访问 combine-operator-chain 中使用的值

Question

我想知道是否可以访问组合运算符链中的值，这些值将在链中进一步使用。

例如，如果您有一个字符串数组，并且您为每个字符串下载了一个资源，是否可以访问该字符串本身？

也许一个伪代码示例将有助于更好地理解：

let strings = ["a", "b", "c"]

strings.publisher
    .compactMap{ str in
        URL(string: "https://someresource/\(str)")
    }
    .flatMap { url  in
        URLSession.shared.dataTaskPublisher(for: url)
    }
    .map { [=10=].data}
    .map { data in
        // here I would need the "str" string from above
    }

非常感谢帮助

谢谢，格雷戈尔

Answer 1

杰克写下并删除的答案是正确的。我不知道他为什么删除它；也许他觉得他不能用示例代码来支持它。但答案是完全正确的。如果稍后在管道中需要初始 str 值，则由 you 将其传递到每个步骤。您通常通过在每个阶段传递值的元组来做到这一点，以便字符串使其在链中足够远以便被检索。这是 Combine 编程中非常的常用策略。

举个简单的例子，看一下本文中间部分的Combine代码：

https://www.biteinteractive.com/swift-5-5-asynchronous-looping-with-async-await/

正如我在文章中所说：

You’ll observe that, as opposed to GCD where local variables just magically “fall through” to nested completion handlers at a lower level of scope, every step in a Combine pipeline must explicitly pass down all information that may be needed by a later step. This can result in some rather ungainly values working their way down the pipeline, often in the form of a tuple, as I’ve illustrated here.

But I don’t regard that as a problem. On the contrary, being explicit about what’s passing down the pipeline seems to me to be a gain in clarity.

为了进一步说明，这里重写了您的伪代码；这是真实的代码，你可以运行试试看：

class ViewController: UIViewController {
    var storage = Set<AnyCancellable>()
    override func viewDidLoad() {
        super.viewDidLoad()
        let strings = ["a", "b", "c"]
        let pipeline = strings.publisher
            .map { str -> (String, URL) in
                let url = URL(string: "https://www.apeth.com/pep/manny.jpg")!
                return (str, url)
            }
            .flatMap { (str, url) -> AnyPublisher<(String, (data: Data, response: URLResponse)), URLError> in
                let sess = URLSession.shared.dataTaskPublisher(for: url)
                    .eraseToAnyPublisher()
                let just = Just(str).setFailureType(to: URLError.self)
                    .eraseToAnyPublisher()
                let zip = just.zip(sess).eraseToAnyPublisher()
                return zip
            }
            .map { (str, result) -> (String, Data) in
                (str, result.data)
            }
            .sink { comp in print(comp) } receiveValue: { (str, data) in print (str, data) }
        pipeline.store(in: &storage)
    }
}

这不是表达管道的唯一方法，但它确实有效，而且应该为您提供一个起点。

访问 combine-operator-chain 中使用的值

Access used values in a combine-operator-chain

swift

combine