总结swift中模式匹配的用法

看了一些介绍pattern matching的文章，里面有不少种使用用法。总结下来就是两种类型：绑定和判断。模式匹配的起手式是case，这是对传统switch-case的扩展。具体用法如下：

1. 「绑定」即let，它表示匹配的数据绑定到变量上。形式是case let (aVar)或case (let aVar)
   a. 对变量可以添加where约束条件
   b. _ 可以匹配任意数据，但是无法使用
   c. case let aVar? 用于匹配可选类型的.some值
   d. case let .some(aVar) 用于匹配枚举中的一个枚举值
2. 「判断」相当于 ==，它只有true/false结果没有绑定的变量
   a. 一种方式是 case is aType
   b. 一种方式是case (aVar...bVar) = value，相当于range的contains方法
3. ~= 是模式匹配操作符可以通过重载实现自定义模式匹配。系统库提供了range的~=匹配实现

下面是测试用例:

/** 测试数据 */
private let arr = [(1, 1), (2, 4), nil, (1, 3), nil, (2, 2)]

private func optionalMatch() {
    for case let item? in arr {
        print("optionalMatch: \(item)")
    }
}

private func ifMatch() {
    for (idx, item) in arr.enumerated() {
        // if nil = item
        if case let .none = item {
            print("ifMatch-optional nil at \(idx)")
        } else if case let (1, item)? = item {
            print("ifMatch-enum item \(item)")
        }
    }
}

private func tupleMatch() {
    for case let (2, item)? in arr {
        if case (_, 2) = (2, item) {
            print("tupleMatch-case: \(item)")
        } else {
            print("tupleMatch-for: \(item)")
        }
    }
}

private func whereMatch() {
    for case let (a, b)? in arr where a == 1 && b > 1 {
        print("whereMatch where 1: \(b)")
    }
}

private func rangeMatch() {
    for case let item? in arr {
        if case (2..<3, 3...4) = item {
            print("rangeMatch-case, \(item)")
        } else if (3...5) ~= item.1 {
            // if (3...5).contains(item.)
            print("rangeMatch ~=, \(item)")
        }
    }
}

private func switchMatch() {
    for item in arr {
        switch item {
        case nil:
            print("switchMatch nil")
        case let (2, b)?:
            let subA: Any = b
            switch subA {
            case let b as Int where b == 4:
                print("switchMatch as Type \(item)")
            case is Int:
                print("switchMatch is Type \(item)")
            default:
                continue
            }
        default:
            print("switchMatch default \(item)")
        }
    }
}

/** 自定义匹配方法 */
func ~= (pattern: String, value: Int) -> Bool {
    return pattern == "\(value)"
}

private func customMatch() {
    for item in arr.compactMap({$0}) {
        switch item {
        case ("1", "3"):
            print("customMatch \(item)")
        default:
            continue
        }
    }
}

private let printResult: (() -> ()) -> () = { action in
    print()
    action()
    print()
    print("==============================")
}

/** 入口方法 */
func runMatchPatternsDemo() {
    printResult(optionalMatch)
    printResult(ifMatch)
    printResult(tupleMatch)
    printResult(whereMatch)
    printResult(rangeMatch)
    printResult(switchMatch)
    printResult(customMatch)
}

运行结果如下：

optionalMatch: (1, 1)
optionalMatch: (2, 4)
optionalMatch: (1, 3)
optionalMatch: (2, 2)

==============================

ifMatch-enum item 1
ifMatch-optional nil at 2
ifMatch-enum item 3
ifMatch-optional nil at 4

==============================

tupleMatch-for: 4
tupleMatch-case: 2

==============================

whereMatch where 1: 3

==============================

rangeMatch-case, (2, 4)
rangeMatch ~=, (1, 3)

==============================

switchMatch default Optional((1, 1))
switchMatch as Type Optional((2, 4))
switchMatch nil
switchMatch default Optional((1, 3))
switchMatch nil
switchMatch is Type Optional((2, 2))

==============================

customMatch (1, 3)

==============================