Swift 中的装饰器

Question

我是 Swift 的新手，我想知道该语言是否有一些等同于 Python 的装饰器模式。
例如：

import functools


def announce(func):
    """Print a function's arguments and return value as it's called."""
    @functools.wraps(func)
    def announced_func(*args, **kwargs):
        rv = func(*args, **kwargs)
        print('In: {0}, {1}'.format(args, kwargs))
        print('Out: {}'.format(rv))
        return rv
    return announced_func


@announce  # add = announce(add)
def add(a, b):
    return a + b

add(2, 5)
# In: (2, 5), {}
# Out: 7
# 7

也许我还没有找到它，但 Swift 似乎没有办法将任意参数转发给函数或保留包装函数的信息（如 functools.wraps ).

是否有等效项，或者该模式不打算在 Swift 中使用？

Answer 1

你可以使用这个：

func decorate<T, U>(_ function: @escaping (T) -> U, decoration: @escaping (T, U) -> U) -> (T) -> U {
    return { args in
        decoration(args, function(args))
    }
}

let add: (Int, Int) -> Int = decorate(+) { args, rv in
    print("In: \(args)")
    print("Out: \(rv)")
    return rv
}

add(2, 5) // In: (2, 5)\nOut: 7

或announce作为函数而不是闭包，允许重用：

func announce<T, U>(input args: T, output rv: U) -> U {
    print("In: \(args)")
    print("Out: \(rv)")
    return rv
}

let add: (Int, Int) -> Int = decorate(+, decoration: announce)

add(2, 5) // In: (2, 5)\nOut: 7

let length = decorate({(str: String) in str.characters.count }, decoration: announce)
length("Hello world!") // In: Hello world!\nOut: 12

Answer 2

上面的示例更新为 Swift 4+。要求参数形式上是元组。我还添加了一个装饰字符串的示例。

func decorate<T, U>(_ function: @escaping (T) -> U,
                    decoration: @escaping (T, U) -> U) -> (T) -> U {
    return { args in
        decoration(args, function(args))
    }
}

let add: ((Int, Int)) -> Int = decorate(+) {args, rv in
    print("In: \(args)")
    print("Out: \(rv)")
    return rv
}
add((2, 5)) // In: (2, 5)\nOut: 7

func announce<T, U>(input args: T, output rv: U) -> U {
    print("In: \(args)")
    print("Out: \(rv)")
    return rv
}

let multiply: ((Int, Int)) -> Int = decorate(*, decoration: announce)
multiply((2, 5)) // In: (2, 5)\nOut: 10

let length = decorate( { (str: String) in str.count }, decoration: announce)
length("Hello world!") // In: Hello world!\nOut: 12

let greet: ((String)) -> String = decorate( {"Hello, \([=10=])" }, decoration: announce)
greet(("Paul")) // In: Paul\nOut: Hello, Paul

Answer 3

以上示例更新为 Swift 5.1+

@propertyWrapper
struct Announced<T, U> {
    private let announcedFunction: (T) -> U
    var wrappedValue: (T) -> U { announcedFunction }
    
    init(wrappedValue: @escaping (T) -> U) {
        announcedFunction = { args in
            let rv = wrappedValue(args)
            print("In: \(args)")
            print("Out: \(rv)")
            return rv
        }
    }
}

struct Test {
    @Announced static var add: ((Int, Int)) -> Int = { [=10=].0 + [=10=].1 }
    @Announced static var multiply: ((Int, Int)) -> Int = { [=10=].0 * [=10=].1 }
    @Announced static var length : (String) -> Int = { (str: String) in str.count }
    @Announced static var greet: (String) -> String = { "Hello, \([=10=])" }
}

Test.add((2, 5)) // In: (2, 5)\n Out: 7
Test.multiply((2, 5)) // In: (2, 5)\n Out: 10
Test.length("Hello world!") // In: Hello world!\n Out: 12
Test.greet("Paul") // In: Paul\n Out: Hello, Paul

[编辑]其实可以简化一下：

@propertyWrapper
struct Announced<T, U> {
    let wrappedValue: (T) -> U

    init(wrappedValue: @escaping (T) -> U) {
        self.wrappedValue = { args in
            let rv = wrappedValue(args)
            print("In: \(args)")
            print("Out: \(rv)")
            return rv
        }
    }
}

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