Peter Steinberger steipete

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

	// # Why +0 is a better default than +1 for "normal function arguments".
	//
	// My intention here is to show why +0 is better default in a resilient
	// word. Keep in mind that inside individual modules and with inlinable
	// declarations, the optimizer can change conventions at will so from a defaults
	// perspective, these are not interesting. The interesting case is calling
	// non-inlinable functions in other modules.
	//
	// Consider a situation where I have a class Klass and a function foo that calls
	// a function bar in a different module.

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	<string>WebDAV</string>

	import UIKit

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	// UICollectionView Objective-C example

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	#import <objc/runtime.h>

	@interface DebuggingOverlay: NSObject

	@end

	@implementation DebuggingOverlay

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	//
	// NSWindow+Fade.swift
	// BH Bezel Notification
	//
	// Created by Ben Leggiero on 2017-11-09.
	// Translated to Swift 4 from the original (ObjC): https://gist.github.com/indragiek/1397050
	// Copyright © 2017 Ben Leggiero. All rights reserved.
	//

	import Foundation