如何限制并发动URLSessions与结合？

Question 1

我有很多(约200)的网址图像，我需要下载的每一项，然后处理(调整)，然后更新的高速缓存。事实是-我只是想有时max3请求在一次，并且由于图像是沉重的，我也不想一个很大的反应"挂"等待处理的(而采取的记忆...).

TLDR我要叫下一次(第4次)网络的的请求后，才 receiveValue 在 sink 被称为上一个第3请求...(即在该网络的响应和处理的都做了...).

将这种流动的工作，并将就举行对等待的网址并没有把它们放在地板上?

还做我需要那个 buffer() 打电话？我用它看到这个回答： https://stackoverflow.com/a/67011837/2242359

wayTooManyURLsToHandleAtOnce // this is a `[URL]`
    .publisher
    .buffer(size: .max, prefetch: .byRequest, whenFull: .dropNewest) // NEEDED?
    .flatMap(maxPublishers: .max(3)) { url in
       URLSession.shared
           .dataTaskPublisher(for: url)
           .map { (data: Data, _) -> Picture in
               Picture(from: data)
           }
    }
    .tryCompactMap {
        resizeImage(picture: $0) // takes a while and might fail
    }
    .receive(on: DispatchQueue.main)
    .sink { completion
        // handling completion... 
    } receiveValue: { resizedImage
        self.cache.append(resizedImage)
    }
    .store(...)

Question 2

我会用一个主题。这不是一个最佳的解决方案，但它看起来工作和可能会触发其他的想法

var cancellable: AnyCancellable?

var urls: [String] = (0...6).map { _ in "http://httpbin.org/delay/" + String((0...2).randomElement()!) }

var subject: PassthroughSubject<[String], Never> = .init()

let maxConcurrentRequests = 3

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    
    print(urls)
    
    cancellable = subject
        .flatMap({ urls -> AnyPublisher<[URLSession.DataTaskPublisher.Output], URLError> in
            let requests = urls.map { URLSession.shared.dataTaskPublisher(for: URL.init(string: $0)!) }
            return Publishers.MergeMany(requests)
                .collect().eraseToAnyPublisher()
        })
        .print()
        .sink(receiveCompletion: { completion in
            print(completion)
        }, receiveValue: { value in
            print(value)
            if self.urls.count <= self.maxConcurrentRequests {
                self.urls.removeAll()
                self.subject.send(completion: .finished)
            } else {
                self.urls.removeLast(self.maxConcurrentRequests)
                self.subject.send(self.urls.suffix(self.maxConcurrentRequests))
            }
        })
    
    subject.send(urls.suffix(maxConcurrentRequests))
}

Blazej SLEBODA · Answer 1 · 2021-11-24T11:30:11

我会用一个主题。这不是一个最佳的解决方案，但它看起来工作和可能会触发其他的想法

var cancellable: AnyCancellable?

var urls: [String] = (0...6).map { _ in "http://httpbin.org/delay/" + String((0...2).randomElement()!) }

var subject: PassthroughSubject<[String], Never> = .init()

let maxConcurrentRequests = 3

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    
    print(urls)
    
    cancellable = subject
        .flatMap({ urls -> AnyPublisher<[URLSession.DataTaskPublisher.Output], URLError> in
            let requests = urls.map { URLSession.shared.dataTaskPublisher(for: URL.init(string: $0)!) }
            return Publishers.MergeMany(requests)
                .collect().eraseToAnyPublisher()
        })
        .print()
        .sink(receiveCompletion: { completion in
            print(completion)
        }, receiveValue: { value in
            print(value)
            if self.urls.count <= self.maxConcurrentRequests {
                self.urls.removeAll()
                self.subject.send(completion: .finished)
            } else {
                self.urls.removeLast(self.maxConcurrentRequests)
                self.subject.send(self.urls.suffix(self.maxConcurrentRequests))
            }
        })
    
    subject.send(urls.suffix(maxConcurrentRequests))
}

不会叫 self.subject.send(completion: .finished) 在沉结束我的订阅永远吗？ (即忽略未来的价值观中发现)
@AvielGross是的,它不会。我理解你收集的网址创建一次每图控制本/解雇。如果它不是真的那么不发送"。完成了"但是，你有一个触发器就像didSet网址上列或另一重新启动与问题。发送后排空和再填充阵列。

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