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Object-C 入门教程 (三)

更新: 2011-09-16字体: 【大中小】点击: 54

继承、多型（Inheritance, Polymorphism）以及其他面向对象功能
- id 型别
  - Objective-C 有种叫做 id 的型别，它的运作有时候像是 void*，不过它却严格规定只能用在对象。Objective-C 与 Java 跟 C++ 不一样，你在呼叫一个对象的 method 时，并不需要知道这个对象的型别。当然这个 method 一定要存在，这称为 Objective-C 的讯息传递。
  - 基于 "Programming in Objective-C," Copyright ? 2004 by Sams Publishing一书中的范例，并经过允许而刊载。
  - Fraction.h

§ #import <Foundation/NSObject.h>

§ @interface Fraction: NSObject {

§ int numerator;

§ int denominator;

§ }

§ -(Fraction*) initWithNumerator: (int) n denominator: (int) d;

§ -(void) print;

§ -(void) setNumerator: (int) d;

§ -(void) setDenominator: (int) d;

§ -(void) setNumerator: (int) n andDenominator: (int) d;

§ -(int) numerator;

§ -(int) denominator;

@end

- - Fraction.m

§ #import "Fraction.h"

§ #import <stdio.h>

§ @implementation Fraction

§ -(Fraction*) initWithNumerator: (int) n denominator: (int) d {

§ self = [super init];

§ if ( self ) {

§ [self setNumerator: n andDenominator: d];

§ }

§ return self;

§ }

§ -(void) print {

§ printf( "%i / %i", numerator, denominator );

§ }

§ -(void) setNumerator: (int) n {

§ numerator = n;

§ }

§ -(void) setDenominator: (int) d {

§ denominator = d;

§ }

§ -(void) setNumerator: (int) n andDenominator: (int) d {

§ numerator = n;

§ denominator = d;

§ }

§ -(int) denominator {

§ return denominator;

§ }

§ -(int) numerator {

§ return numerator;

§ }

@end

- - Complex.h

§ #import <Foundation/NSObject.h>

§ @interface Complex: NSObject {

§ double real;

§ double imaginary;

§ }

§ -(Complex*) initWithReal: (double) r andImaginary: (double) i;

§ -(void) setReal: (double) r;

§ -(void) setImaginary: (double) i;

§ -(void) setReal: (double) r andImaginary: (double) i;

§ -(double) real;

§ -(double) imaginary;

§ -(void) print;

@end

- - Complex.m

§ #import "Complex.h"

§ #import <stdio.h>

§ @implementation Complex

§ -(Complex*) initWithReal: (double) r andImaginary: (double) i {

§ self = [super init];

§ if ( self ) {

§ [self setReal: r andImaginary: i];

§ }

§ return self;

§ }

§ -(void) setReal: (double) r {

§ real = r;

§ }

§ -(void) setImaginary: (double) i {

§ imaginary = i;

§ }

§ -(void) setReal: (double) r andImaginary: (double) i {

§ real = r;

§ imaginary = i;

§ }

§ -(double) real {

§ return real;

§ }

§ -(double) imaginary {

§ return imaginary;

§ }

§ -(void) print {

§ printf( "%_f + %_fi", real, imaginary );

§ }

@end

- - main.m

§ #import <stdio.h>

§ #import "Fraction.h"

§ #import "Complex.h"

§ int main( int argc, const char *argv[] ) {

§ // create a new instance

§ Fraction *frac = [[Fraction alloc] initWithNumerator: 1 denominator: 10];

§ Complex *comp = [[Complex alloc] initWithReal: 10 andImaginary: 15];

§ id number;

§ // print fraction

§ number = frac;

§ printf( "The fraction is: " );

§ [number print];

§ printf( "\n" );

§ // print complex

§ number = comp;

§ printf( "The complex number is: " );

§ [number print];

§ printf( "\n" );

§ // free memory

§ [frac release];

§ [comp release];

§ return 0;

- - output

§ The fraction is: 1 / 10

The complex number is: 10.000000 + 15.000000i

- - 这种动态链接有显而易见的好处。你不需要知道你呼叫 method 的那个东西是什么型别，如果这个对象对这个讯息有反应，那就会唤起这个 method。这也不会牵涉到一堆繁琐的转型动作，比如在 Java 里呼叫一个整数对象的 .intValue() 就得先转型，然后才能呼叫这个 method。
- 继承（Inheritance）
  - 基于 "Programming in Objective-C," Copyright ? 2004 by Sams Publishing一书中的范例，并经过允许而刊载。
  - Rectangle.h

