The Swift API Design Guidelines are crystal clear.
Brevity in Swift code, where it occurs, is a side-effect of the strong type system and features that naturally reduce boilerplate.
Writing compact code is a non-goal. But, often times, it just happens.
I have been thinking about how to approach this post for almost a week, and in the end, I decided that I am going to do it with honesty. The following are my opinions, and I take full ownership of everything I am going to write. So, fair warning, long rant incoming.
I am sick and tired of hearing bullshit related to unit testing. I have a really hard time figuring out how any developer can be fine with delivering untested code, code that nobody knows if it really works until it is tested in production, with real users. I am sick and tired of hearing always the same arguments, the same excuses, justifying not writing unit tests. And I hate how the idea that unit testing is not indispensable has made its way into some management circles.
These are some of the excuses I have heard to not write unit tests.
This one hits way too close to home pic.twitter.com/M9GkT3i4Zr
— The Practical Dev (@ThePracticalDev) March 25, 2016
Well, when you say that, what I hear is “I don’t know how to write unit tests”.
Why? Because of this:
Unit testing is a software development process in which the smallest testable parts of an application, called units, are individually and independently scrutinized for proper operation.
That’s the first result when googling “unit tests”. And I think it is a very good definition.
A unit test should test a small unit of behaviour. If you expect a class to return a specific value from an specific method when some particular conditions are met, all you have to do is test that you get the value you expect from the method you are interested in after recreating your particular preconditions. That is not that hard. That is not that time consuming.
And if it is, if testing the smallest unit of behaviour in your system is hard and time consuming, then your smallest unit of behaviour is not as small as you think. Back to my argument: you would be testing it wrong.
Well, obviously, if you know that the behaviour and the public API of a module are going to change tomorrow, first thing in the morning, it might not make too much sense to unit test that module today.
But… how many times will you find yourself in that situation? Not many.
Of course, a codebase is similar to a living and breathing organism, that evolves and might change its behaviour over time. Well, since you will have to refactor your production code, your will also have to refactor your testing code to reflect the new behaviour of the system.
But justifying not writing tests because you will refactor your code in the future does not make any sense. Will you also refuse writing code, today, because you might need to refactor that code in the future? I don’t think so.
Well, sorry, but if testing your production code is very difficult, your production code is not properly designed.
You might be trying to test classes with too many responsibilities (violating the Single Responsibility Principle) you might be testing private behaviour, or you might be testing multiple layers at once (therefore, trying to write something that is not actually a unit test). Either way, when testing gets hard, that’s a clear sign that the design of the production code is wrong.
Or, to put it another way, well factored code, with small, highly cohesive and loosely coupled units, is easy to unit test.
This point is not too different from the previous one.
If a behaviour is private, it should not be tested. If you really really want to test it, it means it is not private. So, we are back to swore one: the code is badly designed.
Now, allow me to digress a little bit, and tell you a story. A couple years ago, my team spent an insane amount of time trying to test a class that was performing some network requests, and the cached the behaviour. Cache was supposed to expire after five minutes, if I recall correctly. While caches were valid, the class performing the networking would return cached values, when caches were invalid, it would trigger a new network request. The production code looked like this:
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final class DataService { private let cache = Cache() public func getMovies(completion: SuccessBlock) { if let cachedValue = cache.get("movies") { completion(cachedValue) } else { //Attack remote service, save data in cache completion(freshData) } } } |
The team complained a lot about how difficult it was to test that behaviour, how difficult it was to mock the cache, and spent an insane amount of time researching mocking frameworks.
All we needed was turn to the Dependency Inversion Principle. Inject the dependency like this:
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protocol Cache { func get(key:String) -> [Movies] } final class DefaultCache: Cache { // implementation of the Default Cache protocol, // to be injected into DataService } final class DataService { private let cache: Cache init(cache: Cache) { self.cache = cache } public func getMovies(completion: SuccessBlock) { if let cachedValue = cache.get("movies") { completion(cachedValue) } else { //Attack remote service, save data in cache completion(freshData) } } } |
And now the behaviour of the cache could be tested in isolation. And the behaviour of the class doing the networking could be tested in isolation. And the collaboration between the both of them could be tested in isolation as well.
