Grand Central Dispatch (GCD)

Last year I was unemployed and miserable. Using this sub and resources, I've been full time employed for a year. I did it with all free resources. I wanna share with you how I did it.

Background: Environmental Engineering degree from a University of California. But it doesn't matter. None of my coworkers have engineering degrees.
Position: I'm a mobile developer. I primarily work in iOS with Swift and Objective-C but I also know JavaScript, finished Android boot camp through CodePath.
Ask me questions, I'll write a summary of resources I used.

> Why are you writing this?

I recently celebrated one year since my official full time offer after I worked as an apprentice last year. So in total I have about one year and three months of experience. I've also seen a lot of posts from people struggling and I'd like to provide guidance.
I will not post any links to a YouTube account to get views out of you, I won't try to get you to pay me money. part of the reason I love this community is because software engineers are obsessed with teaching people for free. And I'm all about that life.
As promised, here you are:
Sonny's Roadmap from 0 to iOS Hero for FREE
CS50x on EdX - You can audit the course for free. Take this and finish it. This will change the way you think of programming and David Malan is one of the greatest and most inspirational people I've seen talk about computers. Everything else you take, will teach you how to build things like a software engineer. David Malan teaches you how to think like a software engineer.
iTunes Developing iOS 9 Courses with Stanford University - The course and all materials are free on iTunes. While it's outdated from iOS 10, the concepts and fundamentals are crucial to understanding how to write and develop applications in iOS.
Paul Hegarty, like Professor Malan, is a huge inspiration to me.
Hacking with Swift - You can do the entire Hacking with Swift course free, just disable your ad-block because that's how Paul Hudson makes money off of people who don't buy the books. I bought HwS and Pro Swift, so my ad-blocker is on, sorry Paul. Paul knows the industry, so he's not going to sit around and scold you about using a UITextView instead of a UILabel when you want your text to run on additional lines. He's going to teach you how to build iOS applications. He updated his resources for Swift 3.

Graduate School | Further down the rabbit hole | The Red Pill
At this point, you're honestly ready to start building applications and apply to apprenticeships or jobs, but there's still a lot you don't know. The question is, do you go further down the rabbit hole or just let work experience dictate you from here?
Beyond this point, my recommendations are more specialized. If you have a full time job and a technical background, I highly recommend CodePath iOS Courses hosted at Facebook or Hosted at AirBNB. The only conflict with this is that you HAVE to commit 8 weeks of your time. You can't just give up halfway because you'll be given a team and if you bail on them, you're a dick. And to be honest, they could've given a spot to someone who would've finished.
Another recommendation is Udacity's Intro to iOS Development with Swift or their iOS networking course. You can audit Udacity's courses for free, just make sure to constantly add what you work on to GitHub. They also have a Grand Central Dispatch course which is pretty important to know.
Another really solid resource is Ray Wenderlich's iOS Tutorials. A good majority of them are accessible free and they are very solid iOS developers.
Resources to Avoid
I hate to say things like this, but there's a resource I have to call out because it will make you a bad developer but give you a very false sense of security about knowing what you're doing and that's "Rob Percival's iOS Course on Udemy". You'll see it on sale, for $7.99 or $9.99 and suspicious accounts recommending it here, but let me save you the trouble:
As a full time iOS developer, if you use Udemy from start to finish to learn iOS from Rob Percival, you will almost assuredly fail a technical interview and have your code quality seriously questioned. He doesn't teach proper unwrapping of optionals early onward. He copies and pastes code without explaining fundamental MVC or MVVM structure. He says a lot of "just write it, and you can figure it out later". He doesn't have a verifiable work experience with actual clients or companies and more or less just built his reputation on having the most sold iOS course on Udemy. Almost every "review" you find that is on Google has a "referral link, get 50% off with my link here" which makes it hugely suspect.
I got the course for $4.99 last year, just to add to my resources and now when I look at it, I find myself putting my hand on my forehead a LOT.
If you insist on going with Udemy, I recommend Mark Price.
But even then, you shouldn't touch any of these until you finish CS50X.