DispatchQueue extension for situations where one needs to run an expensive operation in background and then use its result in the main thread

DispatchQueue+Extensions.swift

import Foundation

extension DispatchQueue {
    func asyncConsumeInMainQueue<T>(
        work: @escaping () throws -> T,
        mainSuccess: @escaping (T) -> Void,
        mainError: @escaping (Error) -> Void) {
        async {
            do {
                let result = try work()
                
                DispatchQueue.main.async {
                    mainSuccess(result)
                }
            } catch {
                DispatchQueue.main.async {
                    mainError(error)
                }
            }
        }
    }
    
    func asyncConsumeInMainQueue<T>(
        delay: TimeInterval,
        work: @escaping () throws -> T,
        mainSuccess: @escaping (T) -> Void,
        mainError: @escaping (Error) -> Void) {
        asyncAfter(deadline: .now() + delay) {
            do {
                let result = try work()
                
                DispatchQueue.main.async {
                    mainSuccess(result)
                }
            } catch {
                DispatchQueue.main.async {
                    mainError(error)
                }
            }
        }
    }
}

DispatchQueue.global(qos: .background).asyncConsumeInMainQueue(work: { () -> UIImage in
    return try fetchImage(url: "https://example.com/image.jpeg")
}, mainSuccess: { [weak self] (image) in
    self?.imageView.image = image
}) { (error) in
    handleError(error)
}