So, here’s another something that we’ve been working on.

Captain is a lightweight library that lets you add hooks to your code, using JavaScript. We designed it for two reasons:

• If you’ve got a bunch of objects, each needing a small amount of custom objects, creating a subclass for each variation is cumbersome, and devising a sophisticated data-driven architecture is often overkill and sometimes impossible for your use case.
• Extremely rapid iteration, skipping the compile/install phase, is awesome.

Many of the games we make involve a lot of special-cased behaviour. Like all halfway decent developers, we try to minimise this, but sometimes you really just need a sprite that walks two feet left, three right, and does a twirl, and there’s nothing else in the game that you can generalise into a nice, reusable system.

We’ve already found some great results in allowing field-testers and non-programmers to make modifications to iOS applications by exposing certain resources to iTunes File Sharing. For example, if you’re writing an app that has a lot of sound effects, it makes your sound designer’s life a heck of a lot easier if they can open up iTunes and drop in a replacement sound file, and see how it sounds in your app without having to get another build.

What we ended up doing was using JavaScriptCore, which is the built-in JavaScript runtime on iOS, to create a very loose binding to Objective-C, and then allow Objective-C objects to delegate parts of their behaviour out to JavaScript.

Captain isn’t a scripting bridge. This means that it’s a slightly different thing to libraries like the excellent jscocoa. Most scripting bridges allow you to write entire apps in other languages, which is super cool. However, we really like Objective-C, and find that using the native language for the iOS platform is the best way to approach most things.

Captain, therefore, lets you expose very specific parts of your application’s behaviour to scripts, effectively creating a domain-specific language for your app. Plus, it’s all designed to be super lightweight.

An important note: Apple’s rules say your app isn’t allowed to get scripts from the outside world and run them. This means that you can’t use Captain to build a plugin architecture for your app, but you can use it during development to make your edit-build-compile cycles faster and more flexible.

How to use it

The most basic use case for Captain is when you want to run a JavaScript function, and get back whatever the function returned.

When you work with Captain, you first create a JSContext object, which serves as the execution environment for all of your scripts. You can create as many contexts as you like, but each one is kept separate from each other, and variables won’t be shared between them.

Creating a JSContext looks like this:

Once a context has been set up, you can give it JavaScript code to execute. You do this using the evaluateScript:error: method:

This method returns whatever the result of the script you passed in was. If the JavaScript code threw an exception for any reason, the error variable will contain information about it.

Any values returned by the evaluateScript: method are returned as Objective-C objects, and are automatically converted from their JavaScript type into the most appropriate Objective-C type. Strings are turned into NSStrings, numbers become NSNumbers, and JavaScript objects get turned into NSDictionary objects.

Calling native functions from JavaScript

Basic stuff, right? However, the fun stuff happens when you give the JavaScript code some native functions that it can call.

You can add register functions in the JavaScript context that scripts can call, by creating a block object and giving it to the JSContext. These block objects take one parameter, an NSArray that contains the parameters that were passed in from JavaScript, and return an id, which will be automatically converted back to a JavaScript value.

Example time!

Once you’ve added a function, you can call it from JavaScript.

Calling JavaScript functions from native code

Things get really interesting when your native code can call JavaScript code. Because you can load the new code at run-time, or even replace code without having to re-launch your app, you get a crazy amount of flexibility.

If you run some JavaScript code that returns a function, it will be returned to as a JSFunction block, which can be called just like a function.

Working with native objects in JavaScript

Working in only numbers and strings is boring. What would be really awesome is if Captain could let JavaScript work with Objective-C objects in a clean, simple way that had zero programmer overhead, and let the developer write clean code in both languages.

Wait, you can? BONUS.

Any property that uses the standard compiler-generated setter and getter methods can be modified by JavaScript code.

In addition to working with data, you can also call methods on Objective-C objects from JavaScript.

But doesn’t Objective-C have crazy weird method names that look ugly in other langauges?

That’s right, it does. That’s part of the reason why Captain only exposes certain methods to JavaScript.

If your Objective-C object has a method named handleFoo:, which takes one parameter, it will be exposed through JavaScript as “foo”. Only methods that meet these requirements are callable from JavaScript.

