iRobotlegs is an Objective-C/Cocoa Touch port of the popular Robotlegs MVCS Mircro-architecture for Actionscript 3. It aims to stick as closely as possible to the original framework without fighting against Cocoa-Touch. It should be familiar enough for someone who has used the original framework to find their way around easily, but it should be easy to understand for someone familiar with Objective-C but not the original framework. Where Robotlegs uses SwiftSuspenders for its dependency injection needs, iRobotlegs uses a modified version of the excellent Objection framework. There is an example project; 'FlickrGalleryBuilderMK2' in the examples folder and I aim to add more examples as time allows.
Aims and Benefits
The aim of iRobotlegs is the same as that of robotlegs; to make application development easier and more consistant accross projects, to encourage clear separtion of the application into Model, View, Controller and Service regions,to encourage and enable loose coupling between these regions through the use of dependency injection and Event/Notification based communication, and to lower the cognitive hit from appraising or maintaining understanding of a growing project through consistancy. As with most similar frameworks it is not always the best answer, and for trivial applications it will be overkill. A more concrete list of benefits are:
- Bannish Singletons.
- Minimise dependencies between ViewControllers (Particularly important when writing reusable elements). You no longer need to pass data from viewControllers A-B-C-D just to get it to D.
- Encapsulate functionality in commands, minimising duplication and centralising application flow.
There are two primary differences between the way the framework is implemented in Cocoa Touch and Actionscript. Robotlegs makes heavy use of the DisplayList and events bubbling up through the DisplayList to the Application Root. This bubbling allows Robotlegs to watch for views being added and removed from the stage indirectly and automatically. If a view is added or removed, Robotlegs will hear about it. Whilst Objective-C uses a composite view structure, events do not bubble up and out. This means that the framework needs to be actively informed of relevant view activity through an NSNotificationCenter. Given the role that ViewControllers play in Cocoa Touch, this is not so much of a problem as it would at first seem.
ViewControllers are an interesting animal and perform a wide variety of functionality, both View-facing and Model-facing. A ViewController is responsible for its view's creation and destruction, aswell as managing its lifecycle. In many cases a ViewController may deallocate its view and allocate a new view to take its place numerous times during its own lifecycle to free memory when the view is not in use and recreate it when it is needed again.
At first glance they seem perfectly placed for notifying the framework that the view is being created or destroyed, however direct mediation of the view bypasses the ViewController, leaving it out in the cold. The ViewController is tightly coupled with the view/xib not only through shepherding it from birth to death, but also through outlets and actions. Through this two-way communication with its view (and the components it contains) the ViewController is essentially mediating it. This means the natural boundry between View and Framework that exists in Actionscript is blurred. At the same time, the ViewController is often used as a Delegate for View Components, tying it tightly to the view.
This leaves us with two options, both of which are currently supported by iRobotlegs. Firstly, we can remove the separate Mediator class from the framework altogether, knowing we already have mediation carried out by the ViewController. This means the ViewController must have access to the framework, fulfilling the role of a Mediator, interpreting view events to the framework and framework events to the view. Secondly, we can Mediate the ViewController. Though this effectively results in dual Mediators (a Mediator mediating a Mediator), it has the benefit of removing any need for the ViewController to have access to the Framework. There also seems to be more need for a separate Mediator when the ViewController in question is a Container ViewController, especially one that is responsible for switching between views (eg. UITabViewController, UINavigatorController). I think this is the aspect of the framework that has the most potential for improvement. I have added support for UIStoryboard, meaning you can pass the Context NotificationCenter to it and have it inject this into every ViewController it creates that has a 'notificationCenter' property.
Objective-C has no support for meta-data or any kind of annotations, so I've echoed Objection's use of macros to allow for a kind of annotation of classes. Effectively the single iRobotlegs Macro wraps one Objection's macros. Whilst this approach isn't ideal, there is no other option available before removing the dependency declarations from the classes altogether and declaring all dependencies in some sort of configuration file. This is something I might look at later on down the line, but for the moment a macro which directly registers chosen properties with Objection is a good-enough solution. Unfortunately Objection does not support optional dependencies and there is no way I can add it at the present time. However I have spoken to Objection's author; Justin DeWind who is going to look at supporting this functionality in a forthcoming release.
For a quick look at framework setup take a look at AppContext in the example application. There is also comprehensive documentation for Robotlegs over at http://www.robotlegs.org/ and an excellent book on the framework published by O'Reilly. There are some basic diagrams in the Documents folder which might help if you are new to Robotlegs.
The example FlickrGalleryBuilderMK2 is a simple application that allows you to search Flickr for images uisng a keyword. You can then accept or reject each of the photos returned, building a gallery of your accepted images. These galleries remain editable and are persisted using CoreData. Please note you will need your own Flickr API key and Secret which you will need to enter into the file FlickrAPIKey.m. Keys are available if you sign up to a free Flickr account.
At this point, the most important thing is to get some feedback from people who are using Cocoa-touch to help shape the Framework, however my primary goals at present are:
- I want to focus on improving the view tier, whether supporting Mediators or using ViewControllers as Mediators.
- Once I feel that the framework is fairly stable I will fully document it.
- I plan on moving over to using Kiwi for testing.
- Support for Sparrow Framework
- Full support for UIStoryboard
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