Bob Godwin bobgodwinx

libdispatch efficiency tips

The libdispatch is one of the most misused API due to the way it was presented to us when it was introduced and for many years after that, and due to the confusing documentation and API. This page is a compilation of important things to know if you're going to use this library. Many references are available at the end of this document pointing to comments from Apple's very own libdispatch maintainer (Pierre Habouzit).

My take-aways are:

You should create very few, long-lived, well-defined queues. These queues should be seen as execution contexts in your program (gui, background work, ...) that benefit from executing in parallel. An important thing to note is that if these queues are all active at once, you will get as many threads running. In most apps, you probably do not need to create more than 3 or 4 queues.
Go serial first, and as you find performance bottle necks, measure why, and if concurrency helps, apply with care, always validating under system pressure. Reuse

	import UIKit
	import Foundation
	import CoreImage
	import MetalPerformanceShaders
	// MARK: - Filter Types

	enum FilterType {
	case clarendon
	case gingham
	case juno

	/// `ViewController`
	class ViewController: UIViewController {
	private let bag = DisposeBag()
	private let indicator = PublishRelay<RxMBProgressHUD.State>()
	private let viewModel: ViewModelType

	init(_ viewModel: ViewModelType = ViewModel()) {
	self.viewModel = viewModel
	super.init(nibName: nil, bundle: nil)
	configure()

	/// `ViewModel`
	class ViewModel: ViewModelType {
	let source: Driver<UIImage?>
	let finished: Completable

	init(_ model: ModelType = Model()) {
	self.source = model.datasource
	.map { UIImage.gif(data: $0.data) }
	.asDriver(onErrorJustReturn: nil)

	/// `Model`
	struct Model: ModelType {

	let datasource: Observable<Resource>

	init() {
	let url = URL(string: "https://media.giphy.com/media/5z08WdHr0h9SHZekve/giphy.gif")!
	let request = URLRequest(url: url)
	self.datasource = URLSession.shared.rx
	.response(request: request)

	class RxMBProgressHUD: ObserverType {

	static let shared = RxMBProgressHUD()

	enum State {
	case hide(view: UIView?, animated: Bool)
	case show(view: UIView?, animated: Bool)
	}

	typealias E = RxMBProgressHUD.State

	protocol ViewModelType {
	var source: Driver<UIImage?> {get}
	var finished: Completable {get}
	}

	typealias Resource = (response: HTTPURLResponse, data: Data)

	protocol ModelType {
	var datasource: Observable<Resource> {get}
	}

	target 'LegacyApp' do
	use_frameworks!
	pod 'RxSwift', '~> 4.2.0'
	pod 'RxCocoa', '~> 4.2.0'
	end