§ #import <Foundation/NSObject.h>

§ @interface Rectangle: NSObject {

§ int width;

§ int height;

§ }

§ -(Rectangle*) initWithWidth: (int) w height: (int) h;

§ -(void) setWidth: (int) w;

§ -(void) setHeight: (int) h;

§ -(void) setWidth: (int) w height: (int) h;

§ -(int) width;

§ -(int) height;

§ -(void) print;

@end

- - Rectangle.m

§ #import "Rectangle.h"

§ #import <stdio.h>

§ @implementation Rectangle

§ -(Rectangle*) initWithWidth: (int) w height: (int) h {

§ self = [super init];

§ if ( self ) {

§ [self setWidth: w height: h];

§ }

§ return self;

§ }

§ -(void) setWidth: (int) w {

§ width = w;

§ }

§ -(void) setHeight: (int) h {

§ height = h;

§ }

§ -(void) setWidth: (int) w height: (int) h {

§ width = w;

§ height = h;

§ }

§ -(int) width {

§ return width;

§ }

§ -(int) height {

§ return height;

§ }

§ -(void) print {

§ printf( "width = %i, height = %i", width, height );

§ }

@end

- - Square.h

§ #import "Rectangle.h"

§ @interface Square: Rectangle

§ -(Square*) initWithSize: (int) s;

§ -(void) setSize: (int) s;

§ -(int) size;

@end

- - Square.m

§ #import "Square.h"

§ @implementation Square

§ -(Square*) initWithSize: (int) s {

§ self = [super init];

§ if ( self ) {

§ [self setSize: s];

§ }

§ return self;

§ }

§ -(void) setSize: (int) s {

§ width = s;

§ height = s;

§ }

§ -(int) size {

§ return width;

§ }

§ -(void) setWidth: (int) w {

§ [self setSize: w];

§ }

§ -(void) setHeight: (int) h {

§ [self setSize: h];

§ }

@end

- - main.m

§ #import "Square.h"

§ #import "Rectangle.h"

§ #import <stdio.h>

§ int main( int argc, const char *argv[] ) {

§ Rectangle *rec = [[Rectangle alloc] initWithWidth: 10 height: 20];

§ Square *sq = [[Square alloc] initWithSize: 15];

§ // print em

§ printf( "Rectangle: " );

§ [rec print];

§ printf( "\n" );

§ printf( "Square: " );

§ [sq print];

§ printf( "\n" );

§ // update square

§ [sq setWidth: 20];

§ printf( "Square after change: " );

§ [sq print];

§ printf( "\n" );

§ // free memory

§ [rec release];

§ [sq release];

§ return 0;

- - output

§ Rectangle: width = 10, height = 20

§ Square: width = 15, height = 15

Square after change: width = 20, height = 20

- - 继承在 Objective-C 里比较像 Java。当你扩充你的 super class（所以只能有一个 parent），你想自定义这个 super class 的 method，只要简单的在你的 child class implementation 里放上新的实作内容即可。而不需要 C++ 里呆呆的 virtual table。
  - 这里还有一个值得玩味的地方，如果你企图像这样去呼叫 rectangle 的 constructor： Square *sq = [[Square alloc] initWithWidth: 10 height: 15]，会发生什么事？答案是会产生一个编译程序错误。因为 rectangle constructor 回传的型别是 Rectangle*，而不是 Square*，所以这行不通。在某种情况下如果你真想这样用，使用 id 型别会是很好的选择。如果你想使用 parent 的 constructor，只要把 Rectangle* 回传型别改成 id 即可。
- 动态识别（Dynamic types）
  - 这里有一些用于 Objective-C 动态识别的 methods（说明部分采中英并列，因为我觉得英文比较传神，中文怎么译都怪）：

-(BOOL) isKindOfClass: classObj

is object a descendent or member of classObj
此对象是否是 classObj 的子孙或一员

-(BOOL) isMemberOfClass: classObj

is object a member of classObj
此对象是否是 classObj 的一员

-(BOOL) respondsToSelector: selector

does the object have a method named specifiec by the selector
此对象是否有叫做 selector 的 method

+(BOOL) instancesRespondToSelector: selector

does an object created by this class have the ability to respond to the specified selector
此对象是否是由有能力响应指定 selector 的对象所产生

