A source code on how to implement DispatchQueue mock for unit testing in the iOS app

MockDispatchQueue.swift

// A protocol for mocking DispatchQueue instance
protocol DispatchQueueType {
    func async(execute work: @escaping @convention(block) () -> Void)
}

// Confirming existing DispatchQueue to DispatchQueueType
extension DispatchQueue: DispatchQueueType {
    func async(execute work: @escaping @convention(block) () -> Void) {
        async(group: nil, qos: .unspecified, flags: [], execute: work)
    }
}

// Mocking dispatch queue for injecting into view model during unit test run
final class MockDispatchQueue: DispatchQueueType {
    func async(execute work: @escaping @convention(block) () -> Void) {
        work()
    }
}