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In Bite #294 we learned about annotations in Sourcery. They're a great way to make extra metadata available on our types when accessing them in Sourcery's .stencil templates.

Today we'll take a look at one of the most powerful features of annotations in Sourcery, Key/Value Annotations.

We'll explore these by building a fictional API client powered by a simple Swift Enum. Let's begin!

First, we'll need to create our enum. Then, we'll define some cases to represent each HTTP endpoint we want to be able to call.

For us, these are a classic set of "get all", "get one", "create", "change", and "delete":

enum SpaceshipsAPI {
    case ships
    case ship(shipID: Int)
    case createShip(shipID: Int, ship: Ship)
    case updateShip(shipID: Int, ship: Ship)
    case deleteShip(shipID: Int)

Long-time readers may recognize this enum technique from Bites such as #93 or #150.

Here's where the magic begins though. ๐ŸŽฉ

Sourcery Key/Value Annotations are similar to regular Annotations. Instead of a simple tag though, they allow us to annotate types, enums, enum cases, etc with a set of names and a values.

Key/Value Annotations work everywhere that regular Annotations do. We can annotate types, enums, enum cases, and more.

They look like this:

/// sourcery: key = value
/// sourcery: anotherKey = someOtherValue

They can also be listed in a single line:

/// sourcery: key = value, anotherKey = someOtherValue

Values can contain some basic types:

/// sourcery: maxSpeed = 1500, hasHyperdrive = true
/// sourcery: codename = "blacksaber"

Ok, back to our API client. Now that we know we can add simple metadata like this above things in our code, let's use this technique to describe each API endpoint.

enum SpaceshipsAPI {
    /// sourcery: method = "GET", path = "/spaceships"
    /// sourcery: timeout = 5
    case ships

Nice! Normally, we might put things like method and path into essentially a big switch statement switching on self, and returning the correct method or path.

With Sourcery Key/Value Annotations though, we can let Sourcery generate all of these statements for us.

Let's create a SpaceshipsAPI+Properties.stencil template that extends our enum, generating the computed properties:

extension SpaceshipsAPI {
  public var path: String? {
    switch self {
      {% for c in type.SpaceshipsAPI.cases %}
      {% if c.annotations.path %}
      case .{{ }}: return "{{c.annotations.path}}"
      {% endif %}
    {% endfor %}
    default: return nil

  // ... etc

Success! Now we can work at a much higher level. We can update our annotations and never need to manually edit tedious switch statements.

We'll run sourcery, which creates/updates SpaceshipsAPI+Properties.generated.swift, filling it with all those beautiful switches.

Finally, we can use our new properties like this:


The use cases can begin to boggle the mind a bit here.

We can now add rich metadata to just about anything in our Swift code. Then use it to metaprogram new Swift code that we can use in our project.



#294: Annotations with Sourcery ๐Ÿ”ฎ


We first covered Sourcery back in Bite #292. It's a command-line tool that helps us generate Swift code from .stencil template files.

Today we'll check a specific feature of Sourcery, annotations. Let's dive in.

Nefore we begin, let's do a quick "refresher". The TLDR for Sourcery is Render template files into real Swift code. Full access to the type system using SourceKitten. Useful for simple things (automate tasks like adding counts to enums), or crazy complex things (automatically generate Swift test code). Essentially, metaprogramming. Whew. Ok, moving on.

Annotations are simply small bits of metadata that annotate our properties functions, enums, enum cases and other types.

Then, later we can access this metadata inside our .stencil template files.

Let's try it out.

Before we begin, let's add a new Swift Enum to our project. Nothing special here so far, just regular Swift.

enum SpaceshipKind {
    case cruiser
    case frieghter
    case destroyer

Next, we'll add a new SpaceshipKind+Count.stencil file for Sourcery to render. We'll use the "Empty" option in Xcode when creating a new file, then open the Inspector (right side) and change the file's type to Swift Source so it will read (slightly) nicer in Xcode.

Finally, let's run sourcery:

sourcery source/ templates/ /source/_generated/ --watch --verbose

Passing in the --watch tells Sourcery to run continuously and re-render our templates anytime our code or templates change. Neat.

