Tyler Stromberg AquaGeek

macOS Internals

Understand your Mac and iPhone more deeply by tracing the evolution of Mac OS X from prelease to Swift. John Siracusa delivers the details.

Starting Points

How to use this gist

You've got two main options:

Receive GOES-16 and GOES-17 Images with a Raspberry Pi and RTL-SDR dongle

Also see the original Pieter Noordhuis's guide

Hardware

You need:

Raspberry Pi Model B (or B+) with a MicroSD Card $35-40
An RTL-SDR dongle:

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

Scaling your API with rate limiters

The following are examples of the four types rate limiters discussed in the accompanying blog post. In the examples below I've used pseudocode-like Ruby, so if you're unfamiliar with Ruby you should be able to easily translate this approach to other languages. Complete examples in Ruby are also provided later in this gist.

In most cases you'll want all these examples to be classes, but I've used simple functions here to keep the code samples brief.

Request rate limiter

This uses a basic token bucket algorithm and relies on the fact that Redis scripts execute atomically. No other operations can run between fetching the count and writing the new count.

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,

	extension Error {
	var code: Int { return (self as NSError).code }
	var domain: String { return (self as NSError).domain }
	var userInfo: [String:Any] { return (self as NSError).userInfo }

	func timeAfterWhichToRetry(retryCount: Int) -> TimeInterval? {
	// CloudKit suggests us retry too often, so slow us down as we retry a lot, up to 5 minutes
	if let suggestedTimeout = suggestedTimeAfterWhichToRetry {
	if suggestedTimeAfterWhichToRetry == 0 {
	return 0

	import SwiftUI
	import Combine


	struct State {
	var isCreatingItem: Bool = false
	var partialItemName: String = ""
	var todoItems: [TodoItem] = []
	}

	As of iOS 11/macOS High Sierra, and only including ones in Foundation and CoreFoundation

	Strings:

	_NSCFString - a CFStringRef or CFMutableStringRef. This is the most common type of string object currently.
	- May have 8 bit (ASCII) or 16 bit (UTF-16) backing store
	_NSCFConstantString - a compile time constant CFStringRef, like you'd get with @"foo"
	- May also be generated by dynamic string creation if matches a string in a pre-baked table of common strings called the StringROM
	NSBigMutableString - an NSString backed by a CFStorage (https://github.com/opensource-apple/CF/blob/master/CFStorage.h) for faster handling of very large strings
	NSCheapMutableString - a very limited NSMutableString that allows for zero-copy initialization. Used in NSFileManager for temporarily wrapping stack buffers.

	//
	// CustomFontMetrics.swift
	//
	// Created by Zachary Waldowski on 6/6/17.
	// Licensed under MIT.
	//

	import UIKit

	private extension UITraitCollection {

	defmodule MyApp.Scheduler do
	@moduledoc """
	Schedules a Mix task to be run at a given interval in milliseconds.

	## Options

	- `:task`: The name of the Mix task to run.
	- `:args`: A list of arguments to pass to the Mix task's `run/1` function.
	- `:interval`: The time interval in millisconds to rerun the task.