TweetFollow Us on Twitter

The Road to Code: Patterns in the Sky

Volume Number: 24 (2008)
Issue Number: 08
Column Tag: The Road to Code

The Road to Code: Patterns in the Sky

The Model-View-Controller Design Pattern

by Dave Dribin

Welcome Back

We've come a long way since the first Road to Code. We've covered Foundation and AppKit, the basic frameworks of Mac OS X, and we can write GUI applications. While we've covered the Objective-C language and classes that make up an application, we've glossed over a bit of history. How did Mac OS X applications evolve the way they have? Why did Apple's (and NeXT's) engineers create classes like NSApplication, NSView, and NSControl? The search for these answers takes us down a path the computing history.

Design Patterns

Ever since the dawn of computing, programmers have been trying to make their lives easier and more productive. Today we take high-level languages like Objective-C, C++, Java, Ruby, and Python for granted. Back in the day, programmers had to code in assembly language directly. Grace Hopper wrote the very first compiler in the 1950s to alleviate the pain of writing assembly language. In the 1970s, when the Unix operating system was originally being developed, the designers created their own language, called C, to help make Unix easier to port to new computer systems. C was one of the first and, in retrospect, is arguably the most successful high-level language. The benefit of C over assembly language was two-fold. First, the programmer could write code once that would run on multiple computer systems. All she had to do was recompile the C code for the target system and it would, if written properly, run without modifications. The other benefit of C code is that it was a lot more readable and understandable to the programmer. Both of these benefits greatly simplified the programmer's job.

C is still in active use today, either directly or in one of its derivatives forms, such as C++ and Objective-C. C also brought with it the ability to put commonly used code into libraries. Code - such as displaying text to the user and receiving input to the user and manipulating strings - can be re-used by many programmers and applications. The benefit is that each developer can spend their time writing their own application instead of having to re-invent the wheel, so to speak. Reusing code through libraries is also a big boon for the programmer.

But as good as reusing code through libraries is, it does have limitations. Sometimes, different applications perform many common tasks that may not be codified into libraries. Or sometimes these common tasks transcend the particular language you are using and apply to, say, any object-oriented programming languages. These higher-level reusable tasks are called design patterns. Design patterns are not unique to software development. The term "pattern" as a reusable idea originated from an architect named Christopher Alexander.

Software design has some similarities to traditional architecture and engineering, and design patterns are one of these similarities. The term design pattern in the software industry was popularized by the seminal book Design Patterns: Elements of Reusable Object-Oriented Software. The book was written in 1995 by four authors: Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides - now infamously known as the "Gang of Four". The Gang of Four (GoF) patterns book cataloged and described many design patterns that the authors used, or had observed in object-oriented applications. While the examples are in C++, the patterns themselves are still relevant to this day. In fact, there are now many books covering design patterns in specific languages, but the concepts are generally the same.

Aside from providing solutions to common software design problems, the benefit of using patterns is to create a common language and vocabulary that all programmers can use to help describe these problems. By providing a common vocabulary, anyone familiar with it will understand when someone says, for example, "this object implements the Adapter pattern to interface with the legacy database" or "you should use the Factory pattern to ensure future flexibility." The GoF book describes the Adapter pattern and Factory pattern in detail.

Obviously design patterns are a large topic and not something that I could do justice to in a single magazine article. I highly recommend you pickup (or borrow) a copy of the GoF book and read it. Even though it may be a little dated and does not apply directly to Objective-C, it will provide you with knowledge that will benefit you for years to come.

The Model-View-Controller Design Pattern

One common design pattern for developing GUI applications called the model-view-controller pattern or MVC pattern for short. The MVC pattern, while not described directly in the GoF book, is an old pattern with roots in Smalltalk from the early 1980s. It has proven such a successful pattern that it is prominent not only in modern Mac OS X applications in Cocoa, but also in Ruby on Rails web applications, and now in native iPhone applications, as just a few examples.

The MVC pattern breaks objects in an application into three roles: model objects, view objects, and controller objects. Objects in each role serve a particular function, and a single object should not perform multiple roles. The idea is to reduce dependencies, or coupling, between the components of an application. This loose coupling promotes reusability of each of the individual components.

Apple chose the MVC pattern as the basis for all Cocoa applications based on the success of the MVC pattern in Smalltalk and lessons learned in classic Mac OS. Apple used this experience to make programming for Mac OS X as easy as possible. They created the classes in AppKit and Foundation based on this collective experience, and these classes are now available for your use.

The Model

Model classes and objects are the core of your application. Think of them as your application, without a user interface. If, for example, you were developing an address book application, you would probably have objects to represent each contact, as well as objects to represent groups of contacts. Often, model objects are saved to disk and loaded in future invocations. Again, for an address book application, you would want to store all the contacts and groups on disk somewhere, so they could be loaded when your application next runs.

