Proxy design pattern in iOS

My thoughts on the proxy design pattern in iOS

What this article is all about

I might have to disappoint you, but this is not going to be tutorial on using the proxy design pattern. It’s going to be more like a discussion about how it is used internally in the Cocoa framework and how we can take that as example and consider that advice (or not) in our projects. So if you are just interested in what a Proxy is and how it’s used… Honestly, I have no idea what you are doing here. Just read the Wikipedia page or something šŸ˜›

The proxy design pattern

Just so we are on the same page on this, lets look at the definition for the proxy design pattern:

A proxy, in its most general form, is a class functioning as an interface to something else.

This is taken right from that Wikipedia page I mentioned. Reading it, though, gives a very vague definition of the proxy design pattern. A person with a wilder imagination would say many things in programming are proxies. Hmm, and maybe he’d be right.

Anyway, I like to think of a proxy as a “wrapper” that keeps the iterface from the orginal class. Naturally, who am I to say what a proxy is, and what not. I’m sure there are at least 5 counterexamples that prove my definition is incorrect. It’s overly simplistic, but I think it’s good to start with.

Mentioning wrappers… are they actually a design pattern? Turns out… they are! Check it out in Wikipedia! Ok, yeah, it’s just an alias for the Adapter design pattern, but it’s something!

I hate seeing memes in blog articles, so feel free to open it externally

The problem with proxies

I have a problem with the second part of my definition:

A proxy is like a wrapper that keeps the interface from the original class

— Me, one paragraph above

For simple classes, or more specifically classes with simple and short interfaces, that’s fine. If you look at the example given in the Wiki article, the public interface for the class is only one method. It’s extremely easy to mimic the same interface in the proxy. But what if we wanted to proxy a class that is way more complex. For instance UIView. We would have to copy every public method from UIView. That’s quite a lot. And it’s fragile – new methods can appear with every SDK release. And most of those copied methods would probably do the same thing – just call the real functionality. And I don’t think this example is exaggerated in any way. After all, It could be often useful to proxy a large object.

One possible solution would be to extract the interface as a protocol and have the original class and the proxy conform to it. But, really, that doesn’t make it better. The thing about protocols is, you still need to provide implementations for those methods. Again, there is a lot of copying going on.

Note I know protocols have default implementation in Swift 3, and that’s really cool, but that doesn’t really solve the problem.

It seems at this point we have hit a rock and we start to understand why proxies are not so common in our coding life. Or have we?


The story about proxies ended for me for quite a while. I couldn’t find a solution to the problem of the common interface. It was recently that I remembered it while working on my previous article about app branding. If you check the documentation for UIAppearance, it says:

Use the UIAppearance protocol to get the appearance proxy for a class.

right from the start. Well, what do you know – here’s a prime example of the proxy design pattern. Moreover, UIAppearance proxies large, complex objects like UILabel and UIView. Basically most UI classes. So not ony does it mimic the interface of a complex class, but even several of those. Now this is worth some additional investigation.

What kind of object is UIAppearance?

The experiment

We cannot just open UIAppearance’s source code, but at least we can look at it’s declaration. I found the first instance of UIAppearance in my code and clicked on “Jump to definition”:

    public protocol UIAppearance : NSObjectProtocol {

    /* To customize the appearance of all instances of a class, send the relevant appearance modification messages to the appearance proxy for the class. For example, to modify the bar tint color for all UINavigationBar instances:
        [[UINavigationBar appearance] setBarTintColor:myColor];

        Note for iOS7: On iOS7 the tintColor property has moved to UIView, and now has special inherited behavior described in UIView.h.
        This inherited behavior can conflict with the appearance proxy, and therefore tintColor is now disallowed with the appearance proxy.
    public static func appearance() -> Self

Hm, so it is actually a protocol. That’s something, but doesn’t really tell us a lot about who and how implements it. But what is that at the end of the snippet?

public static func appearance() -> Self

That can lead to somwhere. If you ever worked with UIAppearance, you know you typically call it using:

UILabel.appearance().<set something>

In this example, of course UILabel is only one of many classes that support UIAppearance. And if you go to UILabel’s header, you’d find that it conforms to the UIAppearance protocol. Interesting! So if you call UILabel.appearance()… you get a UILabel instance back. Indeed, the line let labelAppearance:UILabel = UILabel.appearance() compiles fine and in runtime, we can inspect that labelAppearance is really an object of type UILabel. Additionally, it is always the same object.

Well that’s a little disappointing. Does it mean that UILabel has a static label variable that holds all the appearance information and all set properties there are also reflected on new instances when they are created?