The reasoning behind this is that Captain is intended to be a simple way to expose limited amounts of domain-specific functionality to a scripting environment, rather than a comprehensive scripting bridge. By only having to support a limited amount of interaction between native code and the script, life is made easier for both worlds.

Example! Here’s an Objective-C object:

And here’s code that interacts with it:

Loading scripts

The final piece to this is in loading a large number of JavaScript functions, which native objects can use.

Let’s say you’ve got a JavaScript file named “UsefulFunctions.js”, and it contains this:

You can register the entire set of functions contained in the script using the loadScriptNamed:error: method.

Doing this causes the JSContext to look for the file “UsefulFunctions.js”, first in the Documents folder, and then in the built-in bundle resources. Because it looks in this order, you can modify the app’s functionality by simply dropping in a replacement file with the same name in iTunes File Sharing, and re-launching the app.

Delegating behaviour to JavaScript

The functions that you load in can also be used by Objective-C objects to delegate some of their behaviour to.

For example, say you have a JavaScript file ‘Foo.js’, containing the following code:

Once Foo.js is loaded in, you can call the function, and additionally provide it with an object to use for the this variable.

With this method, your classes can easily call out to JavaScript when they need some work done.

Conclusion

We hope you enjoy using Captain, and we’d love to see what else you do with it. Captain is on GitHub! We welcome pull requests, and if you’ve got any questions, shoot us an email at lab@secretlab.com.au

Enjoy!

Jon from Secret Lab presenting at Swipe Conference 2012

Last week in Sydney was the second ever Swipe Conference –– an Australian iOS and OS X developer event. Secret Lab was again in attendance and, as always, the Apple developer community was great fun to hang out with and learn from –– and the organisers, Jake MacMullin, Mark Aufflick and Sean Woodhouse really put on a fantastic event. We were also again fortunate enough to have a chance to contribute to the conference (Paris spoke at the first ever Swipe Conference, in Melbourne last year), with Jon presenting Cocos2D for Fun and Profit, a quick guide to the Cocos2D graphics library. Designed as a fast introduction to this time-saving library, this talk leads the audience from a minimal starting point to a full game.

We're incredibly fond of Cocos2D, as it provides a very nice middle option for people who want more power than what UIKit can provide, but don't want to deal with the.. joy that is OpenGL. With Cocos2D, it's straightforward to create a scene comprising a number of quads, and even more trivial to animate these quads in useful ways.

We've uploaded the code and slides to GitHub and Speakerdeck, and encourage you to take a look! We had a number of people approach us after the talk and mention that they were now interested in making games with this library, so hooray, we're sharing the love!

We also took a lot of photos at Swipe Conference –– you can find them on Flickr.

This month we were lucky enough to sponsor and attend PyCon Australia, hosted (for the first time) in our hometown of Hobart! PyCon Australia is the national conference for users of the Python programming language. This was the third year that PyCon Australia has run since being founded in Sydney. We greatly enjoyed meeting members of the Australian and international Python communities, hearing their stories, tips and interests. All the sessions we attended were fabulous, but amongst the many highlights were:

• CodeWars – entertaining programming-as-a-spectator-sport problem solving to kick the conference off. Hosted and designed by our talented friends Tony and Josh, respectively. A great place to spot some amazing nerd gear!
• What to build. How to build it. – the first keynote, presented by Mark Ramm, took us on an entertaining romp through the idea of applying experimental methodologies to product design. Great stuff!
• Virtual robotic car racing with Python and TORCS – Python and virtual robot cars! What's not to like?
• Python for Humans – the second keynote, presented by Kenneth Reitz, was a thorough and useful discussion of making Python more user (programmer) friendly. Exactly the sort of stuff that should be a keynote!
• Lightning Talks – we always love lightning talks, and these were no exception. Lots of quick talks, mostly useful, and highly entertaining!

