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".

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Bookends 12.8 - Reference management and...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Apple iTunes 12.6 - Play Apple Music and...
Apple iTunes lets you organize and stream Apple Music, download and watch video and listen to Podcasts. It can automatically download new music, app, and book purchases across all your devices and... Read more
Default Folder X 5.1.4 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click on... Read more
Amazon Chime 4.1.5587 - Amazon-based com...
Amazon Chime is a communications service that transforms online meetings with a secure, easy-to-use application that you can trust. Amazon Chime works seamlessly across your devices so that you can... Read more
CrossOver 16.2 - Run Windows apps on you...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Adobe Creative Cloud 4.0.0.185 - Access...
Adobe Creative Cloud costs $19.99/month for a single app, or $49.99/month for the entire suite. Introducing Adobe Creative Cloud desktop applications, including Adobe Photoshop CC and Illustrator CC... Read more
MegaSeg 6.0.2 - Professional DJ and radi...
MegaSeg is a complete solution for pro audio/video DJ mixing, radio automation, and music scheduling with rock-solid performance and an easy-to-use design. Mix with visual waveforms and Magic... Read more
Bookends 12.8 - Reference management and...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Adobe Creative Cloud 4.0.0.185 - Access...
Adobe Creative Cloud costs $19.99/month for a single app, or $49.99/month for the entire suite. Introducing Adobe Creative Cloud desktop applications, including Adobe Photoshop CC and Illustrator CC... Read more
Default Folder X 5.1.4 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click on... Read more

The best deals on the App Store this wee...
Deals, deals, deals. We're all about a good bargain here on 148Apps, and luckily this was another fine week in App Store discounts. There's a big board game sale happening right now, and a few fine indies are still discounted through the weekend.... | Read more »
The best new games we played this week
It's been quite the week, but now that all of that business is out of the way, it's time to hunker down with some of the excellent games that were released over the past few days. There's a fair few to help you relax in your down time or if you're... | Read more »
Orphan Black: The Game (Games)
Orphan Black: The Game 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Dive into a dark and twisted puzzle-adventure that retells the pivotal events of Orphan Black. | Read more »
The Elder Scrolls: Legends is now availa...
| Read more »
Ticket to Earth beginner's guide: H...
Robot Circus launched Ticket to Earth as part of the App Store's indie games event last week. If you're not quite digging the space operatics Mass Effect: Andromeda is serving up, you'll be pleased to know that there's a surprising alternative on... | Read more »
Leap to victory in Nexx Studios new plat...
You’re always a hop, skip, and a jump away from a fiery death in Temple Jump, a new platformer-cum-endless runner from Nexx Studio. It’s out now on both iOS and Android if you’re an adventurer seeking treasure in a crumbling, pixel-laden temple. | Read more »
Failbetter Games details changes coming...
Sunless Sea, Failbetter Games' dark and gloomy sea explorer, sets sail for the iPad tomorrow. Ahead of the game's launch, Failbetter took to Twitter to discuss what will be different in the mobile version of the game. Many of the changes make... | Read more »
Splish, splash! The Pokémon GO Water Fes...
Niantic is back with a new festival for dedicated Pokémon GO collectors. The Water Festival officially kicks off today at 1 P.M. PDT and runs through March 29. Magikarp, Squirtle, Totodile, and their assorted evolved forms will be appearing at... | Read more »
Death Road to Canada (Games)
Death Road to Canada 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: Get it now at the low launch price! Price will go up a dollar every major update. Update news at the bottom of this... | Read more »
Bean's Quest Beginner's Guide:...
Bean's Quest is a new take on both the classic platformer and the endless runner, and it's free on the App Store for the time being. Instead of running constantly, you can't stop jumping. That adds a surprising new level of challenge to the game... | Read more »

Price Scanner via MacPrices.net

2.6GHz Mac mini on sale for $559, $140 off MS...
Guitar Center has the 2.6GHz Mac mini (MGEN2LL/A) on sale for $559 including free shipping. Their price is $140 off MSRP, and it’s the lowest price available for this model. Read more
SSD Speeder RAM Disk SSD Life Extender App Fo...
Fehraltorf, Switzerland based B-Eng has announced they are making their SSD Speeder app for macOS publicly available for purchase on their website. SSD Speeder is a RAM disk utility that prevents... Read more
iPhone Scores Highest Overall in Smartphone D...
Customer satisfaction is much higher among smartphone owners who use their device to operate other connected home services such as smart thermostats and smart appliances, according to the J.D. Power... Read more
Swipe CRM Free Photo-Centric CRM Sales DEal C...
Swipe CRM LLC has introduced Swipe CRM: Visual Sales 1.0 for iPad, an app for creating, managing, and sharing visually stunning sales deals. Swipe CRM is targeted to small-and-medium creative... Read more
13-inch 2.0GHz Apple MacBook Pros on sale for...
B&H has the non-Touch Bar 13″ 2.0GHz MacBook Pros in stock today and on sale for $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray (... Read more
15-inch Touch Bar MacBook Pros on sale for up...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
Apple’s iPhone 6s Tops Best-Selling Smartphon...
In terms of shipments, the iPhone 6s from Apple bested all competitors for sales in 2016, according to new analysis from IHS Markit, a world leader in critical information, analytics and solutions.... Read more
Logitech Rugged Combo Protective iPad Case an...
Logitech has announced its Logitech Rugged Combo, Logitech Rugged Case, and Logitech Add-on Keyboard for Rugged Case for Apple’s new, more affordable $329 9.7-inch iPad, a complete solution designed... Read more
T-Mobile To Offer iPhone 7 and iPhone 7 Plus...
T-Mobile has announced it will offer iPhone 7 and iPhone 7 Plus (PRODUCT)RED Special Edition in a vibrant red aluminum finish. The introduction of this special edition iPhone celebrates Apple’s 10... Read more
9-inch 128GB iPad Pros on sale for $50-$70 of...
B&H Photo has 9.7″ 128GB Apple WiFi iPad Pros on sale for up to $70 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 9″ Space Gray 128GB WiFi iPad Pro: $649 $50... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
Starte Dein Karriere-Abenteuer in den Hauptst...
…mehrsprachigen Teams betreust Du Kunden von bekannten globale Marken wie Apple , Mercedes, Facebook, Expedia, und vielen anderen! Funktion Du wolltest schon Read more
*Apple* Retail - Multiple Positions- Chicago...
SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.