Next experiment – if UILabel.appearance() is a label itself, can we actually put it on screen and see what gets displayed:

    override func viewDidLoad() {
        // Do any additional setup after loading the view, typically from a nib.
        let labelAppearance = UILabel.appearance()
        labelAppearance.frame = CGRect(x: 20, y: 50, width: 100, height: 40)

Easily enough, the code compiles and runs. But then:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSMethodSignature getArgumentTypeAtIndex:]: index (2) out of bounds [0, 1]'
*** First throw call stack:
    0   CoreFoundation                      0x000000011290e34b __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x000000010fc7e21e objc_exception_throw + 48
    2   CoreFoundation                      0x00000001128cbd5c -[NSMethodSignature getArgumentTypeAtIndex:] + 204
    3   UIKit                               0x0000000110ab14af +[NSObject(UIAppearanceAdditions) _installAppearanceSwizzlesForSetter:] + 293
    4   UIKit                               0x0000000110ab517f -[_UIAppearance _beginListeningForAppearanceEventsForSetter:] + 248
    5   UIKit                               0x0000000110ab53f9 -[_UIAppearance _handleSetterInvocation:] + 188
    6   CoreFoundation                      0x0000000112893a2e ___forwarding___ + 526
    7   CoreFoundation                      0x0000000112893798 _CF_forwarding_prep_0 + 120
    8   UIKit                               0x00000001103b0d75 -[UIView(Internal) _addSubview:positioned:relativeTo:] + 584

I guess the theory about UILabel.appearance() really being a label is slowly fading away. But that doesn’t make the reality any less disappointing. Lets look at the stack trace.

First of all, the exception description looks like a standard array index out of bounds assertion. Already, it seems UIAppearance just keeps an array of applied styles. But how?

Continuing on, we can start looking at the method calls. On 8th place, we see a call to addSubview. This is no surprise since the documentation specifically says that UIAppearance styles are applied when a view is added to a window. Next, 3 to 5, seems interesting. Invocation… Swizzles…?

Invocations probably refer to NSInvocation, a class encapsulating method calls. It’s a bit obscure, but if you’ve had to save methods along with parameters for later calls in (lets say) a dictionary, you might have had to use NSInvocation.

Swizzling, on the other hand, is something many of us know from the Objective-C runtime. Swizzling refers to the process of replacing a class’s method implementation with another one, effectively changing the class in runtime.

The results

At this point, we can make a pretty good guess as to how UIAppearance does its magic. It collects styling calls as NSInvocation objects in an array. Then, it uses method swizzling to override setters and apply those invocations to all new instances of the styled class. To make the compiler happy, especially in Swift, the appearance method from the UIAppearance protocol makes sure to return an object of the same class. That way all the class’ methods are accessible and statements such as:

UINavigationBar.appearance().barTintColor = themeColor

become possible. But in reality the underlying object is not an instance of that class, but a proxy collecting invocations, ready to apply them to any subsequent instances.

Next steps

We didn’t need to get too deep into Cocoa in order to understand how UIAppearance was implemented. Just some fiddling with its declaration and lldb was enough. Of course, for a lot of things we had to use our imagination and make assumptions. For instance, we inferred implementation details based on method names and undeterministic behavior.

However, a more powerful investigation method that works especially good in Objective-C, is looking at the class dumps. As you probably know, Objective-C is relatively easy to decompile. So there are tools available that will generate the source code based on compiled binaries. Moreover, for built-in libraries, dumps are readily available in Github. So before those libraries and frameworks get re-written in Swift, we can take advantage.

If you want to confirm our findings about UIAppearance or you still have questions regarding its implementation, a good place to start would be UIKit’s source code, and UIAppearanceProxy specifically, that you can find here

Thanks to Shoumikhin for suggesting that in the comment section!


The key takeaway for me here is that there is no clean easy solution to the problem at hand. When you proxy a class, it is your responsibility to carry the interface over to the proxy. And if that interface is non-trivial, the practicality of this approach starts to fade away. And while the proxy design pattern in iOS is definitely possible and also somewhat endorced by the Cocoa framework, it is not a simple and elegant implementation. It heavily relies on Objective-C and its ability to “shapeshift” at runtime.

So whether or not you would use it, heavily depends on how comfortable and open to the Objective-C runtime you are. I, personally, avoid it in my code and I get really suspecious about third party frameworks using it without a really good reason.

App Branding

App branding and UIAppearance


Supporting several application variations sharing the same code base is a programming topic that affects lots of developers. Especially when you think back several years ago with the explosion of holiday themed games, lite and premium version apps and re-skinning. Not that companies have stopped doing those things, but it seems, at least to me, that it’s no longer that prominent.

But it’s not only that. We also have white label solutions and enterprise apps specifically tailored for a company. The point is that there are lots of uses cases for apps that need to change at build time according to external requirements, and share 99% of their code at the same time.

The problem with programming though, is that 99% still means you have to support two distinct configurations. Also, what starts off as 99% similar tends to slowly diverge to 95, then 90 and then even less.

There are a lot of ways to tackle this app branding problem. And I don’t think anyone is in a position to say which way is the correct or the best one. It all depends on the specific circumstances of the project at hand. It’s all about enticipating the scope of the version differences and in what direction requirements might shift in the future.

However, even with that in mind, there are certain best practices and platform fatures we can leverage to have a more successful branding implementation.


We are going to go over several techniques that can be used to achieve app branding. They are all not perfect, but can be useful tools in the right circumstances. These are:

  • Branching
  • Build Targets
  • Subclassing
  • Configurations
  • UIAppearance

Often times several of those items will have to work together, in order to get the job done. Especially build targets are paramount to most solutions. By themselves, they are rarely enough, but coupled with others, they are probably the most powerful tool for app branding.