-(id) performSelector: selector

invoke the specified selector on the object
唤起此对象的指定 selector

- - 所有继承自 NSObject 都有一个可回传一个 class 物件的 class method。这非常近似于 Java 的 getClass() method。这个 class 对象被使用于前述的 methods 中。
  - Selectors 在 Objective-C 用以表示讯息。下一个范例会秀出建立 selector 的语法。
  - 基于 "Programming in Objective-C," Copyright ? 2004 by Sams Publishing一书中的范例，并经过允许而刊载。
  - main.m

§ #import "Square.h"

§ #import "Rectangle.h"

§ #import <stdio.h>

§ int main( int argc, const char *argv[] ) {

§ Rectangle *rec = [[Rectangle alloc] initWithWidth: 10 height: 20];

§ Square *sq = [[Square alloc] initWithSize: 15];

§ // isMemberOfClass

§ // true

§ if ( [sq isMemberOfClass: [Square class]] == YES ) {

§ printf( "square is a member of square class\n" );

§ }

§ // false

§ if ( [sq isMemberOfClass: [Rectangle class]] == YES ) {

§ printf( "square is a member of rectangle class\n" );

§ }

§ // false

§ if ( [sq isMemberOfClass: [NSObject class]] == YES ) {

§ printf( "square is a member of object class\n" );

§ }

§ // isKindOfClass

§ // true

§ if ( [sq isKindOfClass: [Square class]] == YES ) {

§ printf( "square is a kind of square class\n" );

§ }

§ // true

§ if ( [sq isKindOfClass: [Rectangle class]] == YES ) {

§ printf( "square is a kind of rectangle class\n" );

§ }

§ // true

§ if ( [sq isKindOfClass: [NSObject class]] == YES ) {

§ printf( "square is a kind of object class\n" );

§ }

§ // respondsToSelector

§ // true

§ if ( [sq respondsToSelector: @selector( setSize: )] == YES ) {

§ printf( "square responds to setSize: method\n" );

§ }

§ // false

§ if ( [sq respondsToSelector: @selector( nonExistant )] == YES ) {

§ printf( "square responds to nonExistant method\n" );

§ }

§ // true

§ if ( [Square respondsToSelector: @selector( alloc )] == YES ) {

§ printf( "square class responds to alloc method\n" );

§ }

§ // instancesRespondToSelector

§ // false

§ if ( [Rectangle instancesRespondToSelector: @selector( setSize: )] == YES ) {

§ printf( "rectangle instance responds to setSize: method\n" );

§ }

§ // true

§ if ( [Square instancesRespondToSelector: @selector( setSize: )] == YES ) {

§ printf( "square instance responds to setSize: method\n" );

§ }

§ // free memory

§ [rec release];

§ [sq release];

§ return 0;

- - output

§ square is a member of square class

§ square is a kind of square class

§ square is a kind of rectangle class

§ square is a kind of object class

§ square responds to setSize: method

§ square class responds to alloc method

square instance responds to setSize: method

- Categories
  - 当你想要为某个 class 新增 methods，你通常会扩充（extend，即继承）它。然而这不一定是个完美解法，特别是你想要重写一个 class 的某个功能，但你却没有原始码时。Categories 允许你在现有的 class 加入新功能，但不需要扩充它。Ruby 语言也有类似的功能。
  - 基于 "Programming in Objective-C," Copyright ? 2004 by Sams Publishing一书中的范例，并经过允许而刊载。
  - FractionMath.h

§ #import "Fraction.h"

§ @interface Fraction (Math)

§ -(Fraction*) add: (Fraction*) f;

§ -(Fraction*) mul: (Fraction*) f;

§ -(Fraction*) div: (Fraction*) f;

§ -(Fraction*) sub: (Fraction*) f;

@end

- - FractionMath.m

§ #import "FractionMath.h"

§ @implementation Fraction (Math)

§ -(Fraction*) add: (Fraction*) f {

§ return [[Fraction alloc] initWithNumerator: numerator * [f denominator] +

§ denominator * [f numerator]

§ denominator: denominator * [f denominator]];

§ }

§ -(Fraction*) mul: (Fraction*) f {

§ return [[Fraction alloc] initWithNumerator: numerator * [f numerator]

§ denominator: denominator * [f denominator]];

§ }

§ -(Fraction*) div: (Fraction*) f {

§ return [[Fraction alloc] initWithNumerator: numerator * [f denominator]

§ denominator: denominator * [f numerator]];

§ }

§ -(Fraction*) sub: (Fraction*) f {

§ return [[Fraction alloc] initWithNumerator: numerator * [f denominator] -

§ denominator * [f numerator]