The --verbose flag tells Sourcery to print a bunch of useful debug info to the console while it's working. Helpful at first, but once we're more comfortable with Sourcery, we can probably leave this off.

Whew! Now we can finally start using Annotations.

Let's add an annotation to our enum:

enum SpaceshipKind {
    case cruiser
    case frieghter
    case destroyer

Then in our template, let's generate a count property for all our enums:

{% for enum in types.enums %}
extension {{ }} {
  static var count: Int { return {{ enum.cases.count }} }
{% endfor %}

This works great:

extension SpaceshipKind {
  static var count: Int { return 3 }

But what if we were farther along in our project? What if we had many more Enums in our code base, and wanted to exempt a few of them from generating a count property.

We'll add another new enum:

enum EngineKind {
    case hyperdrive
    case sunsail

Since we added some code, Sourcery's --watch kicks in and re-generates our template:

extension SpaceshipKind {
  static var count: Int { return 3 }
extension EngineKind {
  static var count: Int { return 2 }

Let's pretend we don't need that second enum to get a count property.

There's a few ways we could approach this in Sourcery, but this gives us a nice way to learn about and understand how annotations work.

We'll add one above EngineKind in our code:

/// sourcery: skipCount
enum EngineKind {
    case hyperdrive
    case sunsail

Annotations are simply special comments that Sourcery parses, and makes available in our .stencil templates.

The cool part is it's location aware, so we can put annotations above types, and they'll be available on those types in our .stencil templates! Same goes for annotations on properties, enum cases and more!

Now, we can access this annotation in our template:

{% for enum in types.enums %}
extension {{ }} {
  {% ifnot enum.annotations.skipCount %}
  static var count: Int { return {{ enum.cases.count }} }
  {% endif %}
{% endfor %}

Finally, our generated code is back to just the one enum:

extension SpaceshipKind {
  static var count: Int { return 3 }


We can also apply annotations to multiple things at once using :begin and :end tags:

/// sourcery:begin: skipCustomSetter
var captainID: Int
var maxSpeedInParsecs: Int
/// sourcery:end

This is just the beginning. Come back next time to learn about key/value pair annotations.

Learn more about Sourcery at (also in Bite #292)

AttributedStrings are a wonderful way for us to describe and add rich, styled text to our apps.

We've covered a few different approaches and solutions for composing AttributedStrings over the years.

Today we'll check out at a new library, Attributed by Nicholas Maccharoli, which stands out as a modern approach to the task. Let's take a look.

We'll start by looking at the "standard" way to use AttributedStrings in Foundation:

let font = UIFont(name: "AvenirNext", size: 18.0)!

NSAttributedString(string: "Han Solo", attributes: [
  NSFontAttributeName: font

Not too bad, but we can do better. Let's try this same thing but with Attributed:

"Han Solo".attributed(with: Attributes { $0.foreground(color: .black).font(font) })


"Why not just use the AttributedString API that ships with Foundation?"

This is a fair question.

Imagine building an app that had many different attributed string styles. With Attributed, we're given a strongly API, allowing us to omit a couple of types. We're also able to omit the long, verbose key names.

Finally, (and perhaps most importantly) we're using a sort of "closure composition" technique.

This involves accepting a Swift closure in an initializer, giving us the anonymous argument $0 to play with. Then, we can chain function calls on to $0 to add attributes.

All of this leads to a dramatic decrease in manual typing (even with autocomplete in Xcode, writing tons of attribute collections stops being fun, quickly).

It also allows us to work more effeciently, and accurately. We can lean on our syntactic shorthand and trust in the Swift's type system to get us to the finish line.


Before we go, let's talk about one of the most common hiccups we can run into when writing AttributedString-related code. Inheritance.

We've all been there. We're composing an AttributedString to go into a UILabel. We're pumped because we've got the style neatly translated from the original design into code. Then we see it. The design calls for one of the words in the label to be a different color, for emphasis.

Dun, dun dun.