What do you mean by “unit testing a view controller”? Or “unit testing an activity”? Because what I have found people usually mean by that is something along the lines of “how am I going to test that when I tap a button the text in a textfield is updated with new content dowloaded from a server”?
The UI layer should be thin. Very thin. It should only render data, and propagate down user interaction. Period. If it does something else, the design is wrong. Period.
Some people call it The Humble Object Pattern, but I don’t think we really need a name for it. The UI layer should not do any “business” logic. If the UI layer needs to coordinate state between UI elements, that logic should be extracted to different units, because of modularity, and encapsulation, and good separation of concerns, but also because as an added benefit, it will be easier to test.
By now, I am sure you see the pattern: well designed code is easy to test. But the opposite is even more accurate: poorly designed code is difficult to test. And the more difficult it is to test a particular unit of code, the poorer its design.
Do you want to ship code that works? Or do you want to just toss a dice?
Did this post offended you? If that’s the case, to be honest, I don’t care.
1 CommentThis is a post I have been meaning to publish for months, but I am always hesitant to do so for multiple reasons.
Firstly, because giving advice to others about how they should do their job is not the most humble thing in the world, and secondly because I have learnt most of what I am going to discuss in this post from my own mistakes.
So let me start with an apology to all those developers that had to deal with me doing the actual opposite of what I am going to suggest now. You know who you are. Sorry, guys,
Let’s begin by setting some common ground. What defines a developer as senior? This is my premise:
- A senior developer has confidence.
- A senior developer sees the big picture.
I have been lucky to work in three different countries, with three extremely different work cultures and ethics. But there is a trend I have seen everywhere, a significant misconception of what defines a developer as senior.
Obviously what makes a developer senior is not the number of years in the craft. There is a difference between having five years of experience, and having the same year of experience five times. And it hurts deeply when I see hiring managers that refuse to see it that way.
What makes a developer senior is not the amount of encyclopaedic knowledge a developer might have about a specific language, API or framework either.
The difference between a senior and a junior developer is confidence . A senior developer makes faster and better decisions. But not only because they have more experience, but because a senior developer is capable of seeing the big picture.
Now, it can be argued, with reason, that confidence can be a liability more than an asset. And I tend to agree with that as well.
So allow me to clarify: confidence is not refusing to be open to other people’s opinions, it is not thinking that you already have all the answers, it is not thinking that you are the smartest person in the room.
A confident developer will be open to fresh ideas, when not actively pursuing them, that challenge his current knowledge, and confident developers will always approach a technical discussion trying to find better solutions than their own to solve the problem at hand.
Confidence is also what makes senior developers want to share their knowledge and experience with others.
A senior develop must be humble and respectful, honest and transparent, and willing to challenge others and himself.
A senior developer would assume that his knowledge of certain areas of his daily practice is outdated. And senior developers can be sure that there will always be another developer in the team that knows more than them about one or more of those areas.
And you know what? That is great. Remember, this software thingy is not a pissing contest, this is about building great products, with code of the highest possible quality. A senior developer, a team/technical lead in particular, should be able to recognise when others know more, and encourage them to use that knowledge for the greater good (the final product).
We all learn by making mistakes. A senior developer has, most likely, reached that point in her/his career due to all the mistakes he/she has made in the past.
Also, due to the aforementioned confidence, a senior developer won’t be scared of making more mistakes, because the senior developer knows that what matters is not writing perfect code, but recovering from the inevitable mistakes fast and with elegance.
By being honest and transparent, but alway thinking out loud in the open, by discussing his approach with the rest of the team, by showing her/his thought process in the most possible honest and open way, the senior developer will help others understand that what matters is having an actual thought process, and that what matters is taking time to consider the trade-offs before starting to write code like crazy.
I believe there is nothing more valuable for a junior developer that seeing someone, with way more experience, struggle with the same issues that he/she struggles with.
A senior developer always attacks problems with an analytic approach, and a senior developer always has in mind those things that junior developers are usually not aware of yet: business requirements, trade-offs, and the big picture. And makes the importance of those constraints crystal clear.
Because a senior developer always sees the big picture. The senior developer always has a long term plan. The senior developer is relentlessly moving towards the goal posts, no matter what it takes.
Constructing that big picture, that master plan, and setting the placement of the goal posts openly, in front of everyone, rationalising and explaining every single decision is the best that the senior developer can do to mentor others.
