zxing barcode scanner javascript The Model-ViewController Design Pattern in Objective-C

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The Model-ViewController Design Pattern
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Objective-C for iPhone Developers: A Beginner s Guide
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Key Skills & Concepts
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Understanding the Model-View-Controller Design Pattern Using Property Lists Sharing Data Between Views
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nybody can write a simple program However, crafting a well-designed program that works well is a much more difficult endeavor In this chapter you learn about the Model-View-Controller design pattern and how to apply it to your iPhone development efforts using Objective-C The Model-View-Controller design pattern is arguably one of the biggest advances in computer science, so you would do well by learning it
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The Model-View-Controller Design Pattern
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The MVC design pattern separates a program s objects based upon responsibility The model is responsible for your program s logical objects These classes hold an application s data and its business logic Recall the silly classes used in previous chapters Duck, Driftwood, and Glutton are all examples of model classes They are logical objects with data and behavior Their behavior is unrelated to how your application s user interface operates Instead, logic was related to their business functions; hence the term business logic Nowhere do these objects define an application s appearance These are logical objects that reside behind the scenes Now consider the UIView and UIViewController classes that you extended in previous Try This projects The UIView is your view, and the view controller is your application s controller The view s responsibility is to control the application s appearance The view controller s responsibility is to handle the view s life cycle methods and serve as a middleman between the view and the model Figure 12-1 illustrates a typical MVC architecture The view displays your application s interface to its users When a user interacts with the view, those events are forwarded to the controller The controller then updates the state of any related model objects and updates the view Figure 12-2 illustrates the MVC and an iPhone application A UIView displays the iPhone application s user interface A UIView has an associated UIViewController that responds to a user s interactions with its UIView For instance, suppose you had a simple game where a penguin waddles across a screen
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12: The Model-View-Controller Design Pattern
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Figure 12-1
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Model-View-Controller (MVC) architecture
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When a user taps the penguin, it places a large red X on the penguin and the penguin stops waddling When tapped again, the application removes the X and the penguin starts waddling again A Penguin class represents the penguin, which is a model class In Figure 12-3 a penguin waddles across the screen When a user clicks the penguin, the view calls the view controller s shootPenguin: action The penguin, a property of the view controller, is killed and the penguin s state is updated to dead The view controller also updates the view s display by stopping the animation and placing a red X on the penguin
Figure 12-2
MVC architecture and iPhone applications
Objective-C for iPhone Developers: A Beginner s Guide
Figure 12-3
Hypothetical penguin game
Try This
Creating a Simple MVC Game
1 Create a new View-based application named PenguinHunting 2 Create a new class called Penguin (Listings 12-1 and 12-2) Have the Penguin
implement the shoot and revive methods
3 Add a property called state to Penguin and define two constants, STATE_ALIVE
and STATE_DEAD
4 Open PenguinHuntingViewControllerh in Xcode and add an IBOutlet for a
UIButton named penguinImageButton (Listing 12-3) Add an NSTimer named myTimer and a Penguin class named myPenguin Make all three properties
5 Open PenguinHuntingViewControllerm and implement as in Listing 12-4 6 Save and open PenguinHuntingViewControllerxib in Interface Builder 7 Connect the penguinImageButton in File s Owner to a button on the view s
canvas
12: The Model-View-Controller Design Pattern
8 Select the button and change its type to custom and image to tuxpng (Figure 12-4) 9 Connect the button s Touch Up Inside event to the shootThePenguin: method 10 Build and Run the application
Figure 12-4
Creating a custom button to display the penguin
(continued)
Objective-C for iPhone Developers: A Beginner s Guide
Listing 12-1
Penguinh
#import <Foundation/Foundationh> #define STATE_ALIVE 1 #define STATE_DEAD 2 @interface Penguin : NSObject{ int state; } @property (nonatomic, assign) int state; - (void) shoot; - (void) revive; @end
Listing 12-2
Penguinm
#import "Penguinh" @implementation Penguin @synthesize state; - (id) init { id toRet = [super init]; selfstate = STATE_ALIVE; return toRet; } - (void) shoot { NSLog(@"penguin is shot"); '"" selfstate = STATE_DEAD; } - (void) revive { NSLog(@"penguin is revived"); selfstate = STATE_ALIVE; } @end
Listing 12-3
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