Beyond if let

I've given my Beyond if let talk based on my How I Handle Optionals blog post about handling optionals in Swift twice this week. I was probably a more dynamic speaker in the NSLondon talk but the content is pretty similar. I may have been a bit slower and more methodical at the Swift London one though. Let me know if anything isn't clear in either of them or if you have any tips or suggestions (speaking  or Swift).

Vimeo of talk at NSLondon. Questions from 13:30. [Updated to replace Livestream feed with this.]

You Really Don't Need Protected Members

When Swift's access controls were introduced I wrote about how they worked like C's with good use of internal and external headers. I've recently been asked about how to do access private properties when subclassing and thought it would be worth sharing the conversation.

If you are new to Swift's access controls I suggest you look at the previous post (same link as above) first as this is the refinement and conclusion (in my mind) of an issue that I an open question when I wrote that. If you are new to programming then this post is probably one to skip as it is more about comparing how things can be done in Swift that people are used to doing a certain way in Java than something meant as an introduction to the concepts.

I’ve a question in context of open/closed principle (Software entitles should be open for extension but closed for modification.), coming from Java, AS3 background highly influenced by the Design Patterns book by the Gang of Four, the way I see protected is so that work can be extended by other people. 

How can I fully extend a class in it’s entirety without access to it’s private members, surely I can override methods but that won’t be enough.

The only way I could think is to approach this by declaring everything public or have a getter setter for each but then I’m violating “Encapsulation” principle
The short version is I would design the system in such a way to avoid requiring any inheritance apart from the specifically designed to inherit from system classes (NSObject, UIView, UIViewController etc.).

How I Handle Optionals In Swift

Updated to add using map over optionals section - 7th January 2015

Update (26th January 2015) See Beyond `if let` for video of me presentating of much of this content.

Dealing with optionals is unavoidable in any Swift program that interfaces with the Cocoa or CocoaTouch libraries. Realistically that is all of them with a very few limited exceptions. Handling them is often annoying a although I am very glad of the rigour that they enforce on me. In fact I like the rigour so much that I never use implicitly unwrapped optionals (variable declared with !) and I use force unwrapping only in very limited circumstances.

In almost all cases I use combinations of optional chaining and nil coalescing operators with `if let` used where they don't do the job. In this post I'm going to explain how and where I use the different options and give some examples.

Note that this is very much a "How I do it" post and not a "How you should do it" post. It is also what I do this week which may evolve further in future. I take a somewhat radical approach and others may prefer approaches that fail earlier rather keep running in error conditions to the choices I take which are intended to keep running in most circumstances. Do note that there are definitely cases where it is better to stop than continue in an error condition especially where there is risk of corrupting data, losing money or even more serious consequences. Please choose a domain appropriate response. My recent domain has been a consumer app handling network data and not complicated and corruptible user documents or financial or safety critical and the choices you make about operating in unexpected state are different in each one.

theAnswer = maybeAnswer !! "Or a good alternative"

AssertingNilCoalescing Operator !!

I believe that Swift needs one more operator to deal with optionals, in this post I present my concept to make dealing with them better and safer. It isn't a beginners introduction to optionals or Swift. There are plenty of good ones out there already.

infix operator !! { associativity right precedence 110 }
public func !!<A>(lhs:A?, rhs:@autoclosure()->A)->A {
    assert(lhs != nil)
    return lhs ?? rhs()
}

How this code works will be explained near the end of this post so don't feel you need to understand it now. It adds an additional operator !! to a Swift project for dealing with optionals. It is designed to retain the benefits of strong typing proving the validity of some aspects of the code while documenting the expectation that the left hand argument will not be nil and exposing (in debug builds only) if that belief about the code is wrong.

Unlike other operators it behaves differently in a debug build than a release build. In release builds it is exactly the same as the built in ?? nil coalescing operator return the argument on the left unless it is nil in which case the right hand argument will be returned.

Swift Arrays are not Threadsafe

Swift arrays are not necessarily contiguous blocks of memory (although they may be in some cases so some code may work in some cases but not in others). Some operations may trigger them to be converted to a contiguous block of memory and I believe that is what can particularly cause issues when there is access from multiple threads.

[Update - This post is more popular than I expected (thanks Reddit). I've added some additional notes at the bottom. Also the previous post about optimisation is probably more interesting and valuable.]

Even the code as described here may not work (and I haven't tested at all) when there are multiple reads or writes that access the same elements of the array. All the code I used was either reading from a constant array (safe from multiple threads/queues) or writing into non-overlapping ranges with no reads occurring (only seems safe with the technique described here).

.withUnsafeMutableBufferPointer

Swift Optimisation Again - Slides from Swift London 21st October 2014

Update (23rd October 2014) - Now with video link HERE.

Swift optimisation presentation refined and developed further.

Please contact me if you want the source version and please let me know if you make use of it or find it useful.

Description of the demo from a previous event. Note that for Xcode 6.1 you will need to use the 0x, 1x, 2x and fullSpeedXC61 branches instead of the 0, 1, 2 and fullSpeed branches respectively. Likewise master currently has an Xcode 6.1 version although I will probably merge that to the real master shortly.


Core Location - Significant Location Updates Requires Background Permissions

If you've watched the WWDC video you will already know this but if you just look at the class references it isn't obvious that . iOS 8 introduced the possibility to request permission to access location only while the App is in the foreground which should encourage users to grant permission without fear of being tracked inappropriately however the foreground only mode only works with some of the modes. This slide from the talk (WWDC 2014 What's New In Core Location - Session 706) is the key one:

Bluetooth Low Energy in the Background - iOS8 Followup

I posted about the results of some experiments I had done using the iOS background modes for Bluetooth. At the time I hadn't updated any devices to iOS8 (and they would have been betas anyway). This is a quick update to confirm that there are no changes in these areas that I can see in iOS8.

If you do know of any iOS8 changes or workarounds to the issues described please let me know

Basic Findings (repeated from earlier article)

  1. Apps on BLE capable iOS devices can be woken from the background when a device with the correct UUID is detected even when they have been completely removed from memory due to other apps needing the memory.
  2. This background listening/advertising state is stopped if the app is deliberately closed by the user swiping it up from the task switcher.
  3. The background listening/advertising state is stopped if the device is switched off or runs out of battery. It isn't restarted until the App is run again. (I haven't actually retested running out of battery but am guessing it is the same as manually powering off I will try to test again when the battery on something is getting low)


Please see the previous article for discussion of the impact on on app design and some potential workarounds.

London Swift Presentation - Swifter Swift

This is the Swifter Swift presentation I gave at the London Swift Meetup in Clapham today (15th September 2014). Apologies for the stock photos/standard template, I focussed on the content.

I also showed the optimisation levels as described in this blog post.

Things I Forgot To Mention

Use constants - declare with let unless avoidable, allows extra optimisation. Test before changing to mutable state within a function for performance reasons.

Currently the optimiser can do better within a particular file although this will be fixed at some point.

Github Repos

https://github.com/josephlord/GrayScott - Optimised version of Simon Gladwell's Cellular Automata project.

Async.legacy - My iOS7 / OS X 10.9 compatible fork.

Async - Original by Tobias Duemunk