In a source controlled environment, you might be tempted to create a different branch for every branded version of your application. Especially if the new version requires a lot of changes, it might be easier to move to a new branch and implement changes without worrying too much about the main project – you cannot break your generic app if you’re working in a different branch. However, the price for this decision is quite big. With time your branches will diverge a lot. And even if they didn’t, keeping them in sync as new features and fixes get added, would be tedious and error-prone.

Another similar idea would be to extract the common code into a framework, submodule or whatever is convenient for the prticular case, and then have different projects, using those components and implementing the different, “brand-specific” parts. This might seem like a nice idea, and if done correctly might be genuinly awesome, but it is, in my opinion, difficult to achieve. For one thing, the distinction between “common” and “brand-specific” is too vague. There is rarely a single inflex point where you can draw the line. So often, you will have either too little “common” code and end up duplicating features in the “brand-specific” section, or the opposite – you will have too much specifics in you “common” code, leaving you desperate to find ways to change that behavior depending on a version.

So unless you have a really good idea of how you might separate the code and how new requirement might affect that idea, I think you should treat this approach with a lot of suspicions.

Build Targets

This is Xcode specific, but every IDE should have a way to achieve something similar. As you probably know, an Xcode project can have several targets. The ones you usually see, are targets for the application, for unit tests and for UI Tests. But of course, you can add your own. Each target has different build settings and can work with a different subset of source files. What makes targets perfect for app branding is the fact that you can use different info.plists in order to have different images, bundle IDs etc, different source files (you can choose which source file is included in which targets), different artwork, different Interface builder files etc.

This makes it really easy to have different resources for your different app variations. You can even have files with the same name, but included in different targets. In fact, this is a common solution to branding problems. On the other hand, managing which item goes to which target is a bit error-prone. Especially after a while, when app branding is not something you actively think about during feature implementation, it is easy to make a mistake which is going to lead to obscure build error or even worse, crashes and bugs.

So, even though targets are a powerful tool, they have their caveats so you should take special care while using them.

There’s still a lot to discuss about build targets, however most will come up while discussing the next app branding techniques. So lets continue…


The idea behind subclassing is simple. You create a generic (maybe even abstract) class that implmenets an interface for a particular feature. Then, you create a subclass for every app variation you need. This subclass actually implements the functionality necessary for that variation. Of course, then you need to make sure the right subclass is used. It might be some sort of factory method, or what we just duscussed – targets.

At first, subclassing seems like a perfect option. In fact, that’s why object-oriented languages have subclassing – to extend a base implementation. But upon closer examination, you often see that this is not true. To the contrary – you are going against OOP principals. Especially Liskov’s substitution principle. Extending the class quickly turns into slaughtering the base implementation and constantly trying to do something completely different in the subclasses. At some point every class in the hierarchy is so different that the only reason for them to co-exist is because they should get called at the same locations in code. And that’s where it gets ugly.

At least that’s my experience. Maybe I’m doing something wrong, but up until now I’ve failed to see a well working app branding solution that uses subclassing extensively.

That being said, subclassing is sometimes unavoidable. And it can generally be ok as long as it’s kept to a minimum.

Another variation to the subclassing technique (that I like better) is using classes with the same name, but in different targets. This is now especially easy in Swift since you most often don’t need import statements. What I like about this variation is that it completely ditches the idea of inheritance. It acknowledges the fact that those classes are not really similar so you don’t have to constantly fight against inheriting some base functionality. It’s just 2 (or more) classes with completely different implementation, that happen to have the same name.


Configurations are not like the previous techniques we discussed. In fact, they often rely on subclassing and build targets for their functioning. The basic idea is to hold constants that toggle functionality and hold values for properties, specific to an app variation. But that implies having different configurations for each variation. And to differerentiate between them, of course you need something like targets and/or subclassing.

On the implementation level, a “configuration” can be, for instance, a XML, JSON, plist file (essentally a resource file) or a source file full or constants (in Swift probably a file full of structures and enums). Then, the easiest way would be to have the different files only in their respective target.


Now we are getting to the good stuff. And also the bad stuff. Definitely the interesting stuff.

For a general overview of what UIAppearance does, check out the UIAppearance reference page. But in short, UIAppearance is a protocol that allows you to change the default values lots of the UI classes in UIKit use to visualize themselfs. The way that works is that you, for instance, specify that the text color for UILabels is green. And from then on, every UILabel instance that is created, gets that green color by default. Pretty neat!

Note: UIAppearance sets the default value, but if the same property is changed after that, the value gets overriden. And while this rule makes total sense, it is sometimes annoying and creates subtile bugs. For instance, if you set the text color of a label in Interface Builder, the UIAppearance value is overriden because Interface builder applies its changes after the styling from UIAppearance is done. You need to make sure to leave the default option in Interface Builder.

