Connecting to a faulty url, does not return an error

[MustaphaQadri]

Describe the bug

Connecting to a server while it is down, mistyped url, or no internet connection does not result in an error. The WebSocket does not emit any event to inform its delegate that the connection failed. I found that urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) is not implemented in NativeEngine. This is the place where those errors are fired.

Steps to Reproduce

Detailed steps to reproduce the problematic behavior:

        // not an existing url
        var request = URLRequest(url: URL(string: "wss://socket.test.k2.nl/match/2xj5crhn5vhy6vmys0stkqi3s8")!)
        request.setValue(url.host, forHTTPHeaderField: "Host")
        webSocket = WebSocket(request: request)
        
        webSocket.onEvent = { event in
            print(event)
        }
        
        webSocket.connect()

• run this code and break on print(event) this will not be triggered
• disabling internet connection and connecting to a valid url will also not trigger an event

Expected behavior

Connecting to an unreachable server should trigger an error event.

Environment:

• OS/Version: iOS 14.4
• Starscream Version: 4.0.4
• Xcode version: 12.5

Additional context

Add any other context about the problem here.

[StartingZhou]

We also encounter this problem，I found the Starscream use NWConnection，when use unreachable URL, the NWConnection does not call it's handler, below code in WebSocket.swift:

public convenience init(request: URLRequest, certPinner: CertificatePinning? = FoundationSecurity(), compressionHandler: CompressionHandler? = nil, useCustomEngine: Bool = true) {
        if #available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *), !useCustomEngine {
            self.init(request: request, engine: NativeEngine())
        } else if #available(macOS 10.14, iOS 12.0, watchOS 5.0, tvOS 12.0, *) {
            self.init(request: request, engine: WSEngine(transport: TCPTransport(), certPinner: certPinner, compressionHandler: compressionHandler))
        } else {
            self.init(request: request, engine: WSEngine(transport: FoundationTransport(), certPinner: certPinner, compressionHandler: compressionHandler))
        }
    }

The class TCPTransport use NWConnection for data transport, so just delete the else if as whole, or change TCPTransport to FoundationTransport

[eric-clover]

I'm also seeing this issue. The fix in #821 fixes this for me and allows me to gracefully connect/reconnect when the server comes back online.