Because mentoring is not lecturing, mentoring is not only suggesting using an obscure class in the system frameworks, mentoring is showing, with your own actions, day after day, that a master plan, and a thought process to fulfil it, are necessary.
A senior developer cares deeply about what he does, and cares deeply about the end result of his practice. That means a senior developer cares about excellence.
A senior developer has to be a living and breathing example of how excellence does not happen by accident. Because excellence requires work, dedication, commitment, and the will and drive to actually achieve it.
A senior developer will always challenge himself. A senior developer is never satisfied with the current state of the code, and is always willing to improve it in some way. And he is open, even vocal, about it.
A senior developer will also challenge junior developers by asking them challenging questions. There is nothing worse than telling a junior developer, straight away, that something he implemented is not good enough, or plain wrong.
However, there is nothing better than reading carefully a junior developer’s code and ask the kind of questions that will make that developer understand that the code can still be improved.
A senior developer will not provide direct solutions, but gently steer the junior developer towards those spots in the implementation that do not align with the big picture, the masterplan. Because, remember, the senior developer always has the masterplan in mind.
A senior developer will always end a code review with some open ended question or suggestion to the junior developer. A simple “do you think the functionality of this particular part of the system would be easy to extend?” makes wonders. A “do you think there is a way to make this code simpler”, accompanied by a “by the way, have you heard about the builder pattern?” will provide a great learning opportunity to another developer.
And a senior developer will carefully consider when to let others make mistakes. Because mistakes and the lessons learnt from them, are what made the senior developer, well, senior.
Note:
Edited on May 6th to (try to) remove some gender specific pronouns, as pointed out by @fbartho. My apologies if anyone felt this article was gender-specifc. I am not a native English speaker, and the best I can do as a senior developer is try to recover from my mistakes the best I can. If you still feel this post is gender-specific, please let me know.
Allow me to start using a big word: cyclomatic complexity
Let me illustrate a typical example of cyclomatic complexity. Imagine you have some entities similar to these:
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struct Image { let image: UIImage? } struct Movie { let url: NSURL? } struct TextMessage { let message: String } |
Now, imagine we need to render a table view containing Images, Movies, and TextMessages. Notice how each one of those entities provides a description of itself (the data that will need to be rendered) of a completely different type. In plain English, the table cells will need to display a UIImage if the object to render is of type Image, a NSURL if the object to render is of type Movie, and a String if the object to render is of type TextMessage.
So, the obvious implementation, the one that first comes to my mind, the one I have seen in a thousand different projects in the good old Objective-C days (yes, that was sarcasm), would be declaring a method typing the parameter as id, and then cast and check the type of whatever is passed. Translated to Swift, it would look like this:
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final class Cell: UITableViewCell { func render(data: Any) { if let image = data as? Image { print(image.image) } else if let movie = data as? Movie { print(movie.url) } else if let message = data as? TextMessage { print(message.message) } } } let image = Image(image: UIImage(named: "myIcon")) let movie = Movie(url: NSURL(string: "http://ctarda.com")) let textMessage = TextMessage(message: "The amazing adventures of a sse") let cell = Cell(frame: .zero) cell.render(image) cell.render(movie) cell.render(textMessage) |
Well, that was bad in the good old Objective-C days (sarcasm again), but now, in the bright Swift present, code like that is just unacceptable.
Here is where the cyclo-thingy kicks in.
That method is very apparently very simple, but in reality is very complex. Or, to be more accurate, it has the potential to become very complex, very fast.
First, as soon as we start to create different logical branches, we are starting to add different behaviours to somethign that, in fact, is just one single behaviour (in this case, rendering data in a cell). Since we are not dealing with a single behaviour anymore, it is very likely that the behavious in each one of those logical branches will start diverging more and more as time goes by.
But that is not the only reason. As soon as we start creating different logical branches, and each branch starts behaving differenly, the code starts to become more difficult to understand, which makes it more difficult to maintain.
Also, when the code is more difficult to understand, specially because it does multiple, very different things, it is easier to make mistakes. Never understimate the importance of this. This, in my experience, is the first source of bugs in every single codebase I have ever touched.
