To inject, mock, spy and stub DispatchQueue (sync/async).

AsyncDispatcher.swift

import Foundation

protocol AsyncDispatcher {
    func async(
        group: DispatchGroup?,
        qos: DispatchQoS,
        flags: DispatchWorkItemFlags,
        execute work: @escaping @convention(block) () -> Void
    )
    func async(execute work: @escaping @convention(block) () -> Void)
}

extension AsyncDispatcher {
    func async(execute work: @escaping @convention(block) () -> Void) {
        async(group: nil, qos: .unspecified, flags: [], execute: work)
    }
}

extension DispatchQueue: AsyncDispatcher { }

AsyncDispatcherSpy.swift

final class AsyncDispatcherSpy: AsyncDispatcher {
    
    var asyncCalled = false
    var asyncAfterCalled = false
    
    func async(
        group: DispatchGroup?,
               qos: DispatchQoS,
               flags: DispatchWorkItemFlags,
               execute work: @escaping @convention(block) () -> Void
    ) {
        asyncCalled = true
        work()
    }
}

SyncDispatcher.swift

protocol SyncDispatcher {
    func sync<T>(execute work: () throws -> T) rethrows -> T
}

extension DispatchQueue: SyncDispatcher { }