URL Verification Publisher

Question

I'm trying to write a URL Validator in Swift w/ Combine and having a SwiftUI view subscribe to it. It seems to work fine in the simulator but crashes on my dev phone (running 13.1).

Scenario... User types into a UITextField which is connected to the urlString var in the view model. When that changes I clean the string, create a URL, and then do a HEAD test with URLSession. This all works in the sim, but on tap of the text field on the device it crashes the app and I'm not getting any good stack traces. Any ideas?

static func testURLPublisher(string: String) -> AnyPublisher<URL?, Never> {

        let validatedURL = try? validateURL(string: string)

        guard let urlToCheck = validatedURL else {
            return Just(nil).eraseToAnyPublisher()
        }

        var request = URLRequest(url: urlToCheck)
        request.httpMethod = "HEAD"

        let publisher = URLSession.shared.dataTaskPublisher(for: request)
            .handleEvents(receiveSubscription: { _ in
                networkActivityPublisher.send(true)
            }, receiveCompletion: { _ in
                networkActivityPublisher.send(false)
            }, receiveCancel: {
                networkActivityPublisher.send(false)
            })
            .tryMap { data, response -> URL? in
                // URL Responded - Check Status Code
                guard let urlResponse = response as? HTTPURLResponse, ((urlResponse.statusCode >= 200 && urlResponse.statusCode < 400) || urlResponse.statusCode == 405) else {
                        throw URLValidatorError.serverError("Could not find the a servr at: \(urlToCheck)")
                }
                        return urlResponse.url?.absoluteURL
            }
        .catch { err in
            return Just(nil)
        }
        .eraseToAnyPublisher()
        return publisher
    }

The view that is using it looks like this...

class NewSiteViewModel: ObservableObject {

    @Published var validatedURL: URL?

    //@Published var secretKey: String?
    @Published var urlString: String = ""

    @Published var isValidURL: Bool = false

    private var cancellable = Set<AnyCancellable>()

    init() {
        $urlString
        .dropFirst(1)
            .throttle(for: 0.5, scheduler: DispatchQueue(label: "Validator"), latest: true)
            .removeDuplicates()
            .compactMap { string -> AnyPublisher<URL?, Never> in
                return URL.testURLPublisher(string: string)
            }
            .switchToLatest()
            .receive(on: RunLoop.main)
            .sink { recievedURL in
                guard let url = recievedURL else {
                    self.validatedURL = nil
                    self.isValidURL = false
                    return
                }
                self.validatedURL = url
                self.isValidURL = true

            }
            .store(in: &cancellable)
    }
}

Answer 1

13.1beta4 does not implement .throttle like the earlier implementation in 13.0. Now giving DispatchQueue("..") as an argument crashes on that line without any usable error text. One argument that works is RunLoop.main.

It works over here. You probably have to add Allow arbitrary Loads as described in Transport security has blocked a cleartext HTTP to your info.plist. It might not be enforced in Simulator, but it definitely is on devices.

You can leave out the NSExceptionDomains-part.

Example with working .throttle argument

import SwiftUI
import Combine

struct URLTesterView: View {
    @ObservedObject var model = NewSiteViewModel()

    @State var networkActivity = false

    var body: some View {
        VStack{
            TextField("url string", text: $model.urlString)
            Text("Is valid: \(model.isValidURL ? "true" : "false")")
            Text("Validated URL: \(model.validatedURL?.absoluteString ?? "")")
            Text("Network activity: \(networkActivity ? "true" : "false")")
        }.onReceive(networkActivityPublisher
            .receive(on: DispatchQueue.main)) {
            self.networkActivity = $0
        }
    }
}

class NewSiteViewModel: ObservableObject {

    @Published var validatedURL: URL?

    //@Published var secretKey: String?
    @Published var urlString: String = ""

    @Published var isValidURL: Bool = false

    private var cancellable = Set<AnyCancellable>()

    init() {
        $urlString
        .dropFirst(1)
            .throttle(for: 0.5, scheduler: RunLoop.main, latest: true)
            .removeDuplicates()
            .compactMap { string -> AnyPublisher<URL?, Never> in
                return URL.testURLPublisher(string: string)
            }
            .switchToLatest()
            .receive(on: RunLoop.main)
            .sink { recievedURL in
                guard let url = recievedURL else {
                    self.validatedURL = nil
                    self.isValidURL = false
                    return
                }
                self.validatedURL = url
                self.isValidURL = true
            }
            .store(in: &cancellable)
    }
}

func validateURL(string: String) throws -> URL {
    guard let url = URL(string: string) else {
        throw URLValidatorError.urlIsInvalid(string)
    }
    return url
}

let networkActivityPublisher = PassthroughSubject<Bool, Never>()

enum URLValidatorError: Error {
    case serverError(_ string: String)
    case urlIsInvalid(_ string: String)
}

extension URL {
    static func testURLPublisher(string: String) -> AnyPublisher<URL?, Never> {

        let validatedURL = try? validateURL(string: string)

        guard let urlToCheck = validatedURL else {
            return Just(nil).eraseToAnyPublisher()
        }

        var request = URLRequest(url: urlToCheck)
        request.httpMethod = "HEAD"

        let publisher = URLSession.shared.dataTaskPublisher(for: request)
            .handleEvents(receiveSubscription: { _ in
                networkActivityPublisher.send(true)
            }, receiveCompletion: { _ in
                networkActivityPublisher.send(false)
            }, receiveCancel: {
                networkActivityPublisher.send(false)
            })
            .tryMap { data, response -> URL? in
                // URL Responded - Check Status Code
                guard let urlResponse = response as? HTTPURLResponse, ((urlResponse.statusCode >= 200 && urlResponse.statusCode < 400) || urlResponse.statusCode == 405) else {
                        throw URLValidatorError.serverError("Could not find the a servr at: \(urlToCheck)")
                }
                        return urlResponse.url?.absoluteURL
            }
        .catch { err in
            return Just(nil)
        }
        .eraseToAnyPublisher()
        return publisher
    }
}