UIAppearance is really nice for applying application-wide values. Much like setting the tintColor value in your root view, but much more flexible and complicated. And that’s what makes it great for branding. Especially if you what to change your app’s theme color, UIAppearance is a great option.

The good

Think about it – if you had to change a color or something similar in your whole application, it would be a nightmare. Tracking down every view that uses it, always mssing some cases. With UIAppearance, you only need to set that color once, and then it will apply for every instance of that particular class after that. Something as simple as this:

UILabel.appearance().textColor = themeColor

You can even go one step further and extract those values in a resources file – a plist for instance. Then, as your application is loaded, you can read that file and apply the styling. That could be really nice, although I don’t think a situation where you can give that resource to your non-tech colleagues and have them change it themselves, is realistic. I just think that at least a minimal tech background is needed to grasp the concept of what works and what doesn’t.

The bad

UIAppearance is no doubt a great idea. But it’s not for the short tempered, at least not in the beginning. There’s just a lot of subtleties around it. As I mentioned, UIAppearance properties get applied to all new instances of the particular class. But anything that works on those same properties after that, will override the UIAppearance values. And while this is definitely the right behavior, it is not always what you want to happen. This often leads to obscure bugs. So at the start, you will find yourself juggling between UIAppearance, Interface builder and code to figure out why something is not properly applied.

Another common issue you will encounter, is applying a rule too liberally. Sometimes you think a property is universal and should be applied everywhere, but in reality in turns out there are exceptions. Your first instinct might be to override that value after UIAppearances are applied, but that’s also not ideal. It’s just too implicit and hard to maintain. Luckily, this is a place where subclassing can help!

Subclassing and UIAppearance

I previously “preached” using subclassing with caution, and yet here we have a whole section dedicated to that. What makes UIAppearance different?

Truth be told, my previous statement still holds – you should still avoid subclassing whenever possible. However, it can still be quite helpful when using UIAppearance. Lets return to the small example above:

UILabel.appearance().textColor = themeColor

It seems harmless, but I would argue that UILabel is often a terrible candidate for UIAppearance. Even if you want to apply an overall theme to your application, there are usually labels that should have a different style. Sometimes, elements around those labels need to be branded, but the text should remain the same. For instance, you might want to brand a navigtion bar in red, but the custom navigation title inside should be white for contrast.

This poses a question. How do you handle UI elements that don’t have a single style, but several?

Since UIAppearance uses classes to differentiate between elements, a possible solution would be to make those elements instances of different classes. Getting back to the example, it would change to something like this:

BrandedLabel.appearance().textColor = themeColor

BrandedLabel would be an UILabel subclass that is actually quite empty. It needs to behave exactly as a normal label, but it… kind of need to be not UILabel. After that, you only need to make sure all labels that need to change are BrandedLabel instances.

It’s not super clean, but gets the job done with not a lot of added complexity. Empty subclasses are not a really nice architecture, and I’m sure they violate some OOP principles. However, they hardly make the code harder to read, especially after you understand their purpose and learn to ignore them.


App Branding is an inherently difficult problem to solve. There’s no perfect solution and it’s almost always a pain. I cannot claim that the techniques we discussed here are the right one. Definitely they are not an exhaustive list of solutions.

They are just a collection of items I found to be useful and have helped me a lot on the past. At the end of the day, every project is different and calls for different strategies. So you will have to do a bit of experimenting. Good luck!

iOS/Mac open source world

This post is part of Issue01 of the DevMonogolue Magazine. Whereas the following post contains all the information, you might consider looking into the whole issue for better formatting, context and additional articles.

In this article, we will delve deeper into the world of open source projects, specifically for iOS and Mac. And by that, I mean mostly iOS. On Mac, many of the really good open source applications are actually cross platform and more native to Linux than Mac OS.

Going to GitHub and starting a search for either Swift or Objective-C yields mostly frameworks and chunks of reusable code. Whereas this is really useful to other developers for learning and sharing, here we will focus on complete applications (with some exceptions). Something you can actually run and see.

Lets get started and see some prime examples of the Apple open source community:


Artsy is definitely an exemplary company in regards to open source. Most of what they do is out in the open. It’s always on GitHub and it’s always free and open source. They even use the MIT license, so you are free to do whatever you like with their code.

It probably takes a lot of courage to let go of all your intellectual property like Artsy does, but I guess it worked out in the end. Even regarding this kind of structural aspect of the company, they are really open to sharing. You can find all kinds of information on their blog about virtually any aspect of their operation.

For example, you can find their thoughts on going open source in this article. They bring up a lot of good points regarding the threats and opportunities of open source software. Having a management team with a technical background definitely helps selling the idea, but the text makes a very good argument about it.

Companies revolve around ideas, and understanding what your core value is important. A company who make money purely off selling their apps could be easily copied, and OSS by default won't work for them. Artsy is a platform, but OSS by default can work for us because a technical platform is just one aspect of what we offer.
-- OSS expectations, Artsy blog

This is a really solid point, especially regarding mobile. Users are expecting to get apps for free and paid applications are getting less and less popular. Most companies don’t really expect revenue from the App Stores. Their applications are solely there to complement their business. So why hide it? It’s a code base specifically tailored for a propriatary service in mind and doesn’t give much (if any) advantage to your compatition. Unfortunately, looking at the majority of apps in the AppStore, most companies don’t seem to share my vision.