It took me years to figure it out, but once I got it, I decided to make this my only non-negotiable rule when it comes to writing code:
The Single Resposibility Principle applies to every single task in the daily practice of a software engineer, from organizing code in files, to structs, classes, methods, functions, or any other construct.
Again, let me be clear about this: to me this rule is non-negotiable. Sowftare engineering is all about trade-offs except when it comes to the Single Responsibility Principle.
One file should contain only one abstraction (a struct, a class, an enumeration, a protocol, you name it), one method should only do one thing (either render a movie, or render an image, but never, ever, render a movie or an image), one class should do just one thing, one module should deal with only one high-level concern, and so on.
Now that we got that out of the way, let’s look again at the implementation of the Cell class, because there is another violation of the SOLID principles: the Cell class is not open for extension and closed for modification. If we want to render another type (say, Audio), and since Cell is already rendering three different tyes it wouldn’t be that surprising if we had to add yet another one, we would need to edit the source code of the Cell class. And we’d need to edit it to add yet more logical branches, more complexity.
In other words, we would need to edit to make even more brittle. Not good.
Well, protocol extensions make composing behaviours very easy.
Now, it could be argued that the solution I am going to discuss is not the only possible solution. And that would be true. And we have already discussed an alternative solution to a problem very similar to this, by relying on plain-old polymorphism. So, one thing we could do here would be having a bunch of Cells, all of them implementing a protocol, or all of them subclasses of a base Cell class. However, I believe there is a neater way to implement this.
So, let’s start again with the same entities:
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struct Image { let image: UIImage? } struct Movie { let url: NSURL? } struct TextMessage { let message: String } |
Now, let’s declare a protocol, and declare the render method in it:
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protocol Populable { associatedtype T func render(data: T) } |
The next step would be declaring a Cell class:
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final class Cell: UITableViewCell {} |
Notice how this class is actually empty, does not provide any behaviour at the moment. Nowm let’s start composing behaviours in extensions and protocol extensions. The first extension would make the Cell class implement the Populable protocol, and provide a “default behaviour” for it:
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extension Cell: Populable { func render(data: Cell) { print("Don't render yourself, dummy!. On second thought, why not?") } } |
To be honest, having to provide a “default behaviour” is the only part of this solution I don’t like. As you can see, I typed the parameter as Cell, assuming a cell won’t have to render itself, and that way, I am providing a defaul tbehaviour that I do not expect to be necessary. And, again, that makes me feel uncomfortable.
Anyway, now I can start providing the rendering behaviour that I expect from the Cell, in separate extensions, one for each type of data I want to render:
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extension Populable { func render(data: Image) { print("Render Image \(data.image)") } } extension Populable { func render(data: Movie) { print("Render Movie \(data.url)") } } extension Populable { func render(data: TextMessage) { print("Render TextMessage \(data.message)") } } |
Notice how Swift’s type inferrence makes the final code clean of any extra type information: it just works:
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let image = Image(image: UIImage(named: "myIcon")) let movie = Movie(url: NSURL(string: "http://ctarda.com")) let textMessage = TextMessage(message: "The amazing adventures of a sse") let cell = Cell(frame: .zero) cell.render(image) cell.render(movie) cell.render(textMessage) |
Now, here is the actual beauty of this solution. If I want to render a new type, all I have to do is add an extension to the Populable protocol with the logc necessary to deal with that specific type:
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struct Person { let idCardNumber: String } extension Populable { func render(data: Person) { print("Render Person \(data.idCardNumber)") } } let me = Person(idCardNumber: "Will-not-tell") cell.render(me) |
And, as added bonus, Xcode, with the help of the Swift compiler, will provide me extra documentation, in the form of code hints, about what types can be rendered by an instance of Cell:
In object-oriented programming, polymorphism (from the Greek meaning “having multiple forms”) is the characteristic of being able to assign a different meaning or usage to something in different contexts – specifically, to allow an entity such as a variable, a function, or an object to have more than one form.
By relying in strict typing, the single responsibility principle, favouring composition over inheritance, and using protocol extensions in our advantage, we can implement very robust solutions to complex problems, with code compliant with the Single Responsibility Principle, and therefore simple to read, understand, and maintain. And, as an added benefit, easy to test.
7 CommentsImagine that every Sunday you have the opportunity to design your week schedule.
Now, imagine that you are only allowed to design your schedule by combining pre-built blocks, and that you are given blocks like those in set A.
