在官方的《Swift编程语言》指南中，它是关于开关的情况的：“ ...如果情况包含与控件表达式匹配的多个模式，则这些模式均不能包含常量或变量绑定。”
包含多个模式是什么意思？
这意味着具有多个模式的案例标签无法声明变量。
这是允许的：
let somePoint = (1, 1)
switch somePoint {
// Case with multiple patterns without binding
case (0, _),
(_, 0):
println("(\(somePoint.0), \(somePoint.1)) is on an axis")
default:
println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}
这也是允许的：
let somePoint = (1, 1)
switch somePoint {
// Case with single patterns with binding
case (0, let y):
println("(0, \(y)) is on an axis")
case (let x, 0):
println("(\(x), 0) is on an axis")
default:
println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}
但是，这是禁止的：
let somePoint = (1, 1)
switch somePoint {
// Case with multiple patterns that have bindings
case (0, let y),
(let x, 0):
println("(\(x), \(y)) is on an axis")
default:
println("(\(somePoint.0), \(somePoint.1)) is not of an axis")
}
上面产生一个错误：
error: 'case' labels with multiple patterns cannot declare variables
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