§ denominator: denominator * [f denominator]];

§ }

@end

- - main.m

§ #import <stdio.h>

§ #import "Fraction.h"

§ #import "FractionMath.h"

§ int main( int argc, const char *argv[] ) {

§ // create a new instance

§ Fraction *frac1 = [[Fraction alloc] initWithNumerator: 1 denominator: 3];

§ Fraction *frac2 = [[Fraction alloc] initWithNumerator: 2 denominator: 5];

§ Fraction *frac3 = [frac1 mul: frac2];

§ // print it

§ [frac1 print];

§ printf( " * " );

§ [frac2 print];

§ printf( " = " );

§ [frac3 print];

§ printf( "\n" );

§ // free memory

§ [frac1 release];

§ [frac2 release];

§ [frac3 release];

§ return 0;

- - output

1/3 * 2/5 = 2/15

- - 重点是 @implementation 跟 @interface 这两行：@interface Fraction (Math) 以及 @implementation Fraction (Math).
  - （同一个 class）只能有一个同名的 category，其他的 categories 得加上不同的、独一无二的名字。
  - Categories 在建立 private methods 时十分有用。因为 Objective-C 并没有像 Java 这种 private/protected/public methods 的概念，所以必须要使用 categories 来达成这种功能。作法是把 private method 从你的 class header (.h) 档案移到 implementation (.m) 档案。以下是此种作法一个简短的范例。
  - MyClass.h

§ #import <Foundation/NSObject.h>

§ @interface MyClass: NSObject

§ -(void) publicMethod;

@end

- - MyClass.m

§ #import "MyClass.h"

§ #import <stdio.h>

§ @implementation MyClass

§ -(void) publicMethod {

§ printf( "public method\n" );

§ }

§ @end

§ // private methods

§ @interface MyClass (Private)

§ -(void) privateMethod;

§ @end

§ @implementation MyClass (Private)

§ -(void) privateMethod {

§ printf( "private method\n" );

§ }

@end

- - main.m

§ #import "MyClass.h"

§ int main( int argc, const char *argv[] ) {

§ MyClass *obj = [[MyClass alloc] init];

§ // this compiles

§ [obj publicMethod];

§ // this throws errors when compiling

§ //[obj privateMethod];

§ // free memory

§ [obj release];

§ return 0;

- - output

public method

- Posing
  - Posing 有点像 categories，但是不太一样。它允许你扩充一个 class，并且全面性地的扮演（pose）这个 super class。例如：你有一个扩充 NSArray 的 NSArrayChild 物件。如果你让 NSArrayChild 扮演 NSArray，则在你的程序代码中所有的 NSArray 都会自动被替代为 NSArrayChild。
  - 基于 "Programming in Objective-C," Copyright ? 2004 by Sams Publishing一书中的范例，并经过允许而刊载。
  - FractionB.h

§ #import "Fraction.h"

§ @interface FractionB: Fraction

§ -(void) print;

§ @end

- - FractionB.m

§ #import "FractionB.h"

§ #import <stdio.h>

§ @implementation FractionB

§ -(void) print {

§ printf( "(%i/%i)", numerator, denominator );

§ }

@end

- - main.m

§ #import <stdio.h>

§ #import "Fraction.h"

§ #import "FractionB.h"

§ int main( int argc, const char *argv[] ) {

§ Fraction *frac = [[Fraction alloc] initWithNumerator: 3 denominator: 10];

§ // print it

§ printf( "The fraction is: " );

§ [frac print];

§ printf( "\n" );

§ // make FractionB pose as Fraction

§ [FractionB poseAsClass: [Fraction class]];

§ Fraction *frac2 = [[Fraction alloc] initWithNumerator: 3 denominator: 10];

§ // print it

§ printf( "The fraction is: " );

§ [frac2 print];

§ printf( "\n" );

§ // free memory

§ [frac release];

§ [frac2 release];

§ return 0;

- - output

§ The fraction is: 3/10

The fraction is: (3/10)

- - 这个程序的输出中，第一个 fraction 会输出 3/10，而第二个会输出 (3/10)。这是 FractionB 中实作的方式。
  - poseAsClass 这个 method 是 NSObject 的一部份，它允许 subclass 扮演 superclass。
- Protocols
  - Objective-C 里的 Protocol 与 Java 的 interface 或是 C++ 的 purely virtual class 相同。
  - 基于 "Programming in Objective-C," Copyright ? 2004 by Sams Publishing一书中的范例，并经过允许而刊载。
  - Printing.h

