk" ]

if let oldItem = shoppingList.updateValue("honey", forKey: "item2") {
    println("old value was \(oldItem)")
}

// This prints out "old value was milk"

但是，如果我这样做

var shoppingList = ["item1": "bread", "item2": "milk" ]

let oldItem = shoppingList.updateValue("honey", forKey: "item2")
println("old value was \(oldItem)")
// This would print out "old value was Optional("milk")

if oldItem != nil {
   println("old value was \(oldItem)")
}

// And this prints out the same "old value was Optional("milk")

为什么会出现在这里，而不是出现在第一个示例的 if 语句中？

注意:我正在 playground 版本 6.1.1 (6A2008a) 中对此进行测试。

Answer 1

var shoppingList = ["item1": "bread", "item2": "milk" ]

if let oldItem = shoppingList.updateValue("honey", forKey: "item2") {
    println("old value was \(oldItem)")
}

// This prints out "old value was milk"

因为此代码(if let 语句)解包了键“item2”的值，并将解包后的值存储在 oldItem 中。

如果你只是像这样打印它:

let oldItem = shoppingList.updateValue("honey", forKey: "item2")
println("old value was \(oldItem)")

它将打印Optional("old value was milk")

但是如果键 item2 没有任何值，那么程序就会崩溃。