We also thoroughly enjoyed the dinner (a cruise to Peppermint Bay, followed by a fabulous meal and cruise back to Hobart!) as well as the coffee (which was a tasty blend called "African Swallow", designed specifically for PyCon Australia by Ritual Coffee). The coffee for all attendees was sponsored by Secret Lab, and we were proud to have our logo on every cup. The attendees all really seemed to enjoy the coffee, and we're very proud to have been involved in a small way in the running of such an excellent conference in Tasmania. PyCon Australia returns to Hobart next year as part of their 2-year cycle. We can't wait!

You can find the rest of our photos from PyCon Australia 2012 on Flickr and the videos from PyCon on YouTube. Congratulations to our friend, and frequent collaborator, Chris on the meticulous organisation he and his team undertook to bring us PyCon Australia!

We're very pleased to announce the availability of the official iPhone app for the Tasmanian events of the National Science Week 2012 festival. National Science Week is Australia's largest national festival and includes hundreds of events held all over Australia.

The app features:

• full program of National Science Week events in Tasmania
• day by day Event Calendar
• maps showing the location of events
• search by region or current location
• automatic schedule updates

Grab the app on the iTunes App Store, and please let us know what you think!

Our next scheduled general enrolment developer training is scheduled for November 23 to 25 in Melbourne, Australia. For more information, or to make a booking, head to our training page. For enquiries, please email events@secretlab.com.au

You want to write an iOS or OS X app that quickly and easily retrieves some data from a server and converts it to an Objective-C class. Also, you want to write the server using something standard, like Django or Ruby on Rails.

The standard way that you’d do this would be using NSCoder, where your client application would receive the data and you’d unpack it like so:

However, the file format for serialised objects is internal to iOS and OS X, and is also a binary format that’s very difficult to read while in transit.

We recently had a similar problem, where we wanted to have our server send data in JSON format, which we like because JSON’s pretty human-readable. We could implement our own custom implementation of NSCoder, but we were after a more lightweight solution.

Key-value coding

This is where key-value coding comes in. Key-value coding is a feature of Cocoa that lets you access properties and instance variables of class at runtime by name, rather than simply at compile time.

For example, if you had a class that had an NSString property called “name”, you could access it like this:

Or you could access it with key-value coding, like this:

Likewise, you can set the name property like this:

But you could also set it using key-value coding, like this:

The advantage of key-value coding is that you can generate the keys at run-time. Your code doesn’t have to know about the properties and variables kept inside an object in order to attempt to get and set the values.

This is where our lightweight serialisation comes in. Let’s say that we’ve received some JSON data that looks like this:

In this example, we’ve also got an Objective-C object that has matching properties:

Converting the JSON data into a dictionary is pretty easy.

You can then create the Objective-C class from this dictionary by creating a new Employee object, and then iterating over each of the keys in the dictionary, using setValue:forKey: to set the values.

This works even for non-object properties like Employee’s income property, which is an int. In this case, the dictionary loaded from the JSON contains an NSNumber object for the income field; when setValue:forKey: is used for that key, it automatically extracts the int from the NSNumber and applies it to the class.

There’s an even easier way to set all of the values:

Loading objects when you don’t know the class

Another possible use case is one where you receive a chunk of JSON, but you don’t know ahead of time what kind of object the JSON should be turned into. This is where NSClassFromString comes in handy.

If your JSON object contains a field called, say, class, you can use that to create the empty object. Like so:

The reason we use NSObject in the above example is because all NSObjects support the key-value coding methods. Even though loadedObject is treated as a generic NSObject, the fact that it was created with whatever class ObjectClass is determined to be means that the object will be what you specify.

Dangers and Caveats

This technique only applies for simple properties, like strings and numbers. More complex data types or references to other Objective-C objects need to be handled differently.

If you use setValue:forKey: on a key that doesn’t exist in the target object, the object will throw an exception and crash. There are a couple of ways you can handle this; one is to implement the setValue:forUndefinedKey: method in your classes.

This method is called when you try to set a value for a property or instance variable that doesn’t exist. The default implementation throws an exception and crashes; you can implement your own that leaves a warning, for example.

A final note: this is a very quick-and-dirty solution. If you want more reliable behaviour, you’ll need to add more checks and validation code. For simply loading objects, it works pretty well.