Ideally, model classes are completely independent of the user interface. This allows these classes to be reused in other applications and with other user interfaces. For example, you could use the same classes in a GUI, command line, and iPhone application. Or, you may be writing automated unit tests to ensure that the model objects work as desired. The reason these objects can be used in so many different situations is because they have no ties to the user interface. In technical terms, the model objects are not coupled to the user interface.

In our previous applications, the Rectangle class would be considered a model object. In fact, we have already reused this class in multiple applications. We started out using it in a command line application, and then used the exact same class when we started writing GUI applications.

The View

View objects and classes are at the other end of the spectrum. They represent the user interface and display information to the user. However, they are not responsible for storing the data they display. For GUI applications, we've already seen a number of view classes. For example, NSButton, NSTextField, and NSWindow are view classes. The NSApplication class is also considered part of the view as it represents the hub of a GUI application. Again, the main purpose of making generic view classes is to make them reusable in many applications. You could say that much of the AppKit framework is comprised of view classes, and the fact that AppKit provides so many pre-made view classes is one reason why it is easy to develop GUI applications for Mac OS X.

By designing AppKit using the MVC pattern, Mac OS X applications get to stand on the shoulders of giants. As programmers, we get to build on Apple's wealth of experience creating and designing GUI applications.

The Controller

Controller objects sit between the model and view classes and shuffles data back and forth. The GoF book describes a Mediator pattern where an object works like a real-world mediator communicating between two separate parties. The controller objects often follow this Mediator pattern.

One of the controller's responsibilities is to keep the model and view in sync. Figure 1 shows how the models, views, and controllers interact with each other. You can see the user interaction from the view goes through the controller. This user interaction is often in the form of targets and actions. For example, when a user clicks a button, the action method is on a controller class. The controller class is responsible for taking data out of the view and updating the model class. Conversely, if the model object changes without user interaction, it notifies the controller. It then updates the view accordingly. The controllers also generally act as the delegate and data source for views.


Figure 1: Model-View-Controller interaction

Hello World as MVC

Let's look at some of our programs through the MVC prism. The GUI from our original HelloWorld rectangle application is shown in Figure 2. The user interface is made up of standard Cocoa controls: a window, some text fields, and a button. These are all considered part of the view.


Figure 2: Rectangle application window

The rest of the application was implemented in two classes: HelloWorldController and Rectangle. As I mentioned above, the Rectangle class is considered a model class. The name I chose for HelloWorldController may not have made sense back then, as we didn't know about MVC, but it should be clear now. Since it performs the role of a controller class, I named it accordingly. Let's take a quick look at the interface for HelloWorldController in Listing 1.

Listing 1: HelloWorldController.h

#import <Cocoa/Cocoa.h>
@class Rectangle;
@interface HelloWorldController : NSObject
{
    IBOutlet NSTextField * _widthField;
    IBOutlet NSTextField * _heightField;
    IBOutlet NSTextField * _areaLabel;
    IBOutlet NSTextField * _perimiterLabel;
    
    Rectangle * _rectangle;
}
- (IBAction) calculate: (id) sender;
@end

This controller class has outlets to the text field, a calculate: action, and a _rectangle instance variable. Outlets and actions are generally how controllers and views are hooked up. Actions are used by views to notify the controller of a user's action. The controller uses outlets to get current values from the view and update the view to new vales of the model.

When the button is clicked and the action method is performed, the controller updates the _rectangle instance variable from the width and height text fields, and finally it updates the area and perimeter text fields:

- (void) updateAreaAndPerimeter
{
    [_areaLabel setFloatValue: _rectangle.area];
    [_perimiterLabel setFloatValue: _rectangle.perimeter];
}
- (IBAction) calculate: (id) sender
{
    _rectangle.width = [_widthField floatValue];
    _rectangle.height = [_heightField floatValue];
    [self updateAreaAndPerimeter];
}

You can see even in this simple example how the model view controller works. Data flows from the view to the model and vice versa through the controller. The controller receives a user action and updates both the model and the view to keep them in sync.

Model-View-Controller in Cocoa

You are highly encouraged to follow the MVC design pattern when writing your own applications. If you do, you will find that the Cocoa environment will help you get your job done faster. Apart from providing a rich set of view classes, there are some other benefits to following MVC. Some of the more advanced Cocoa technologies rely on MVC:

Document Architecture - Document-based applications follow the MVC pattern with your NSDocument subclass playing a controller role. When we changed our single window application to a document-based application that could save and open rectangle documents, we modified the Rectangle to implement the NSCoding protocol. By giving the Rectangle class the ability to convert itself to and from a sequence of bytes, it stays consistent with the role of a model class. However, it was the controller class that interacted with the view to perform the actual saving and loading to and from a file.