And now imagine you can choose from blocks like those in set B
Which set of blocks will allow you to build a more flexible schedule? Would the schedule built with blocks from set A be more or less difficult to adapt to change that the schedule built with blocks from set B?
That’s modularity. And that’s why the single responsibility principle matters. And that’s why methods should not be long. And that’s why classes should not be long. And that’s why view controllers that control views, do networking, and contain logic to deal with navigation are bad.
Leave a CommentSo, this has been my “facepalm moment” of the week. Not the only one I had this week, but the most remarkable one.
I was trying to explain what the Open Closed Principle is all about to a colleague. You know, “modules should be open for extension, but clsoed for modification”. You know, the good stuff.
As usual, I was trying to find a good example to illustrate the point of the OCP, and as usual, I was having a hard time finding it. Side note: finding good examples to illustrate abstract concepts is hard; if the example is too simple, everyone misses the point, if the example is too convoluted, everyone falls asleep.
And suddenly, the duh-facepalm-eyerolling moment! How can I be so dumb, the perfect example of the OCP is sitting right there, looking at us!
Extensions!
So, here comes the example (not too simple but yet not too convoluted) to illustrate my point.
Imagine a model object:
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struct Contact { let id: String let name: String let phoneNumber: Int } |
Now imagine we have a mechanism for transporting model objects across the wire, based on JSON:
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protocol JSON { func to_JSON() -> [String: Any] } final class JSONDataSender { func send(data: JSON) { print("Sending data ", data.to_JSON()) } } |
Well, we want to be able to send Contacts using this JSONDataSender, so we should make Contact implement the JSON protocol, right? True, but devil is in the details, and in this case, the simplest solution would require modyfing the source code of the Contact struct to add the new behaviour:
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struct Contact: JSON { let id: String let name: String let phoneNumber: Int func to_JSON() -> [String: Any] { return [ "id": id, "name": name, "phoneNumber": phoneNumber] } } |
Which would be a blatant violation of the Open Closed Principle (the object had to me modified in order to add behaviour to it). Tsk, tsk, tsk…
What’s the problem with that approach? Well, we might want to serialize to a protocol buffer, instead of JSON. Or write Contacts to a plist file, or to a SQLite database, or whatever. So every time we wanted to add that extra behaviour, we would need to edit the Contact source. Again, tsk, tsk, tsk.
However, if we don’t modify the original Contact, but add an extension:
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struct Contact { let id: String let name: String let phoneNumber: Int } extension Contact: JSON { func to_JSON() -> [String: Any] { return [ "id": id, "name": name, "phoneNumber": phoneNumber] } } |
We would have extended the object without actually modifying it. Again, the object would be open for extension and closed for modification. And adding subsequent behaviours would be a matter of adding more extensions. So, to me, and from now on, this counts as the perfect example to illustrate the Open Closed Principle (in Swift).
Leave a CommentLately, I have been dipping my toes into the world of reactive programming. To be honest, I should probably say that I’m up to my neck into it: ReactiveCocoa, MVVM, “everything is a stream”, and all that.
In my spare time I’m looking also into RxSwift. Even though this blog is mostly about Swift, I spend a significant amount of time writing code in other languages (mostly Java, Scala and C#), so it makes sense to learn what seems to be a more cross-platform approach than the one ReactiveCocoa follows. Platform agnosticism FTW!
But, back to the point of this post. Remember the Ring we discussed in the previous post? If you don’t, well, it’s basically a UIView subclass, that exposes a few public properties to make it configurable. Today I am going to focus on just one of them: progress.
The Ring class is part of the UI of a little pet project of mine, where I want to display some data I fetch from an async service. After fetching that data, I want to update a progress ring (my Ring class) and a label.
So, in the good old days, before my reactive enlightenment I would probably do something like this: (for context, this is the view controller that manages the ring and the label)
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private func fetchData() { dataService.stepCount(NSDate(), endDate: NSDate(), completion: { steps in label.text = "\(steps)" ring.progress = CGFloat(steps) / Constans.dailyGoal }) } |
For the sake of completing the example, the signature of the method in the data service would be like:
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func setpCount(initalDate: NSDate, endDate: NSDate, completion: ()->Int) {} |
So, not too bad, I guess. An async service, a completion closure as a parameter, and done.
