Swift中where的使用场景

Swift中可以使用where对模式匹配增加进一步的匹配条件。常见的5种使用场景：swiftch语句、for循环语句、协议中关联类型的约束、函数的适用条件、扩展中。

func testWhere() {
    // 1.
    var data = (20, "dandy")
    switch data {
    case let(age, _) where age > 10: // where给case增加条件
        print(data.1, "age > 10")
    default: break
    }
    // 2.
    var ages = [10, 20, 30, 40]
    for age in ages where age > 30 {
        print(age)
    }// 40
    
}

// 3.
protocol Stackable { associatedtype Element }
protocol Container {
    associatedtype Stack: Stackable where Stack.Element: Equatable // 关联类型的元素要遵守Equatable协议
}

// 4.
func equal<S1: Stackable, S2: Stackable>(_ s1: S1, s2: S2) -> Bool
    where S1.Element == S2.Element, S1.Element: Hashable {
        return false
}

//5
extension Container where Self.Stack.Element: Hashable {}