Last night, I spent a couple hours setting up nREPL support. The most flexible way to communicate with it is via TCP with “bencoded” messages. So I had to write an encoder/decoder in ObjC for this.
If you’re not familiar with bencoding, it only takes 3 minutes to learn. Here’s the entire spec, in just a few (mostly fluff) paragraphs.
Writing an encoder is dead-simple. When you know what type you have, you just encode it and concat them together in order.
But the decoder was tricky! Especially because the most painless way to do TCP in ObjC is using CocoaAsyncSocket, which is purely asynchronous.
AsyncSocket works like this: you have a single callback that takes a string and a user-defined tag, which gets called no matter what data you just asked for. And to ask for data, there are two methods: one takes a string-delimiter to read until, and one takes a length to read until. Both also take a tag.
If I had the bencoded string right up front in its entirety, it would be easy to decode. You just use recursion, letting the call-stack map to the inner-most level of the nested structure that you’re in. As you recurse back up the stack, your container variables capture whatever was decoded below, and you return the containers upwards. It’s pretty simple.
But how do you write a recursive algorithm when you have to wait for the data to come in via a callback? That was the hard part!
My first attempt was to use libdispatch to make some kind of thread-safe queues that I could block while reading from. But that turned out to be uglier than I’m comfortable with.
Then I figured it out! I can still use the stack and do recursion, just in an unconventional way! Instead of function calls, I would add anonymous-functions to an instance-variable array. These closures would hold a strong reference to whatever container they were adding to, and because they’re stored in an array, I still get the stack-like behavior that the call-stack would have given me! Then when the end-delimiter is reached for a given type, it would pop the next closure off the stack and call it with this newly-formed object.
This involved taking advantage of the tag-parameter of the ask/receive methods to know what I’m waiting for. But due to the linear nature of the format, it’s safe and striaght-forward.
I’m pretty happy with how this implementation turned out. The whole
@implementation is about 115 lines of pretty readable code.
Update: Here’s the source code to my implementation.
My name is Steven Degutis, and I've been writing software professionally for a decade. During that time, I've written many apps and websites, quite a few technical articles, and kept up-to-date with the rapidly evolving software industry.
If you have software needs for web, mobile, or desktop, and are looking for a seasoned software professional, please reach out to me at email@example.com to set up a phone call.
- Self-employed – present
- Clean Coders – 5 years
- 8th Light – 2 years
- Big Nerd Ranch – 1 year
- Self-employed - 1 year
- Web: full-stack
- iOS (UIKit)
- macOS (Cocoa)
- REST APIs
- AWS / EC2 / ELB
- HTML5 / CSS
Over the past decade, I've written a total of 169 technical articles on various programming languages, frameworks, best practices, and my own projects, as I kept up-to-date and active in the software industry.
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- 2017 — "Clean code" isn't actually clean
- 2017 — Passion in your field is overrated
- 2017 — What I learned in 5 days of writing an experimental website
- 2014 — Age of the Polyglot
- 2013 — How to Program
- 2013 — Ignore the Naysayers
- 2013 — Writing Clearly
- 2012 — Reinvent the wheel
- 2010 — Good usability
- 2009 — Twitter is the wrong tool
- 2009 — We're all pretty bad at driving
- 2008 — Why I Code
|March||Notes on Haskell Extensions|
|February||Second thoughts on front-end tools|
|February||First thoughts on front-end tools|
|February||Some thoughts on GUIs|
|February||First thoughts on OCaml|
|February||First thoughts on Haskell|
|August||Age of the Polyglot|
|August||The history of Mjolnir|
|August||Quitting the GUI wars|
|June||Lua: my new favorite extension language|
|January||My programming life-goals|
|January||Lingua Latina, Pars I|
|January||Allocating an AST on the stack|
|April||Ruby Accessors Considered Pernicious|
|March||Reinvent the wheel|
Here are some of the projects I'm most proud of. They were created using a variety of technologies, running on several different platforms and OSes. They're all finished products, and many of them are open source.
I made Docks in 2009 for users who wanted to swap out icons in their Dock with a single click. Its unique functionality and design aesthetic attracted the attention of Apple, Engadget, MacWorld, and led to an acquisition of my start-up by Big Nerd Ranch.
This toy was made in a weekend to entertain my 1 year old daughter. It lets you create bubbles with your fingers, which then simulate physics by bumping into each other and falling.
When I couldn't find an app in the App Store that let me make very simple lists extremely quickly, I made one myself. I use it almost every day to organize and track my activities.
I created this app to increase my productivity by letting me move windows around in macOS using keyboard shortcuts. It grew into a community-driven highly extensible app, using Lua for its plugin system.
Implementing this elite social network gave me experience integrating both Apple Pay and credit card payments (via Stripe.com) seamlessly into web apps, for a frictionless and pain-free payment experience.
This isn't just any chatroom. In this web app, you can see what everyone is typing while they type it. I made this in order to scratch my itch for making real-time apps and games, and learned how to use WebSockets in the process.
This was written in 2009, before the time of Slack, when IRC was the main way for programmers to get short-term assistance from each other. Its purpose was to be a beautiful app with an emphasis on simplicity and usability over technical power.
This is an app I actually use every single day. It lets you move windows with global keyboard shortcuts. Since it uses Vim-like key bindings, it should feel pretty natural to any programmer. There's no configuration needed; it Just Works™.
As an evolution of Phoenix, Hydra was my first attempt at embedding a full Lua virtual machine into an Objective-C app, to make a lightweight and efficient window manager that focused on speed, low memory usage, low CPU usage, and overall being gentle on laptop batteries.
These may be tiny, but they're interesting technical feats.
|Lua4Swift||Swift framework for embedding Lua with a native Swift API.|
|choose||Command line fuzzy-matching tool for macOS that uses a GUI|
|music||Command line music daemon for macOS that only speaks JSON|
|hecto||Command line text editor with an embedded Lua plugin system|
|ZephSharp||Window manager for Windows using Clojure for scripting|
|management||Minimalist EC2 configuration & deployment tool in Ruby.|
|go.assert||Assertion helper package for writing tests in Go.|
|go.shattr||Go library for printing shell-attributed strings to stdout.|
|OCDSpec2||Objective-C based testing framework with Xcode integration.|