Andrzej Naglik anaglik

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

	struct Property<T> {
	let name: String

	let _getter: () -> T
	let _setter: T -> ()

	var value: T {
	get {
	return _getter()
	}

	//
	// CollectionViewDataSource.swift
	// Khan Academy
	//
	// Created by Andy Matuschak on 10/14/14.
	// Copyright (c) 2014 Khan Academy. All rights reserved.
	//

	import UIKit

	BOOL PSPDFSystemFunctionOverriddenInCategory(Class theClass, SEL selector, NSString __autoreleasing categoryName) {
	NSCParameterAssert(theClass);
	NSCParameterAssert(selector);

	Dl_info info;
	if (dladdr(class_getMethodImplementation(theClass, selector), &info)) {
	// /System/Library/Frameworks is a common denominator, some methods are in /usr/lib/libobjc.A.dylib
	// Custom app is in /private/var/mobile/Containers/Bundle/Application/<UID>/PSPDFCatalog.app/PSPDFCatalog
	if (!strstr(info.dli_fname, "/System/Library") && !strstr(info.dli_fname, "/usr/lib")) {
	if (categoryName) *categoryName = @(info.dli_sname);

	struct User {
	let id: Int
	let name: String
	let email: String?
	}

	extension User: JSONDecodable {
	static func create(id: Int, name: String, email: String?) -> User {
	return User(id: id, name: name, email: email)
	}

	//
	// MultiDirectionAdjudicatingScrollView.swift
	// Khan Academy
	//
	// Created by Andy Matuschak on 12/16/14.
	// Copyright (c) 2014 Khan Academy. All rights reserved.
	//

	import UIKit
	import UIKit.UIGestureRecognizerSubclass

	import Foundation

	final class SafeSyncQueue {

	struct QueueIdentity {
	let label: String
	}

	let queue: DispatchQueue

	public struct TextSize {

	fileprivate struct CacheEntry: Hashable {
	let text: String
	let font: UIFont
	let width: CGFloat
	let insets: UIEdgeInsets

	fileprivate var hashValue: Int {
	return text.hashValue ^ Int(width) ^ Int(insets.top) ^ Int(insets.left) ^ Int(insets.bottom) ^ Int(insets.right)