No worries though, with Attributed we can solve this problem quickly:

let base = Attributes().font(UIFont(name: "AvenirNext", size: 18.0)!)

let highlighted = base.foreground(color: .red)

"Han Solo is the captain of the ".attributed(with: base)
    + "Millennium Falcon".attributed(with: highlighted)

Very cool. We're able to define a base set of attributes in Attributes(), then compose new sets that inherit all the attributes of our base set.

Then, we're leaning on Attributed's extension to String and String's support of the + operator to write some super clean code.

Pro Tip: This functionality also allows us to easily build up a re-usable set of Attributes(), keeping them in one spot, then sprinkling them throughout our code.

Learn more about Attributed at

We cover plenty of libraries and developer tools here on LBOC. Many are useful not just on their surface, but also in terms of how they allow us to learn from our fellow developers.

Through this process, we collectively explore new approaches and techniques. New ideas emerge all the time.

Every now and then, one of these ideas stands out.

Sometimes, an idea makes too much sense, or is simply too useful to ignore.

In modern times, things like Fastlane, CocoaPods, and Carthage come to mind. Slightly more seasoned folks may remember the emergence of Pull to Refresh, or BWToolkit.

Sourcery from Krzysztof Zabล‚ocki is the latest addition to this set.

It brings the concept of "meta-programming" to Swift, and it is definitely too useful to ignore.

Let's peer into our crystal ball, and see what it can do.

At its core, Sourcery generates Swift code from template files.

It elegantly brings together two other great developer tools: SourceKitten (for introspecting our code), and Stencil (for templates).

It aims to solve a few problems:

  • Reduce time spent writing so-called "boilerplate", or repetitive/obvious code.
  • Provide a way to reason about the types in our code, and their properties.
  • Reduce simple human errors, caused by things like typos.

Ok, enough introduction. Here's how Sourcery works:

  • First, we'll write some code that looks almost like regular Swift code into "template" (.stencil) files.
  • Then, we'll run the sourcery command-line tool. This will "render" our .stencil files into .swift files that we'll add to our project.
  • Finally, when we build our app, our "generated" .swift files will get compiled just like any other .swift files we might add.

Immediately some ideas of how to use this begin coming to mind. Here's a few specific tasks that might cause us to reach for Sourcery:

  • Conforming our types to NSCoding.
  • Conforming to Equatable or Hashable
  • Writing JSON de-serialization code

Maintaining each of these implementations is a never-ending task. Anytime we add, remove, or change a property we'll need to potentially revist each of these bits of code as well. Bummer.

Ok. Let's try this thing.

First we'll need to get the sourcery command-line tool. The simplest way to do this is to download the latest release binary here.

Let's cd into the root directory of the download and copy the tool over to somewhere permanent:

cp bin/sourcery /usr/local/bin

(Note: /usr/local/bin is a common place to put command line tools on macOS thanks largely to the fact that Homebrew puts things there, so it's likely already in our $PATH).

Neat. Now we can use it anywhere.

We could also have simply copied the tool into our project, and added it to source control. Any approach is fine, we just need to be able to run it in the root directory of our project somehow.

Now, let's head into that root directory of our project, and create a couple directories:

mkdir templates
mkdir generated

We're almost ready to try things out. First though, we'll need a template to generate from.

Let's add a new file in the templates directory called Enum+Count.stencil. Then, we'll write our first template code:

{% for enum in types.enums %}
extension {{ }} {
  static var count: Int { return {{ enum.cases.count }} }
{% endfor %}

The {{'s, }}'s, {%'s, and %}'s are Stencil template tags.

Stencil deserves a full Bite of it's own, but for now we just need to know that statements within these tags get evaluated by the sourcery command line tool, and iterated or replaced when generating Swift code.

The rest of the content is regular Swift code.

Anyone who has worked on a web app in recent years should feel right at home with this technique. Instead of generating HTML though, we're generating Swift code, neat!

Let's break down what's happening in our template:

First, we want to iterate through all the enums in our project's code:

{% for enum in types.enums %}

{% endfor %}

Then, for each enum we find, we want to extend it to have a new static property called count.

extension {{ }} {
  static var count: Int { return {{ enum.cases.count }} }

This property will return the number of cases in the enum. (Providing us a piece of functionality currently missing from Swift itself).