Cocoa Bindings - Bindings is a technique that allows you to remove a lot of repetitive code from of your controller classes. This is an advanced technique that in turn relies on key value coding (KVC) and key value observing (KVO). We will cover these topics in due time.

Core Data - Core Data allows you to easily create complex model objects and save them to disk. This allows you to remove a lot of repetitive code from your model classes. Again, we will cover Core Data in a future article.

Conclusion

The Model-View-Controller design pattern was identified as a way to design GUI applications that promotes loose coupling and reusability of its components. Apple chose the MVC pattern as the foundation for AppKit based on its previous success in Smalltalk. Because of this, you should design your Mac OS X applications with the MVC pattern in mind. It ensures your code is as reusable as possible, which is always a good thing, as you never know when you may need to use your model classes in another application. You will be glad you followed the MVC pattern when you do. As we will see in upcoming articles, Cocoa also rewards those following the MVC pattern.

Bibliography

Gamma, Erich; Richard Helm, Ralph Johnson, and John Vlissides. Design Patterns: Elements of Reusable Object-Oriented Software. 1995.


Dave Dribin has been writing professional software for over eleven years. After five years programming embedded C in the telecom industry and a brief stint riding the Internet bubble, he decided to venture out on his own. Since 2001, he has been providing independent consulting services, and in 2006, he founded Bit Maki, Inc. Find out more at http://www.bitmaki.com/ and http://www.dribin.org/dave".

 
AAPL
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LaunchBar 6.1 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
BBEdit 10.5.12 - Powerful text and HTML...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more

Latest Forum Discussions

See All

Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »
Baseball Battle Review
Baseball Battle Review By Jennifer Allen on August 29th, 2014 Our Rating: :: SIMPLE HITTINGUniversal App - Designed for iPhone and iPad Simple and cute, Baseball Battle is a fairly fun baseball game for those looking for something... | Read more »
Checkmark 2.1 Update Released, and it’s...
Checkmark 2.1 Update Released, and it’s on Sale for a Limited Time Posted by Jessica Fisher on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Labor Day Weekend MacBook Pro sale; 15-inch m...
B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for up to $125 off MSRP. Shipping is free, and B&H charges NY sales tax only. They’ll also include free copies of Parallels Desktop... Read more
Labor Day Weekend iPad mini sale; $50 to $100...
Best Buy has the iPad mini with Retina Display (WiFi models) on sale for $50 off MSRP on their online store for Labor Day Weekend. Choose free shipping or free local store pick up. Price is for... Read more
13-inch 1.4GHz MacBook Air on sale for $899,...
Adorama has the new 2014 13″ 1.4GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY & NJ tax only. Their price is $100 off MSRP. Read more
It’s Official: Apple Issues Invitations To Se...
Apple has issued one of its characteristically cryptic press invitations for a special event to be held at the Flint Center for the Performing Arts in hometown Cupertino on Sept. 9, 2014 at 10:00 am... Read more
Tablet Shipments To See First On-year Decline...
TrendForce analyst Caroline Chen notes that when the iPad launched in 2010, it was an instant hit and spurred a tablet PC revolution, with tablets so popular that that notebook PC sales stagnated and... Read more
SOBERLINK Releases Apple iOS Compatible Handh...
Cypress, California based SOBERLINK, Inc., creator of the first handheld Breathalyzer designed to improve recovery outcomes, continues to show prominence in the mobile alcohol monitoring space with... Read more
New 21″ 1.4GHz iMac on sale again for $999, s...
Best Buy has the new 21″ 1.4GHz iMac on sale for $999.99 on their online store. Their price is $100 off MSRP. Choose free shipping or free local store pick up. Price is for online orders only, in-... Read more
Smartphone Outlook Remains Strong for 2014, U...
According to a new mobile phone forecast from the International Data Corporation (IDC) Worldwide Quarterly Mobile Phone Tracker, more than 1.25 billion smartphones will be shipped worldwide in 2014,... Read more
Save up to $60 with Apple refurbished iPod to...
The Apple Store has Apple Certified Refurbished 5th generation iPod touches available starting at $149. Apple’s one-year warranty is included with each model, and shipping is free. Many, but not all... Read more
12-Inch MacBook Air Coming in 4Q14 or 2015 –...
Digitimes’ Aaron Lee and Joseph Tsai report that according to Taiwan-based upstream supply chain insiders, Apple plans to launch a thinner MacBook model either at year end 2014 or in 2015, and that... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.