Another great contribution from Artsy to the open source world, is the way they segment their code into libraries and more specifically how they manage those library’s lifecycle. Getting a chunk of code and making it reusable by turning it into a library is a great way to encourage other developers to integrate it for their own projects. But you can also say it’s more about code quality than being open and sharing knowledge. And that’s surely true. By no means is open sourcing only benefiting others. By thinking about the community, you also better stucture your own code. And that by itself is quite powerful. This is actually something the Artsy team talked about in their How we Open Source’d Eigen article.

3 months on the way we operate has changed. We're a lot more organized, and the Eigen repo is easily the most well run project on the mobile team. It has active milestones, that represent long term goals and the current sprint. We discuss a lot of the interesting cultural choices publicly on issues and in our mobile team repo. Having this app in the open, and the experience of doing so has also improved our workflow on other apps.
-- "How we Open Source'd Eigen" post, Artsy blog

I guess people start working differently if they know someone could be watching. It’s probably the same difference between working alone in a dark room with your monitor facing the wall and working in the middle of a huge open office with everyone looking into your screen. It’s just harder to hide something embarracing so you tend to avoid it.

But going back to extracting code into frameworks, Artsy does something that would make many managers cringe. When a developer commits to creating a library out of some pile of code, they do so in their own name in GitHub. Effectively, this developer owns the code, not the company. It sounds strange at first, but first of all the code is open source either way, so everyone can use it. And secondly, their idea is that even if that developers decides to leave the team, he is still the owner of that library’s code. So maybe after resign, they will continue working on the code base. And since it’s 100% open source, Artsy can still use it. Awesome! You can read all about that in Issue 22 of

So, in conclusion, there’s a lot we can learn about open source from Artsy. It’s an excellent example of a company that is not scared of open source, but actually leverages it to create a dynamic and vibrant development environment. I didn’t write much about what their business is about, but I encourange you to [go to their website][Artsy website], download their apps and start using their great technology.

Companies like Artsy can show us a lot about monetarising open source software and doing business in the open. I hope it inspires people to be less scared of uploading their code for everyone to see. But that’s not what open source is entirely about. It’s about the little people. The developers that are driven by desire to create something for themselves but also shared it with others so that everyone can use it. The projects that are not made because of money, but because writing code can be fun. Which brings us to another amazing project:


Are you using a password manager? If not, you should. A lot of people nowadays use 1Password for that, but an older, and more open source alternative, is KeePass. So, why would you use it?
* It’s cross platform
* It’s secure
* It’s awesome

And why would you not use it?
* It’s not as flashy as other solutions
* Doesn’t sync your database (at least out-of-the-box)

Without going too deep into a review of the product, KeePass is a really popular, cross platform password manager solution that I personally use and love. But one question remains… does cross platform also include mobile? That’s where MiniKeePass comes in!

MiniKeePass is a mostly independant (from KeePass itself) project that brings the popular password manager to iOS and Android. It’s almost exclusively written by Jason Rush and John Flanagan. It is released under GPL (General Public License). If you wonder what that means, refer to the Open Source Licenses article of this issue. Basically it’s a relatively conservative license that protects the code from being used commercially in closed source projects.

The projects is not in very active development, but it is by no means abandoned. it receives updates with fixes whenever an iOS release breaks something and also gets support for new features as the iOS operating system evolves and gives applications more freedom.

Apart from the usual benefits of an open source project, I think MiniKeePass is an excellent way to learn a little bit about security and encryption. Just clone the repository and see how a world class password manager protects your accounts.

Artsy and MiniKeePass are both great showcases, but they miss one really amazing phenomenon associated with open source software. The community! One of the most powerful aspect of open sourcing your code is to leverage the power of the community and start receiving contributions from all over the world (and become famous among nerds). However, MiniKeePass is a too small project, several developers is all it needs and Artsy is too tied to its own services to benefit the people so much that they start making many pull requests. Of course, both projects have contributors, but it’s not enough for a whole community.

And now, you guessed it! We can going to look into projects with a community around them (which are also nerdy enough to get my attention).


Kodi logo

Kodi, or XBMC, is one of those projects that is so big that you don’t know where to start. At its core, it is a media center / home theatre software. Or at least that’s how it started. Howadays, it’s full of so many features, that it is difficult to explain. It’s compiled for almost all the platforms you can imagine, iOS included (but for jailbroken devices mostly). As far as I remember, it also had an AppleTV version several years ago (Yes, before tvOS). Kodi supports all the codecs and protocols you can imagine. It plays radio, tv, connects to (or creates) a DLNA server and is completely extendable and skinnable.

