合并:发布者有时会失去价值并完成 [英] Combine: Publisher sometimes loses value and completes

问题描述

我有一个简单的 Deferred Publisher ，它可以从磁盘读取数据，并在 SwiftUI列表中显示数据， Publisher 效果最佳有时，但是 有时 表现不佳，只是失去了它的值(这是 Model 对象的数组)并完成了完成消息.我已经尝试过提及

如果我在使用者函数 readData()中的 sink 之前添加任何下游发布者，例如 combineLatest ，将异步发布者(readFromBundle)与同步发布者( combineLatest )链接在一起，将导致该值根本无法在 iOS 13.3 + 设备上交付，有时会在解决方案

看起来像是赛车类问题，请尝试以下操作(仅通过阅读代码即可)

1)明确使用后台队列

  private let readQueue = DispatchQueue(label:"ReadQueue"，qos:.background，属性:.concurrent) 

2)将发布程序安排在此队列中，而不是在其上接收

  .subscribe(on:self.readQueue) 

I have a simple Deferred Publisher that reads data from disk and I display the data in a SwiftUI List, the Publisher works well most of the time, but sometimes it doesn't behave well, it just loses its value (which's an array of Model objects) and completes with finished message. I've tried a workaround mentioned here to use the buffer operator to keep the value in buffer because I believe the Combine's Publisher by design won't pass the data downstream if there is no demand requested by subscribers and hence dropping this data and completes, however using buffer didn't solve the issue.

The code I have:

enum FileError: Error {
    case someError
}

class ViewModel: ObservableObject {
    @Published var modelArray = [Model]()
    private var subscriptions = Set<AnyCancellable>()
    func readData() {
        DataSource()
            .readFromBundle(resource: "Sample", type: "json")
            .receive(on: DispatchQueue.main)
            .sink(receiveCompletion: { completion in
                print("Completion: \(completion)")
            }) { array in
                self.modelArray = array
        }.store(in: &subscriptions)
    }
}
struct ContentView: View {
    @ObservedObject var viewModel: ViewModel

    var body: some View {
        VStack {
            List(self.viewModel.modelArray) { model in
                Text("\(model.name)")
            }
        }
        .onAppear {
            self.viewModel.readData()
        }
    }
}

struct Model: Codable, Identifiable {
    var id: Int
    var name: String
}

class DataSource {
    private let readQueue = DispatchQueue(label: "ReadQueue", qos: .default, attributes: .concurrent)

    func readFromBundle (resource: String, type:String) -> AnyPublisher<[Model], FileError> {
            Deferred {
                 Future { promise in
                    guard let url = Bundle.main.url(forResource: "Sample", withExtension: "json"),
                      let data = try? Data(contentsOf: url),
                      let modelArray = try? JSONDecoder().decode([Model].self, from: data)
                      else {
                        promise(.failure(.someError))
                        return
                    }
                      promise(.success(modelArray))
                }
            }
           .receive(on: self.readQueue)
           .eraseToAnyPublisher()
        }
}

This is a link to download a working sample project.

EDIT:

Environment: Xcode 11.3.1, iOS 13.3 iPhone 11 Pro Max simulator and device.

gif screenshot (notice the console output)

EDIT2:

if I add any downstream publishers, like combineLatest for example just before sink in the consumer function readData() then a new behavior introduced, which's chaining an async publisher (readFromBundle) with a sync publisher (combineLatest) will result in the value will not deliver at all on iOS 13.3+ devices and will sometimes deliver on devices below iOS 13.3, as stated on this link.

解决方案

It looks like racing-kind issue, please try the following (just by code-reading)

1) use background queue explicitly

private let readQueue = DispatchQueue(label: "ReadQueue", qos: .background, 
    attributes: .concurrent)

2) schedule Publisher on this queue instead of receiving on it

.subscribe(on: self.readQueue)

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