Finally, we can run sourcery.

./sourcery . templates generated --watch

We've passed in the --watch flag to make sourcery watch our template files, and re-generate them anytime it sees a change. Neat.

This will scan our source code for a bit, then produce a new file in the generated directory called Enum+Count.generated.swift.

It will look like this:

extension SpaceshipKind {
  static var count: Int { return 37 }

extension CrewRank {
  static var count: Int { return 10 }

extension HTTP.Method {
  static var count: Int { return 7 }

How cool is that?

Now, we just need to add this generated file to our Xcode project like we would any other file. Its contents will be replaced anytime sourcery runs.

Pro Tip: We can also optionally add a new "Run Script..." Build Phase to our Xcode project to run the sourcery command (without --watch of course) at the beginning of each build of our app. Very cool.

The Sourcery Github repo offers a some very useful example templates for adding things like Equatable and Hashable. These examples are a great way to learn more about what's possible.

We've of course only barely scratched the surface of what's possible with Sourcery. Look out for future Bites where we'll explore much more...

Learn more and find full documentation of Sourcery at

We've covered UI Tests a fair amount over the years, but one thing has always stuck out: Running tests can be slow. Very slow.

Even if our individual tests themselves run quickly, the entire process is essentially "single threaded", slow to start up and complete each pass, and is prone to strange errors.

Today we'll take a look at Bluepill, a new tool from LinkedIn that can help us with all of this by running multiple iOS Simulators in parallel, simultaneously. Let's dive in.

We'll start by cloning the repository, then running the build script that comes inside:


This will build the command line tool. Once it's finsihed, we can copy the tool somewhere permanent:

cp build/Build/Products/Debug/bp /usr/local/bin

Then rename it to something we can more easily identify:

mv /usr/local/bin/bp /usr/local/bin/bluepill

(Note: /usr/local/bin is a common place to put command line tools on macOS thanks largely to the fact that Homebrew puts things there, so it's likely already in our $PATH).

Now that we have Bluepill installed, let's try it out.

We can head back to our project and run something like:

./bluepill -a ./ -s ./SpaceshipsUITests.xcscheme -o ./output/

This is great for quick runs, but ideally we'd be able to configure this sort of thing once and use it each time. Let's make a quick configuration file using JSON. We'll call it bluepill-config.json:

   "app": "./",
   "scheme-path": "./SpaceshipsUITests.xcscheme",
   "output-dir": "./bluepill-logs/"

By default Bluepill will run 4 iOS Simulators simultaneously. Before we run our tests, let's turn that up a notch by adding one more option to our config file:

(This will cause our test to be run in up to 12 iOS Simulators at once. Very cool).

   "app": "./",
   "scheme-path": "./SpaceshipsUITests.xcscheme",
   "output-dir": "./bluepill-logs/",
   "num-sims": 12

Finally, we can start our engines:

./bluepill -c bluepill-config.json

So awesome.

Not only are we saving tons of time this way, but Bluepill also does other helpful things for us, such as automatically retrying when the Simulator hangs or crashes. Neat.

We've only scratched the surface of what's possible with Bluepill. Be sure to check out the README for a full list of options and defaults.

Learn more about Bluepill at

Optimizing for responsiveness is a huge part of making great apps. Before we can optimize though, we'll need to measure. Xcode and Instruments offer some incredible tools to do "deep-dives" for answers (Bite #68, Bite #113), but often we just want to keep an eye on our app's performance and respond as needed.

Today we'll try out a library called GDPerformanceView by Gavrilov Daniil that lets us easily monitor our app's rendering speed and CPU usage in the device's status bar while we use the app. Let's begin.

We'll install GDPerformanceView and head over to our AppDelegate. We'll add a bit of code:

GDPerformanceMonitor.sharedInstance.startMonitoring { (textLabel) in
  textLabel?.backgroundColor = .black
  textLabel?.textColor = .white

Here we're telling GDPerofrmanceMonitor to start its engines, then customizing the look and feel of the label that appears in the status bar.