Kodi multi device screenshot

And it’s all 100% open source. A truly amazing project. Millions of people around the world are using it and looking at their GitHub repository, they have (at the time of writing) 480 contributors. Of course, that’s across a lot of platforms, but it makes it even more impressive. It is a whole ecosystem of enthusiasts from different backgrounds, joining forces to build something beautiful. I strongly recommend looking into Kodi’s organisation. The project is really commited to establishing a strong culture between its members. It’s an exemplary work of maintaining projects descriptions, product landing pages, blogs and forums. Right it the README file, people can find all the important links to get them started, they have specific resources for the ones that want to start contributing and you can find all the information you need in the forums.

It is no doubt that a community doesn’t just happen, it’s built and maintained. And Kodi is an ecosystem that shows how that’s done.

To some, open source software has a reputation of low quality. And unfortunately, often you cannot expect the same level of polish you’d find in commercial products. But this is not the case with Kodi. It is a mature project that not only manages to bring a large number of features to its users, but also delivers them with unprecedented quality and taste. You will hardly find something that doesn’t work well or is not really well thought out.

Kodi’s code is released under GPLv2. It’s not the most permissive license but working with some many dependencies – codecs, network protocols, plugins, it is probably hard to use a more “relaxed” license.

Overall, Kodi is a much better option for your home entertainment than most (actually all) Smart TV operating systems. If you are trying to upgrade your home theatre, get an old PC or a Raspberry Pi and setup Kodi on it. Then downloaded one of the many Kodi Remote mobile applications (I definitely recommend the official one for iOS) and get started.

Kodi Downloads

Do you remember how I wrote that we are only going to discuss complete applications earlier in this article? Yeah… I lied a little. There is one framework in particular that is really close to my heart. A framework that saved me so much work recently and was constantly amazing me how well design it was. So naturally, I have to write something about it.


CorePlot logo

CorePlot is a 2D graph and chart drawing framework built for iOS, Mac OS X and tvOS. And one of not so many, I’d say. Most alternatives I’ve seen are targetted towards a specific plotting task and are not very generic and reusable.

CorePlot on the other hand, supports a wide range of graph types (even pie charts) and each graph is customizable in almost every aspect. And with a current version of 2.1, it is a quite mature project.

Discussing CorePlot right after Kodi, the question of community arises naturally. The project is, I think, popular, but it definitely doesn’t have a whole culture around itself. It only has 20 contributors in GitHub so far. But that doesn’t mean it is not welcoming to newcomers. Looking into the project description and documentation, there is very little to complain about. All the essential information like the license, contribution guide, important links and installation tutorial are very well explained and easy to follow. The documentation is auto generated with Doxygen and it leaves a lot to be desired, but the project contains a sample application which is really excellent. It guides you through most of the framework’s features. If you want to implement something, just find a graph within the sample application, lookup its code and copy it. That simple!

CorePlot sample project

The code is released under an MIT license, unlike many of the projects we saw so far, which is very cool of the authors.

So what sets CorePlot apart from others? We already saw several extremely good projects. Artsy was able to build a successful software business “in the open”. MiniKeePass was an application that extended the legacy of the core KeePass community and brought an excellent password management utility to the mainstream mobile platforms. And Kodi was an entire software suite bringing all the features you might imagine into your living room’s enterntainment system, and also building a massive community of loyal users along the way. What could CorePlot possible bring to the table to compete against the others?

One of the most important aspects of code reusabilty is good architecture, intuitive interface, loose coupling and high cohesion. Obviously, if we want to use the same code in different settings, it has to be abstract and very well segmented. But that’s easier said than done and all developers know that. it’s more of an art than a process. Many have tried and even more have failed.

Looking back at the specific case of CorePlot, the problem that framework is trying to resolve is inherently difficult. There are lots of components in a graph and there are more types and styles of graphs that any of us can imagine. So how do you create a library that’s, at the same time easy to comprehend, fully customizable and well structured? Well, I have no idea… but it seems the CorePlot authors sort of do.

The framework itself is so big and the architecture and core principles behind it so deep that it would be impossible to explain in this article. In fact, it really deserves a whole dedicated issue (coming soon?).

What I can tell you right here is that the framework does its best to give you freedom to access and customize every element in your graph and still keep a consistent inteface in order to keep the API managable and relatively easy to understand.

Not going too deeply into that, the way they achieve that is by incorporating a “style” object into most elements that you see on screen. These styles have mostly the same interface, so you always know how to change a color or increase a font size. And if that is not enough, often you have on option to substitute a CorePlot element with your own CALayer, effectively injecting your own visuals into the graph. This can be a lifesaver when your design requires something quite unusual and custom made.

And while we are still on the topic of layers, CorePlot is entirely CALayer-backed, which makes the graphs highly interactive and animatable.

Overall quite an enjoyable and useful framework. Even if you don’t necessarily need graphs in your app, it is still worth the time checking CorePlot out and thinking about all the design choices made there. It can serve as an inspiration and example next time you need to layout the architecture for a new app.


There are, naturally, many other really nice open source applications for iOS and Mac OS. However, it doesn’t make sense to talk about all of them here. Additional projects that I was considering writing about include WordPress for iOS, Wikipedia for iOS and Hammerspoon – a desktop automation tool for Mac OS. All of those have an interesting story to tell and are an important addition to the open source world, showing that you don’t need to keep your code all to yourself and still get the job done.

