Execute `APIKit.Session.send(_:callbackQueue:handler:)` synchronously.

SendSynchronously.swift

import APIKit
import Result

extension APIKit.Session {
    
    class func sendSynchronous<Request>(_ request: Request) -> Result<Request.Response, APIKit.SessionTaskError> where Request: APIKit.Request {
        return shared.sendSynchronous(request)
    }
    
    /// Execute `send(_:callbackQueue:handler:)` synchronously.
    /// This method should be called on non-main thread.
    func sendSynchronous<Request>(_ request: Request) -> Result<Request.Response, APIKit.SessionTaskError> where Request: APIKit.Request {
        guard !Thread.isMainThread else { fatalError("'\(#function)' should be called on non-main thread.") }
        var result: Result<Request.Response, SessionTaskError>? = nil
        let semaphore = DispatchSemaphore(value: 0)
        send(request) {
            result = $0
            semaphore.signal()
        }
        _ = semaphore.wait(timeout: .distantFuture)
        return result!
    }

}

DispatchQueue(label: "FooBar").async {

    let fooResult = Session.sendSynchronous(FooRequest())
    switch result {
    case .success(let response):
        print(response)
    case .failure(let error):
        print(error)
    }

    let barResult = Session.sendSynchronous(BarRequest())
    switch result {
    case .success(let response):
        print(response)
    case .failure(let error):
        print(error)
    }

}