Marek Hrusovsky xhruso00

System Design Cheatsheet

Picking the right architecture = Picking the right battles + Managing trade-offs

Basic Steps

Clarify and agree on the scope of the system

User cases (description of sequences of events that, taken together, lead to a system doing something useful)
- Who is going to use it?
- How are they going to use it?

How to download and view legacy documentation from Apple (no need to sign in to your dev account)

Download the docset index XML.
Find the docset you want (there are some with URL https://apple.com/none.dmg; ignore them - you will find them again further down the file with a working URL).
Download the dmg. It's probably around a gigabyte or so.
"Install" the .pkg file somewhere on your disk. If you don't trust the installer, do it manually:
1. Find the largest file, named Payload, and extract it using The Unarchiver.
2. This creates a new, even larger file, probably named Payload-1.
3. Extract Payload-1 using The Unarchiver.
After many minutes of extracting, we have our .docset file.

`vs` Questions

http://stackoverflow.com/questions/804115 (rebase vs merge).
https://www.atlassian.com/git/tutorials/merging-vs-rebasing (rebase vs merge)
https://www.atlassian.com/git/tutorials/undoing-changes/ (reset vs checkout vs revert)
http://stackoverflow.com/questions/2221658 (HEAD^ vs HEAD~) (See git rev-parse)
http://stackoverflow.com/questions/292357 (pull vs fetch)
http://stackoverflow.com/questions/39651 (stash vs branch)
http://stackoverflow.com/questions/8358035 (reset vs checkout vs revert)

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

The dynamic wallpaper in MacOS Mojave is a single 114 MB .heic file that seems to contain 16 embedded images.

It also contains the following binary plist data in its metadata under the key "Solar". It's an array of 16 items, each with four keys:

i (integer). This seems to be the image index.
o (integer). This is always 1 or 0. Stephen Radford thinks it indicates dark mode (0) vs. light mode (1).
a (decimal). I’m pretty sure this is the angle of the sun over the horizon. 0º = sunset/sunrise. 90º = sun directly overhead. Negative values = sun below horizon.
z (decimal). This seems to be the cardinal position of the sun relative to the camera. 0º = sun is directly in front of the camera. 90º = sun is directly to the right of the camera. 180º = sun is directly behind the camera.

The attached lldb command pblock command lets you peek inside an Objective-C block. It tries to tell you where to find the source code for the block, and the values captured by the block when it was created.

Consider this example program:

#import <Foundation/Foundation.h>

@interface Foo: NSObject
@end

@implementation Foo

	# 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)

	# /etc/nsmb.conf - macOS 11.3 - 2021-04-29
	#------------------------------------------------------------------------------
	# SMB configuration for macOS 11.3 <-> Synology
	#------------------------------------------------------------------------------
	# Additional information:
	# -----------------------
	# https://support.apple.com/de-de/HT211927
	# https://support.apple.com/en-us/HT208209
	# https://apple.stackexchange.com/questions/309016/smb-share-deadlocks-since-high-sierra
	# https://photographylife.com/afp-vs-nfs-vs-smb-performance

	// Don't forget to prefix your category!

	#import <UIKit/UIKit.h>

	NS_ASSUME_NONNULL_BEGIN

	@interface UIWindow (PSPDFAdditions)

	#if TARGET_OS_UIKITFORMAC

	//
	// Debug.swift
	//
	// Created by Craig Hockenberry on 3/15/17.
	// Updated by Craig Hockenberry on 2/20/24.

	// Usage:
	//
	// SplineReticulationManager.swift:
	//