But, if there is a use case for reactive programming, that would be dealing with async stuff. So, let’s go Rx!
The method in the data service signature could look like this:
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func stepCount(initDate: NSDate, endDate: NSDate) -> Observable |
And now, in my view controller, I could do something like:
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private func fetchData() { dataService.stepCount(NSDate(), endDate: NSDate()).subscribeNext { result in self.label.text = "\(result)" self.ring.progress = CGFloat(result) / Constants.goal }.addDisposableTo(disposeBag) } |
Not too bad! But it could be better. This code is still a little bit imperative. It is not a big deal, but I think it would look better, be more readable, and in a way easier to maintain, if I could bind both the ring and the label to the result emitted by the data service observable.
Binding to a UILabel is easy, RxCocoa provides extensions for that, but binding to a custom property in a custom view is obviously not provided out of the box.
But fear not, because it is very easy to do. As easy as the following:
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extension Ring { public var rx_ring_progress: AnyObserver { return UIBindingObserver(UIElement: self) { view, progress in view.progress = progress }.asObserver() } } |
So now, back to my view controller, I can bind the Observable directly to the ui controls, transforming the values emitted by the data service, in just one go:
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let data = dataService.stepCount(NSDate(), endDate: NSDate()) data.map{"\($0)"}.bindTo(label.rx_text).addDisposableTo(disposeBag) data.map{CGFloat($0) / Constants.goal}.bindTo(ring.rx_ring_progress).addDisposableTo(disposeBag) |
And done!
Leave a CommentA pet project of mine requires replicating one of the progress indicators in the Health app. I want to be able to configure the ring’s thickness, colour, and percentage of completion.
Before launching Xcode and start writing code right away, I wrote down what I knew about the problem I wanted to solve:
With that in mind, I decided to build a subclass of UIView, insert a CAShapeLayer into it, and bind the “progress” property of my view to the layer’s strokeEnd property. Also, I decided to took this as an opportunity to explore @IBDesignable and @IBInspectable, which is something I’d never done before.
And this is the end result:
The source code is available on GitHub, but let’s take a look at some of the highlights.
This one was easier than I expected. I can not claim to be an expert in Interface Builder, so I expected a lot of friction here, but annotating the class as IBInspectable was enough to make Interface Builder aware of it:
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@IBDesignable public class Ring: UIView { ... } |
Making the view configurable through interface Builder (as well as programatically) was also quite easy, just by annotating properties as IBInspectable and providing a default value for them:
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@IBInspectable public var progress: CGFloat = Constants.defaultProgress { didSet { progress = normalize(progress) pathLayer.strokeEnd = progress } } @IBInspectable public var lineWidth: CGFloat = Constants.defaultLineWidth { didSet { pathLayer.lineWidth = lineWidth } } |
There might be a better way to do this -I’m looking at you, UIBezierPath(arcCenter, radius, startAngle, endAngle,clockwise)- but I decided to rely on the fact that UIBezierPath’s strokeEnd is animatable (which means that everytime I cahnge its value, CoreAnimation will animate the changes for me), so I built the path as an oval, and set the strokeEnd to the value of the progress property (which is normalized between 0 and 1).
Of course, that makes the path start at the X axis, so the path’s layer needs to be rotated -PI/2.
After that, it is just a matter of setting the proper frame, and path in the layoutSubviews method. Here is the relevant code:
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override init(frame: CGRect) { super.init(frame: frame) configureRing() } required public init?(coder aDecoder: NSCoder) { super.init(coder: aDecoder) configureRing() } private func configureRing() { pathLayer.frame = bounds pathLayer.lineWidth = lineWidth pathLayer.fillColor = fillColor.CGColor pathLayer.strokeColor = lineColor.CGColor layer.addSublayer(pathLayer) pathLayer.anchorPoint = CGPointMake(0.5, 0.5) pathLayer.transform = CATransform3DRotate(pathLayer.transform, -0.5 * CGFloat(M_PI), 0.0, 0.0, 1.0); } override public func layoutSubviews() { super.layoutSubviews() pathLayer.frame = bounds pathLayer.path = ringPath().CGPath } private func ringPath() -> UIBezierPath { return UIBezierPath(ovalInRect: ringFrame()) } private func ringFrame() -> CGRect { return CGRectInset(CGRect(x: 0, y: 0, width: CGRectGetHeight(bounds), height: CGRectGetHeight(bounds)), lineWidth*2, lineWidth*2) } |
The full source is available on GitHub, and can be pulled in as a dependency through Carthage.
