取消订阅结合框架

Question

当应用程序在Combine 框架中进入后台时，我们有没有办法取消dataTaskPublisher？

Answer 1

Just send .cancel() to subscriber. Here is a demo class

class Downloader: ObservableObject{
    var cancellable: AnyCancellable?
    let url: URL

    init(url: URL){
        self.url = url
    }

    func download(){
        cancellable = URLSession.shared.dataTaskPublisher(for: url)
        // ... other operators code
    }
    
    func cancel() {
        self.cancellable?.cancel()
        self.cancellable = nil
    }
}

Answer 2

If you want to embed the cancellation behavior right into the Combine publisher operator chain then prefix(untilOutputFrom:) can be helpful:

let url: URL = ...
cancellable = URLSession.shared.dataTaskPublisher(for: url)
    .prefix(untilOutputFrom: NotificationCenter.default.publisher(for: UIApplication.willResignActiveNotification))
    .sink(receiveCompletion: {
        print($0)
    },receiveValue: {
        print($0)
    })

This starts the the data task but then cancels it if UIApplication.willResignActiveNotification is triggered. I don't see many people using this operator, probably because it is poorly named, but it is a common pattern in other reactive frameworks such as ReactiveSwift where the operator is more sensibly named take(until:) .