tclementdev

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

gnou

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)

khanlou

import Foundation

final class SafeSyncQueue {

    struct QueueIdentity {
        let label: String
    }

    let queue: DispatchQueue

andymatuschak

//
//  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

andymatuschak

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)
	}

steipete

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);

andymatuschak

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

import UIKit

jspahrsummers

struct Property<T> {
	let name: String
	
	let _getter: () -> T
	let _setter: T -> ()

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

mikeash

#import <Foundation/Foundation.h>

NSDictionary *TestD(void) {
    return @{
        @"string" : @"abc",
        @"number" : @42,
        @"dictionary" : @{
            @"string" : @"abcdef",
            @"array" : @[ @"a", @2 ]
        },

rgcottrell

// NOTE: @schwa figured this out, but I'm claiming some of the credit for asking the question.
// See https://gist.github.com/schwa/ecd5f8c154e60fcb0f58 for the original solution.

// Playground - noun: a place where people can play

import Foundation

// Implementation (repeat as needed for number of parameters).

func unwrap<T1, T2>(p1: T1?, p2: T2?) -> (T1, T2)? {