When we Build & Run, here's what we get:

Neat! By default GDPerformanceView will show the app and device version. This is great in some cases (QA testing, beta builds), but in our case we don't really need it. Let's them both off:

GDPerformanceMonitor.sharedInstance.appVersionHidden = true
GDPerformanceMonitor.sharedInstance.deviceVersionHidden = true

Beautiful. Now we'll always know exactly how well our app is behaving, and we'll be able to identify issues as they happen.

GDPerformanceView has one more trick up its sleeve. ๐ŸŽฉ

We can actually subscribe to performance updates and do whatever we'd like with the data.

Let's try this out. First we'll subscribe to updates by making our AppDelegate conform to the provided GDPerformanceMonitorDelegate protocol:

extension AppDelegate : GDPerformanceMonitorDelegate {
  func performanceMonitorDidReport(fpsValue: Float, cpuValue: Float) {
    // TODO

Then, we'll set it as the delegate for the shared GDPerformanceMonitor in our didFinishLaunching:

GDPerformanceMonitor.sharedInstance.delegate = self

Nice. Now we need to do something interesting with these updates. Let's use the Taptic Engine (Bite #269) to provide some force feedback if we hit the CPU too hard:

func performanceMonitorDidReport(fpsValue: Float, cpuValue: Float) {
  if cpuValue > 50 {
    UIImpactFeedbackGenerator(style: .heavy)

Neat! We could also toss these values into an array somewhere and use it to build charts, etc.

Learn more about GDPerformanceView at

Xcode offers a wealth of great tools for debugging. Sometimes though, we'd like to be able to debug or evaluate the inner-workings of our app without needing to be connected to Xcode's debugger.

Today we'll check out TinyConsole by Devran รœnal, a library that lets us easily display our log messages directly inside our app. Let's take a look.

We'll install TinyConsole into our app, then slightly modify some code in our AppDelegate to set it up.

Instead of assigning our root view controller like this:

self.window.rootViewController = SpaceshipsViewController()

We'll wrap our root view controller in a TinyConsoleController:

self.window = TinyConsoleController(
  rootViewController: SpaceshipsViewController()

That's it. Now all we need to do is add some log messages throughout our code. We can do this with calls like:

TinyConsole.print("spacehip id: \(")

Now, we can launch our app and try it out. When running on a device, we can simply shake the device to show/hide the console view.

(Pro Tip: Press โŒ˜โŒƒZ to simulate a shake in the iOS Simulator).

Neat! TinyConsole doesn't stop there though, we can also use colors:

TinyConsole.print("Crew Member Saved!", color:

and add Markers:


Finally, there's a few more gestures available (in addition to shaking to hide/show).

We can swipe to add a Marker, tap with 2 fingers to log something manually, or tap with 3 fingers to show an Action Sheet that allows us to send our log messages as an email.

Learn more about TinyConsole at

Whether we need sample values while prototyping our app's interface, or some multipliers for our game's logic, random data can be incredibly useful when programming. Today we'll check out a great library from Nikolai Vazquez called RandomKit that makes generating random data both simple and intuitive. Let's begin.

RandomKit is built on a set of Swift Protocols. We're provided Random, RandomWithinRange, RandomToValue, and many more. RandomKit then extends a bunch of types to conform to these protocols.

(This is great because it means we can easily add RandomKit-style functionality to our own types, if we ever need to. Neat.)

Let's try out the basics. Some of the most common things to generate randomly are numbers and booleans. RandomKit has us well covered here, supporting all the major numerical Foundation types (and more):

Double.random() // 0.6472946529383645
Int.random(within: 1...10) // 7
NSNumber.random(within: -5...5)  // -3
CGFloat.random(0...1)  // 0.27969591675319
Bool.random() // false

Neat. Next up, Strings:

String.random(ofLength: 5) // "$-=5t"
Character.random() // "#"

We can also easily grab a random element from any Swift Sequence or Collection:

"Little Bites of Cocoa".characters.random // "o"
["Han", "Luke", "Chewie"].random // "Luke"
NSDictionary(dictionary: [ "firstName": "Han", "lastName": "Solo" ]).random // ("lastName", "Solo")

But wait, there's more! RandomKit is incredibly comprehensive. Let's try some more advanced functionality like Dates:

Date.random(within: Date.distantPast...Date())  // "Feb 7, 472, 5:40 AM"


URL.random() //

...or even UIColors and NSColors:

UIColor.random(alpha: false)  

Last but not least, we can even grab random values for CoreGraphics types:

CGPoint.random(within: 0...50, 0...50) // {x 23.284 y 45.302 }
CGSize.random(within: 0...50, 0...50) // {w 29.475 h 12.863 }
CGRect.random() // {x 3.872  y 46.15  w 8.852  h 20.201}

These examples were just a (sorry) random-sampling of RandomKit's functionality. It has a ton more to offer.