In in order to cut this article short before it gets too long, we will not be covering those projects. I hope that the ones we discussed were enough to get you inspired and encourage you to develop in the open. Unless you have something really special to hide, it is probably worth thinking if you can benefit from sharing your code with the world, both for the sake of helping others, and also helping yourself. Even if you cannot build a community around your product, keeping everything for the world to see, might have a highly positive effect on your business. As they say:

You can write much better code with a monitor facing the entire office than if you are stuck alone in a dark basement.

Open source licenses

This post is part of Issue01 of the DevMonogolue Magazine. Whereas the following post contains all the information, you might consider looking into the whole issue for better formatting, context and additional articles.

This article is about the different available open source licenses and how to choose the one you’re comfortable with.


As a developer, trying to give something back to the community, the first encounter with open source licenses can be overwhelming. There seems to be an endless selection to choose from and it can sometimes be difficult to distinguish between options. Of course, you can alway make your own license, but I’m going to assume most people don’t have the expertise and/or the desire to do so. Leave it to corporations.

The number one thing you should be interested in when choosing a license is how open you want your code to be. More specifically, what are the rules others need to follow if they want to use and/or modify your code.

Commonly used licenses

What better way to explore the world of open source licenses, than going back to the origin – The Open Source Initiative. Heading over to the licenses section of this website, we can see a number of “popular” options.

OSI licenses image

Here’s the list at the time of writing:

  • Apache License 2.0
  • BSD 3-Clause “New” or “Revised” license
  • BSD 2-Clause “Simplified” or “FreeBSD” license
  • GNU General Public License (GPL)
  • GNU Library or “Lesser” General Public License (LGPL)
  • MIT license
  • Mozilla Public License 2.0
  • Common Development and Distribution License
  • Eclipse Public License

What you might find strange here is that you can see company names. As mentioned, you are free to create your own terms, if you have the money and expertise to do that. So some big companies, like Mozilla in this case, do that, but for most people choosing an existing one is fine.

Choosing a license

If you read around the OSI website a little bit, and open the page for some of the licenses shown above, you will notice that, even though it is legal text, it’s not really hard to read. And quite short. For instance, the Apache License has 9 relatively short sections. So I encountage every single one of you to actually read at least the most popular ones.

When choosing a license, there are mostly several things to think about.

First, and definitely most important, are the rules for using the code (and any documentation, configurations files and whatever else you have there). What are the rules, other developers need to obey, if they want to use your hard work. Surely, since you open sourced your code, you welcome people to enjoy it, but still you need to decide how open you want to be about this. For instance, the MIT license, which is a popular choice for “liberal” developers, permits almost every kind of usage scenario. The only thing you cannot do, is claim the code is actually yours. On the other hand, the GNU General Public License allows you to use and change the source, but only if you, in turn share your code and any changes you might have made. So basically this limits the projects appeal for commercial products. It is a fairly popular choice in the Linux world, where people promote free and open software.

Most of the other text within a open source software license is about liability and contribution. Here, there is less to choose since most options are relatively the same. They tend to disclaim any sort of liability obligations. And for good reason. You were happy to share your work with the world, but do you really want to be held responsible for problems that might arise in other people’s projects? Probaby not. So OSI comes to the rescues and makes sure you don’t run into those kinds of problems.

Another liability topic, licenses handle in a similar way, are contributions. If you share your code with others, the best this that could happen is to start getting code back in the form if bug fixes and features. And surely, you think that’s great. But what does your lawyer think? What if tomorrow someone claims that their code is in your project so they should have a piece of the fame? Thankfully, open source licenses don’t allow that. All popular ones claim that contributions that you might get are free of charge and are yours for the taking.

Something cool I found in some licenses (The Apache), is section 9 called “Accepting Warranty or Additional Liability”. It allows you to bring support and consulting to your business model for open source projects. This is something that many companies like Canonical and RedHat already do. It means that whereas the code is free, not providing any type or warrenty, and might actually not allow you to used in commercial project (not the case with Apache), you can still charge money in exchange for support and accepting warrenty.

So the elephant in the room of choosing a license is actually what we discussed in the first paragraph – do you want people to start using your code for profit. It’s to some degree a philosophical question, but also your decision might change from project to project. What I’m seeing in other people, and it also happens to be my personal opinion, is that for small projects and pieces of code, you should choose a more “liberal” lisense. The MIT license for instance, is a great choice in that matter. The truth is, if your project is just a few files of code, who cares if it’s used by an evil corporation. And even if you protect it with a strict license, like GPL (General Public License), the code base is so small, a developer can re-write it base on your ideas and solutions. For bigger projects, however, let’s say for frameworks and whole applications, you might have to weigh in your options more carefully. If you’re building this product just for fun, think about it in a “political” way. Do you accept that people might be profiting using your work, without giving anything back? Are you a strong supporter of open source? And if you’re trying to run a business with that code base, can other companies levarage that in order to gain a competitive advantage over you? Can they use your code, extend it a little without in turn sharing their modifications, to take the upperhand? If so, then a more permissive lisense might not be a good choice for you.

