swift - Swift语言中的switch语句，其中有where的case子句中的执行顺序是什么？

Question

假设我们有以下伪代码片段:

switch some_variable {
    case let v where <condition_checking>:
        do_something...
}

据我了解，当代码执行进入 switch 时，它首先执行第一个 case 语句(我们只有一个)。然后它检查 condition_checking 部分，如果它是真的，那么 let 部分将被执行并且 do_something 将有机会运行。对吗？

我问这个问题是因为我在 Apple 文档中看到了以下代码片段 https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/ControlFlow.html (页面的最后一部分):

let finalSquare = 25
var board = [Int](count: finalSquare + 1, repeatedValue: 0)
board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02
board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08
var square = 0
var diceRoll = 0

gameLoop: while square != finalSquare {
    if ++diceRoll == 7 { diceRoll = 1 }
    switch square + diceRoll {
    case finalSquare:
        // diceRoll will move us to the final square, so the game is over
        break gameLoop
    case let newSquare where newSquare > finalSquare:
        // diceRoll will move us beyond the final square, so roll again
        continue gameLoop
    default:
        // this is a valid move, so find out its effect
        square += diceRoll
        square += board[square]
    }
}
println("Game over!")

注意这个声明case let newSquare where newSquare > finalSquare: , newSquare由 let 定义在这个case陈述。 where正在使用 newSquare直接，似乎let部分首先执行，然后 where部分。这不是我对它的理解。谁能帮忙澄清一下？

Answer 1

As I understand, when code execution goes into switch, it first goes with the first case statement (we only have one). Then it checks the condition_checking part and if it's true, then the let part will be execute and do_something will have a chance to run. Is that correct?

我想你误会了。在这段代码中:

case let newSquare where newSquare > finalSquare:

执行顺序为:

• 将 newSquare 绑定(bind)到 square + diceRoll 的值。
• 评估 newSquare > finalSquare
• 如果为真，则执行带有 newSquare 绑定(bind)的 block

这一行:

case finalSquare:

可以被认为是以下内容的简写:

case let __temp where __temp == finalSquare:

(没有创建真正的 __temp 符号)