Learn more about RandomKit at

Properly responding integrating with the software keyboard is a big part of building a solid iOS app. Traditionally, this would involve observing NSNotificationCenter notifications and some pretty lengthy boilerplate code. Today we'll look at a great little library from Toto Tvalavadze called Typist that can improve all this. Let's check it out.

Typist works by exposing a way for us to easily "opt-in" to certain keyboard events:

  .on(event: .didShow) { (options) in
    // TODO

Yep. That's it. Gone are the wildly verbose notifications and complicated setup.

(Note: We're not required to use Typist's singleton instance, but it's provided for convenience).

Typist provides this same interface for a bunch of useful events:

  .on(event: .willShow) { _ in /* TODO */ }
  .on(event: .didShow) { _ in /* TODO */ }
  .on(event: .willHide) { _ in /* TODO */ }
  .on(event: .didHide) { _ in /* TODO */ }
  .on(event: .willChangeFrame) { _ in /* TODO */ }
  .on(event: .didChangeFrame) { _ in /* TODO */ }

Finally, inside each event closure, we're given a Typist.KeyboardOptions type that contains strongly-typed properties describing the event:

  .on(event: .willShow) { (options) in
    guard options.belongsToCurrentApp else { return }

    updateLayout(for: options.endFrame)
  .on(event: .willHide) { (options) in
    guard options.belongsToCurrentApp else { return }

    updateLayout(for: options.endFrame)

Here we're guard-ing to make sure the keyboard belongs to our app, and returning early if not.

Then, we use the endFrame property to adjust our own views to accomodate the keyboard.


Typist's options type also provides startFrame, animationCurve, and animationDuration properties to help us match our app's animation to the keyboard's.

Learn more about Typist at

Validating data is usually one of the "chores" we encounter while building apps. It can be annoying and boring code to write, and can be a huge source of bugs. Today we'll check out a great library called FormValidatorSwift that can help us avoid all these issues. Let's take a look.

The core of FormValidatorSwift is built around checking if a String passes some sort of test to be deemed 'valid'.

We'll use individual Conditions or combined sets of Conditions called Validators. We'll attach these to controls like Text Fields and Text Views to allow them to be validated.

Let's try out the included ValidatorTextField to validate an email address:

let field = ValidatorTextField(validator: EmailValidator())

Then, we'll configure it to allow the user to begin entering an email address that's invalid like "hello@", and specify that it should only be validated when it loses focus.

field.shouldAllowViolation = true
field.validateOnFocusLossOnly = true

This sets us up perfectly for our final steps: Displaying the validation info.

First, we'll set the validatorDelegate:

field.validatorDelegate = self

Then, we'll implement one function to update the visual state of our field when the validation state changes:

func validatorControl(validatorControl: ValidatorControl, changedValidState validState: Bool) {
  guard let view = validatorControl as? UIView else { return }

  if validState {
    view.backgroundColor = UIColor.white
    errorLabel.hidden = true
  } else {
    view.backgroundColor =
    errorLabel.hidden = false

And finally, we'll add one more function to display the actual reason that validation failed (if it failed):

func validatorControl(validatorControl: ValidatorControl, violatedConditions conditions: [Condition]) {
  var errorText = ""

  for condition in conditions {
    errorText += condition.localizedViolationString

  errorLabel.text = errorText
  errorLabel.hidden = false

Last but not certainly not least, we can group our fields into a provided Form type, then ask if the whole thing is valid:

var form = ControlForm()


print(form.isValid) // true

Learn more about FormValidatorSwift at

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