Anyway, based on how liberal they are towards commercial use, the popular lisences you can see on OSI’s website, can be divided in two:

Open to commecial use

  • MIT license
  • Apache License 2.0
  • BSD license
  • Eclipse Public License
  • Mozilla Public License 2.0
  • Common Development and Distribution License

Open source only

  • GNU General Public License (GPL)

Note: Saying “Open to commercial use”, doesn’t mean you cannot use a GPL license for commercial products. It just means that if you incorporate a GPL licensed project in your own, you need to also open source any modification you make to the code base and the way it’s used. Surely, there a lot of loopholes, but generally, that’s the rule. And that’s what makes it often unsuitable for commercial products.

Digging deeper, beyond distribution terms, most licenses seem similar to the common programmer. I encourage you to read those licenses to see how they differ, but generally, after you choose between those two sections above, you’re almost ready.

But before we move on to “Applying the license”, there are several interesting topics many licenses address. One of them, that addresses the so called “privacy loophole”, makes sure cloud services cannot bypass their responsibility towards your code. Since many options are fairly old (GPL v1 for instance was created in 1989), they don’t address hosted services. Many companies go around strict licenses by saying that they don’t distribute the protected source code – they just deploy it once internally, and then provide it to users as a SaaS (Software as a Service). Since this is not exactly fair and sometimes unacceptable for the authors, several licenses address that issue, namely:

  • Affero GNU Public License
  • Common Public Attribution License 1.0
  • Non-Profit Open Software License 3.0
  • Open Software License 3.0
  • Reciprocal Public License 1.5

Another interesting topic is the way third parties should acknowledge their use of open source software. Normally, the rule is that you should distribute a copy of the license with your product, but that doesn’t mean it needs to be easily accessible. However, authors might choose a license that makes other developers acknowledge their ue of the open source code in a user accessible place within their software (most likely somewhere in the UI). That’s called enhanced attribution. This time the options are:

  • Adaptive Public License
  • Affero GNU Public License
  • Attribution Assurance Licenses
  • Common Public Attribution License 1.0
  • Reciprocal Public License 1.5

Finally, a consideration that I wish anyone of us had, is the no promotion feature. It protects authors from being used as advertisement for a product. It forbids
third parties to use the author’s name to promote their business.

What you go for is largely a matter of choice. For instance, if I wanted to just share some code with other people, I’d use MIT or BSD, since they are so simple and short.

And if I ever release a framework or an application I want to protect better, I’d go for GPL. However, as a disclaimer, I’d like to say I’m no expert so I might be missing something. For best results, just read the different options and make your own choice.

Where to go from here?

An excellent resource for choosing between open source licenses, is the OSS Watch Licence differentiator. There, you can input up to 7 parameters for your particular case and it will give you a list of licenses that that suitable for what you are trying to achive. I recommend that go go check it out. Just explore a little, start playing with the values and you’ll become a open source pro!

Applying a license

When I wanted to release some code “to the wild” for the first time, and I thought about licensing, I started wandering how I can obtain one. Surely, if you want a ligally binding doocument for your project, you need to talk to someone. At that point, several years ago, I remember it wasn’t plainly obvious how you can apply a license to your code. A simple search didn’t yield any useful answers immediately. But soon enough, I found it. The only thing you need to do is… specify which license you’re using somewhere in your project.

Also, if you look into any of the license pages at OSI, you will see at the bottom a should appendix explaining how to apply the license to your code.

APPENDIX: How to apply the Apache License to your work

To apply the Apache License to your work, attach the following boilerplate notice, with the fields enclosed by brackets “[]” replaced with your own identifying information. (Don’t include the brackets!) The text should be enclosed in the appropriate comment syntax for the file format. We also recommend that a file or class name and description of purpose be included on the same “printed page” as the copyright notice for easier identification within third-party archives.

   Copyright [yyyy] [name of copyright owner]
   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   See the License for the specific language governing permissions and
   limitations under the License.

I guess at some point GitHub (the popular hosted repository website) realized as a major problem that developers didn’t know to apply a license to their code and added a license selection step to their “new project” wizard.

To summarize, the only thing you need to do in oder to apply a license to your project is to add that text somewhere in your code. Most of the time, it means adding a LICENSE.txt file to your project and maybe add a comment to the begining of your source code files. For the actual text, refer to your license of choice’s webpage.

Importing other people’s code

What about the opposite? If you had a library you wanted to use, what do you need to do to acknowledge the authors?
Well, normally you need to specifically state that you’re using someone else’s code. And also add their LICENSE and NOTICE files. However, not many people realize, this doesn’t need to be that complicated.

As already discussed above, as long as the code is not proctected by a license with an enhanced attribution policy, all the law says is that these things need to be shipped with the actual product. So for mobile appilication, it is enough to put those files in the package that you upload to (and people download from) the Store. Yes, it’s not easily accessible, but a copy of all that is included in the product. Also, many applications choose not to include the actual acknowledgements for open source code that they are using, but to provide a link to their website where they enumerate all the projects and authors.