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CHAPTER 3: EA Framework Design Patterns
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Does this violate the rule of MVC Not if you consider the rule to be more of a guideline. When in doubt you need to consider the overall goals of your project as bounded by your resources. Most of the time, no matter whether you divide your source code into distinct model, view, and controller files, or if you put something all into a single object file, you still need to design your system conforming to MVC. Only by looking at your system before you start coding can you really see what you need to do. If you have a huge model object with dozens of status fields and calculations, then it s easy to see that it makes sense to separate along the lines of MVC. On the other hand, as in the Pong game, you ve got less than a handful of data elements and only three things moving on the screen two paddles and a ball then packaging everything in one set of code may be the way to go. Looking at this game system for a moment, I clearly talked about the game, the paddles, board, ball, scoring, and so on, but what about the controller How does it fit in Think about it this way does the game work without the controller It absolutely does. In fact, when you build the first iteration, you will not even consider the accessory game controller. You ll build your game as if it is a basic, touch-the-screen-to-play game application. The addition of a hardware accessory controller to the already developed game app depicts one of the strengths of OOP. You change the basic system by adding a realworld piece of hardware, and represent it as a delegate object that you add to the system. Remember when I talked about upgrading the system that ran the manufacturing floor. The simple change of adding a new printer line created havoc and necessitated the complete redesign of a system. I will demonstrate that, through the use of delegation and notification, you can very easily expand your system your game to use a hardware accessory controller with minimal modifications. NOTE: Every system designed contains some form of the MVC design pattern. While you will often choose to put everything in one file, the more clearly the lines are drawn between M, V, and C, the easier you will find it to reuse the parts of the system. The need or desire to reuse parts of your system always arises more than you anticipate. A corollary to that is the ability to reuse parts of a system always turns out to be work that you anticipate.
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Notification
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Of all the design patterns discussed in this book, notifications are far and away my favorite. They are also one of the most overused tools in Cocoa/Objective-C with the possible exception of Dictionaries, because notifications are so easy to use. Start with a simple program containing several objects. Each object contains its own set of properties and methods. You build the system and use method calls to pass
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CHAPTER 3: EA Framework Design Patterns
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information between objects. For example, in the Pong game, you move the ball by a certain amount each time period. When an interval timer expires, say with a period of 10 milliseconds, you might call a moveBall method that multiplies the 10ms times the speed of the ball. That method erases the old ball and displays it at the new position. What I ve just described is the goal of the development process. In an ideal world, you d always use method calls to pass data to objects and get those objects to do something. Unfortunately, a system like this is rarely achievable. One of the reasons for this inability to adhere to a strict OOP philosophy comes from the fact that systems evolve. Even before the initial release of the system, you more than likely will find the need to autonomously pass data between objects. By autonomously, I mean that one object needs to send data somewhere, but the originating object doesn t know where to send it. This is where notifications come in. Notifications allow objects to post information to a notification center that then resends that information to all interested objects. In general object-oriented programming, we refer to capability this as message passing. Some documentation refers to the similarities between notification and delegation; mainly because delegation also uses notification. While I personally don t see too much similarity between the two patterns, a lot of people may, therefore let s define the three key areas where they differ. First, delegation permits only one delegate object to receive a notification, in comparison to using the notification center, where many objects can be addressed. That seems great, but there s one thing to consider; using delegation with just one object being notified permits that receiver to provide a return value. Using notifications to multiple addresses prohibits the use of return values. Second, using delegation, you pass only predefined delegate methods. With notification you can send any message you like through the notification center including user data. Finally, using delegation, the receiving object understands something about the sender object. The receiver knows that the sender exited. There s a connection between the two. Using notification, that connection becomes decoupled. The receiver processes notifications that it has registered for, but doesn t have to understand anything about the sender. It only needs to know how to process the notification that may come from anywhere. While delegation provides a tight coupling between sender and receiver, notifications are termed loosely coupled. Figure 3 6 shows the differences in data flow between delegation and notification.
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