Terriermon Terriermon

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

	public protocol Diffable: Hashable {

	var primaryKeyValue: String { get }

	}

	/// A type-erased diffable value.
	/// The AnyDiffable type forwards diffing, equality comparisons and hashing operations to an underlying diffing value,
	/// hiding its specific underlying type.
	///

	// FAILURE
	var array = [["name": "glen"]]
	for item in array {
	item["rank"] = "advanced" // Generates an @lvalue error
	}

	// Even though array is immutable, it's of type <Array<Dictionary<String,String>>,
	// item is assigned by let automatically.

	// Swift 2.2 syntax / API

	import UIKit

	extension UIBezierPath {

	class func arrow(from start: CGPoint, to end: CGPoint, tailWidth: CGFloat, headWidth: CGFloat, headLength: CGFloat) -> Self {
	let length = hypot(end.x - start.x, end.y - start.y)
	let tailLength = length - headLength