1 CommentThere was an interesting discussion at work today that led to class clusters.
First, I was surprised by the fact that, apparently, class clusters are something only old farts like me are aware of. But the discussion got me wondering: would it be possible to implement a class cluster in Swift?
Well, apparently it is. Let’s say we want to consume this cluster within the module it is declared in. So, by relying on inner types, we coudl do somenthing like this:
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//Cell.swift class Cell { private class GreenBackgroundCell: Cell { override func doYourThing() { print("I'm a green background cell") } } private class RedBackgroundCell: Cell { override func doYourThing() { print("I'm a red background cell") } } static func cellWithGreenBackground() -> Cell { return GreenBackgroundCell() } static func cellWithRedBackground() -> Cell { return RedBackgroundCell() } func doYourThing() { } } |
So, now we could do something like this:
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let cell: Cell = Cell.cellWithGreenBackground() cell.doYourThing() let anotherCell: Cell = Cell.cellWithRedBackground() anotherCell.doYourThing() //The compiler won't allow any of these: //let cell = Cell.GreenBackgroundCell() //let cell = GreenBackgroundCell() |
Now, the question is: would I ever do this in production code? I am not sure. Are there better ways to simulate a class cluster? I assume there would be, so if you know about them, please shout out in the comments!
Anyway, for those interested, there is a sample project on github
Leave a CommentInmmutability is good. Just yesterday, we were discussing how good it is. But just in case, here is another example.
Let’s assume we are building the software to manage a company’s payroll. We have only these requirements:
With those requirements in mind, this the simplest thing we can build. A Person would look like this:
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struct Person { let id: String let name: String } |
And the Human Resources Department would look like this:
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enum HRError: ErrorType { case NotValidID } final class HumanResources { var employees: [Person] = [] func validateId(id: String) -> Bool { return id.characters.count > 3 } func hire(person: Person) throws { guard validateId(person.id) else { throw HRError.NotValidID } employees.append(person) } func fire(person: Person) { if let index = employees.indexOf({$0.id == person.id}) { employees.removeAtIndex(index) } } } |
Notice how we do some fancy validation when hiring a new person, and throw an error if anything goes wrong. Also notice how the employees arrary is public, because HR need to produce it upon request.
We can hire someone:
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let personWithValidID = Person(id: "1234", name: "Valid id") do { try hr.hire(personWithValidID) } catch HRError.NotValidID { print("This ID is not valid") } |
We can try to hire someone without a valid Id, and HR would throw an error:
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let personWithoutValidID = Person(id: "1", name: "Not valid id") let hr = HumanResources() do { try hr.hire(personWithoutValidID) } catch HRError.NotValidID { print("This ID is not valid") } |
But, we can also bypass the ID validation by modifying the employees array directly:
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// After All that work, this is possible: let anotherPersonWithoutValidID = Person(id: "0", name: "Another without valid id") hr.employees.append(anotherPersonWithoutValidID) //Boom |
Boom!
This is, obviously, a very convoluted example. But the thing is that when we do not enforce immutability, bad things may happen, either by distraction, by means of unexpected side effects, or just by means of malicious or misbehaving code.
One way, and it is not the only possible way, to enforce immutability in the precious example might be this:
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final class HumanResources { private var internalEmployeesList: [Person] = [] var employees: [Person] { get { return self.internalEmployeesList.map{$0} } } init() { } func validateId(id: String) -> Bool { return id.characters.count > 3 } func hire(person: Person) throws { guard validateId(person.id) else { throw HRError.NotValidID } internalEmployeesList.append(person) } func fire(person: Person) { if let index = employees.indexOf({$0.id == person.id}) { internalEmployeesList.removeAtIndex(index) } } } |
Notice how we made the mutable collection private, and we provide a readonly property (employees) that returns an immutable collection, built upon request. But, remember, what matters here is not how we create that immutable collection, but the fact that the public API of the HR class provides only immutable data.
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