When
you start programming, you’ll quickly come to the conclusion that there
is more than one “correct” way to do just about everything. Part of the
joy of programming is that it is a creative process that allows you to
be as clever as your imagination allows. This doesn’t mean, however,
that adding structure to the development process is a bad idea. Having a
defined and documented structure means that other developers will be
able to work with your code, projects large and small will be easy to
navigate, and you’ll be able to reuse your best work in multiple
applications.
The
application design approach that you’ll be using on the iPhone is known
as Model-View-Controller (MVC) and will guide you in creating clean,
efficient applications.
Making Spaghetti
Before we get into MVC,
let’s first talk about the development practice that we want to avoid,
and why. When creating an application that interacts with a user,
several things must be taken into account. First, the user interface.
You must present something
that the user interacts with: buttons, fields, and so on. Second,
handling and reacting to the user input. Finally, the application must
store the information necessary to correctly react to the user—often in
the form of a database.
One approach to
incorporating all of these pieces is to combine them into a single
class. The code that displays the interface is mixed with the code that
implements the logic and the code that handles data. This can be a
straightforward development methodology, but it limits the developer in
several ways:
When code is
mixed together, it is difficult for multiple developers to work together
because there is no clear division between any of the functional units.
The
interface, application logic, and data are unlikely to be reusable in
other applications because the combination of the three is too specific
to the current project to be useful elsewhere.
The
application is difficult to extend. Adding features requires working
around existing code. The developer must work around the existing code
to include new features, even if they are unrelated.
In short, mixing code, logic,
and data leads to a mess! This is known as “spaghetti code” and is the
exact opposite of what we want for our iPhone applications. MVC to the
rescue!
Structured Application Design with MVC
MVC defines a clean
separation between the critical components of our apps. As implied by
the name, MVC defines three parts of an application:
A model
provides the underlying data and methods that provide information to
the rest of the application. The model does not define how the
application will look or how it will act.
One or more views
make up the user interface. A view consists of the different onscreen
widgets (buttons, fields, switches, and so forth) that a user can
interact with.
A controller
is typically paired with a view. The controller is responsible for
receiving the user’s input and acting accordingly. Controllers may
access and update a view using information from the model and update the
model using the results of user interactions in the view. In short, it
bridges the MVC components.
The logical isolation created between the functional parts of an application, illustrated in Figure 1, means the code becomes more easily maintainable, reusable, and extendable—the exact opposite of spaghetti code.
Unfortunately, MVC
comes as an afterthought in many application development environments. A
frequent question that I am asked when suggesting MVC design is, “How
do I do that?” This isn’t indicative of a misunderstanding of what MVC
is or how it works, but a lack of a clear means of implementing it.
In the Apple Development
Suite, MVC design is natural. As you create new projects and start
coding, you’ll be guided into using MVC design patterns automatically.
It actually becomes more difficult to program poorly than it does to
build a well-structured app.
