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Notice in Figure 4-9 that there are arrows on both ends of the line connecting Foo with Bar Arrows indicate directionality in a relationship For example, in Figure 4-7 Purchase must communicate with Stock, but Stock does not need to communicate with Purchase, and so an arrow is added to the line end connecting to Stock When both classes must communicate with one another, arrows are either omitted or arrows are added to both line ends Figure 4-8 is an example of arrows having been omitted; both AcademicDepartment and Professor must know about each other In Figure 4-9, the arrows are both included, indicating a bidirectional relationship
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Foo myFoo 1 1 myBar
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Figure 4-9 Two classes in a circular dependency
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(continued)
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1 Create a new View-based Application named Circular 2 Create two new Objective-C NSObject subclasses named Foo and Bar (Listings 4-5,
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4-6, 4-7, and 4-8)
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3 Import Foo in Bar and Bar in Foo, and then implement both classes as in Listings 4-5
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through 4-8
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4 Try building the application and note that the application does not compile
Listing 4-5
Fooh (incorrect)
#import <Foundation/Foundationh> #import "Barh" @interface Foo : NSObject { Bar * myBar; } - (void) sayFooBar; - (NSString *) sayBarFoo; @end
Listing 4-6
Foom (incorrect)
#import "Fooh" @implementation Foo - (void) sayFooBar { myBar = [[Bar alloc] init]; NSLog(@"sayFooBar:%@", [myBar sayBarFoo]); } - (NSString *) sayBarFoo { return @"sayBarFoo"; } @end
Listing 4-7
Barh (incorrect)
#import <Foundation/Foundationh> #import "Fooh"; @interface Bar : NSObject { Foo * myFoo; } - (NSString *) sayBarFoo; @end
4: Classes, Objects, and Messaging
Listing 4-8
Barm (incorrect)
#import "Barh" @implementation Bar - (NSString *) sayBarFoo { myFoo = [[Foo alloc] init]; return [myFoo sayBarFoo]; } @end
1 Modify Fooh to refer to Bar using the @class directive and not import Barh
(Listings 4-9 and 4-10)
2 Modify Barh to refer to Foo using the @class directive and not import Fooh
(Listings 4-11 and 4-12)
3 Try building the application and note that it compiles and runs, but notice the warning
received in both classes implementation (Figure 4-10)
4 Modify both implementations so that they import each other; Foom should import
Barh and Barm should import Fooh
5 Click Build And Debug and the application builds and runs, only this time the
compiler generates no warnings
Listing 4-9 Fooh (correct)
#import <Foundation/Foundationh> @class Bar; @interface Foo : NSObject { --- snip --@end
Listing 4-10
Foom (correct)
#import "Fooh" #import "Barh" @implementation Foo --- snip --@end
(continued)
Objective-C for iPhone Developers: A Beginner s Guide
Figure 4-10
Compiler warning
4: Classes, Objects, and Messaging
Listing 4-11
Barh (correct)
#import <Foundation/Foundationh> @class Foo; @interface Bar : NSObject { --- snip --@end
Listing 4-12
Barm (correct)
#import "Barh" #import "Fooh" @implementation Bar --- snip --@end
Methods and Messaging
Up to now, I have glossed over Objective-C s strange syntax However, this strange syntax is actually quite elegant Figure 4-11 illustrates a typical method declaration in Objective-C A method declaration begins with a + or sign As you learn in the next section, this indicates whether the method is a class method or instance method The declaration then specifies the method s return type For instance, in Figure 4-11 the method returns an NSMutableString Note that if a method does not return anything, you write void in the parentheses, as the following code illustrates:
-(void) buildHelloString:(NSString *) personName;
Return type -(NSMutableString *)buildHelloString:(NSString *)personName; Method type Method name Argument type and name
Figure 4-11
An Objective-C method
Objective-C for iPhone Developers: A Beginner s Guide
Methods taking a parameter follow its name with a colon Methods without a parameter omit the colon This distinction is important, as the colon becomes part of the method s name
NOTE
Objective-C methods that take a parameter use a colon That colon is part of the method s name For instance, the method s name in Figure 4-11 is not buildHelloString, but rather, buildHelloString: this is an important distinction
If a method has a colon, then the parameter s type, in parentheses, follows the colon Following the parameter s type is the parameter s name, followed by a semicolon, which ends the declaration An Objective-C class methods, when called by other objects, appear very different than code calling C functions This difference is because Objective-C uses something called infix notation Infix notation mixes operands and operators Figure 4-12 illustrates an Objective-C message An Objective-C message from one object to another begins with an opening square brace and ends with a closing square brace The message begins with an opening square brace, followed by the object s name (the receiver) The receiver is followed by a space, and then the object s method being called (the message) If a message takes a parameter, then the method s name includes a colon and a parameter value
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