Zoë Smith zoejessica

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

DEPRECATED for Xcode 10 🎈

(check out What's New in Swift at 11:40, slide 42)

Whole Module Compilation Optimizations: Why these terms are sometimes misleading

When you look up how to compile swift faster for debug builds, people very earnestly give advice that seems contradictory: you should "try using the whole module optimization flag," and also "never use whole module optimization for debugging". [^1]

This is confusing because some of us are using these two general words:

compilation: "turning text into an executable program"

	import Foundation

	/// `StringScanner` is a fast scanner for Strings and String-like objects.
	/// It's used to extract structured bits from unstructured strings, while
	/// avoiding making extra copies of string bits until absolutely necessary.
	/// You can build Scanners over Substrings, allowing you to scan
	/// parts of strings and use smaller, more specialized scanners to extract bits
	/// of that String without needing to reuse another scanner.
	public struct StringScanner<Input: StringProtocol> {
	let input: Input

	import Darwin

	@dynamicMemberLookup
	struct Environment {
	subscript(dynamicMember name: String) -> String? {
	get {
	guard let value = getenv(name) else { return nil }

	return String(validatingUTF8: value)
	}

	#!/bin/bash

	set -e

	CONTENTS=$(tesseract -c language_model_penalty_non_dict_word=0.8 --tessdata-dir /usr/local/share/tessdata/ "$1" stdout -l eng \| xml esc)

	hex=$((cat <<EOF
	<?xml version="1.0" encoding="UTF-8"?>
	<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
	<plist version="1.0">

	import Foundation

	extension Data {
	var hexString: String {
	return self.map({ return String(format: "%02hhx", $0) }).joined()
	}
	}

	/*******************************************************************************
	* The MIT License (MIT)
	*
	* Copyright (c) 2017 Jean-David Gadina - www.xs-labs.com
	*
	* Permission is hereby granted, free of charge, to any person obtaining a copy
	* of this software and associated documentation files (the "Software"), to deal
	* in the Software without restriction, including without limitation the rights
	* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
	* copies of the Software, and to permit persons to whom the Software is

	// See http://proxpero.com/2017/07/11/encoding-and-decoding-custom-enums-with-associated-values-in-swift-4/
	enum Barcode {
	case upc(Int, Int, Int, Int)
	case qrCode(String)
	}

	extension Barcode: Codable {
	init(from decoder: Decoder) throws {
	self = try Barcode.Coding.init(from: decoder).barcode()
	}

	public extension MKMultiPoint {
	var coordinates: [CLLocationCoordinate2D] {
	var coords = [CLLocationCoordinate2D](repeating: kCLLocationCoordinate2DInvalid,
	count: pointCount)

	getCoordinates(&coords, range: NSRange(location: 0, length: pointCount))

	return coords
	}
	}

	public struct Promise<T> {
	typealias Fulfiller = (T) -> (Void)
	typealias Rejecter = (Void) -> (Void)
	typealias Resolver = (_ fulfill: @escaping Fulfiller, _ reject: @escaping Rejecter) -> (Void)
	let resolver: Resolver
	init(_ resolver: @escaping Resolver){
	self.resolver = resolver
	}
	func then<U>(_ execute: @escaping ((T) -> U)) -> Promise<U> {
	return Promise<U>({(fulfill, reject) in