Diogo Tridapalli diogot

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

A composable pattern for pure state machines with effects

State machines are everywhere in interactive systems, but they're rarely defined clearly and explicitly. Given some big blob of code including implicit state machines, which transitions are possible and under what conditions? What effects take place on what transitions?

There are existing design patterns for state machines, but all the patterns I've seen complect side effects with the structure of the state machine itself. Instances of these patterns are difficult to test without mocking, and they end up with more dependencies. Worse, the classic patterns compose poorly: hierarchical state machines are typically not straightforward extensions. The functional programming world has solutions, but they don't transpose neatly enough to be broadly usable in mainstream languages.

Here I present a composable pattern for pure state machiness with effects,

Using private Objective-C classes from Swift

In Objective-C, if you want to make use of one of Apple's internal classes, all you have to do is declare the class's interface with the methods you want to use. From there you can create instances of the class by using the NSClassFromString function to obtain a reference to the class object, like so:

[[NSClassFromString(@"_NSSomeClass") alloc] initWithFoo:5]

In Swift, this is not so easy. Referencing the type of the "hollow" Objective-C class interface anywhere in Swift code makes the compiler query the class for type information that is not known at compile time. This presents a huge problem. You cannot dynamically initialize a type either. Though, it seems the following code used to work at one point:

	#import <Foundation/Foundation.h>
	#import <CoreData/CoreData.h>

	@class MHWMigrationManager;

	@protocol MHWMigrationManagerDelegate <NSObject>

	NS_ASSUME_NONNULL_BEGIN
	@optional
	- (void)migrationManager:(MHWMigrationManager *)migrationManager migrationProgress:(float)migrationProgress;

	struct Video {

	let title: String
	let description: String?
	let category: String
	let thumbnailUrl: URL?
	let …

	}

	import Foundation

	enum CardTypeError:Error {
	case TypeNotFound
	}

	enum CardType:CustomStringConvertible {
	case visa
	case mastercard
	case amex

	NSDate *date = [NSDate new];

	NSDateFormatter *dateFormatter = [NSDateFormatter new];
	dateFormatter.locale = [NSLocale localeWithLocaleIdentifier:@"nl_NL"];
	dateFormatter.dateStyle = NSDateFormatterFullStyle;
	dateFormatter.formattingContext = NSFormattingContextDynamic; // this is the important setting

	NSString *dateString = [dateFormatter stringFromDate:date];

	NSString *s1 = [NSString stringWithFormat:@"Foo %@", dateString]; // "Foo dinsdag 13 december 2016"

	# The trick is to link the DeviceSupport folder from the beta to the stable version.
	# sudo needed if you run the Mac App Store version. Always download the dmg instead... you'll thank me later :)

	# Support iOS 15 devices (Xcode 13.0) with Xcode 12.5:
	sudo ln -s /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/15.0 /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport

	# Then restart Xcode and reconnect your devices. You will need to do that for every beta of future iOS versions

	# (A similar approach works for older versions too, just change the version number after DeviceSupport)

	class PersonBuilder {

	private final var name: String
	private final var age: String?
	private final var gender: String?

	init(name: String) {
	self.name = name
	}

	import UIKit

	struct Screen<A> {
	let run: (A -> ()) -> UIViewController
	}

	struct Step<A> {
	let build: (navigationController: UINavigationController, callback: A -> ()) -> UIViewController
	}