§ @protocol Printing

§ -(void) print;

@end

- - Fraction.h

§ #import <Foundation/NSObject.h>

§ #import "Printing.h"

§ @interface Fraction: NSObject <Printing, NSCopying> {

§ int numerator;

§ int denominator;

§ }

§ -(Fraction*) initWithNumerator: (int) n denominator: (int) d;

§ -(void) setNumerator: (int) d;

§ -(void) setDenominator: (int) d;

§ -(void) setNumerator: (int) n andDenominator: (int) d;

§ -(int) numerator;

§ -(int) denominator;

@end

- - Fraction.m

§ #import "Fraction.h"

§ #import <stdio.h>

§ @implementation Fraction

§ -(Fraction*) initWithNumerator: (int) n denominator: (int) d {

§ self = [super init];

§ if ( self ) {

§ [self setNumerator: n andDenominator: d];

§ }

§ return self;

§ }

§ -(void) print {

§ printf( "%i/%i", numerator, denominator );

§ }

§ -(void) setNumerator: (int) n {

§ numerator = n;

§ }

§ -(void) setDenominator: (int) d {

§ denominator = d;

§ }

§ -(void) setNumerator: (int) n andDenominator: (int) d {

§ numerator = n;

§ denominator = d;

§ }

§ -(int) denominator {

§ return denominator;

§ }

§ -(int) numerator {

§ return numerator;

§ }

§ -(Fraction*) copyWithZone: (NSZone*) zone {

§ return [[Fraction allocWithZone: zone] initWithNumerator: numerator

§ denominator: denominator];

§ }

@end

- - Complex.h

§ #import <Foundation/NSObject.h>

§ #import "Printing.h"

§ @interface Complex: NSObject <Printing> {

§ double real;

§ double imaginary;

§ }

§ -(Complex*) initWithReal: (double) r andImaginary: (double) i;

§ -(void) setReal: (double) r;

§ -(void) setImaginary: (double) i;

§ -(void) setReal: (double) r andImaginary: (double) i;

§ -(double) real;

§ -(double) imaginary;

@end

- - Complex.m

§ #import "Complex.h"

§ #import <stdio.h>

§ @implementation Complex

§ -(Complex*) initWithReal: (double) r andImaginary: (double) i {

§ self = [super init];

§ if ( self ) {

§ [self setReal: r andImaginary: i];

§ }

§ return self;

§ }

§ -(void) setReal: (double) r {

§ real = r;

§ }

§ -(void) setImaginary: (double) i {

§ imaginary = i;

§ }

§ -(void) setReal: (double) r andImaginary: (double) i {

§ real = r;

§ imaginary = i;

§ }

§ -(double) real {

§ return real;

§ }

§ -(double) imaginary {

§ return imaginary;

§ }

§ -(void) print {

§ printf( "%_f + %_fi", real, imaginary );

§ }

@end

- - main.m

§ #import <stdio.h>

§ #import "Fraction.h"

§ #import "Complex.h"

§ int main( int argc, const char *argv[] ) {

§ // create a new instance

§ Fraction *frac = [[Fraction alloc] initWithNumerator: 3 denominator: 10];

§ Complex *comp = [[Complex alloc] initWithReal: 5 andImaginary: 15];

§ id <Printing> printable;

§ id <NSCopying, Printing> copyPrintable;

§ // print it

§ printable = frac;

§ printf( "The fraction is: " );

§ [printable print];

§ printf( "\n" );

§ // print complex

§ printable = comp;

§ printf( "The complex number is: " );

§ [printable print];

§ printf( "\n" );

§ // this compiles because Fraction comforms to both Printing and NSCopyable

§ copyPrintable = frac;

§ // this doesn't compile because Complex only conforms to Printing

§ //copyPrintable = comp;

§ // test conformance

§ // true

§ if ( [frac conformsToProtocol: @protocol( NSCopying )] == YES ) {

§ printf( "Fraction conforms to NSCopying\n" );

§ }

§ // false

§ if ( [comp conformsToProtocol: @protocol( NSCopying )] == YES ) {

§ printf( "Complex conforms to NSCopying\n" );

§ }

§ // free memory

§ [frac release];

§ [comp release];

§ return 0;

- - output

§ The fraction is: 3/10

§ The complex number is: 5.000000 + 15.000000i

Fraction conforms to NSCopying

- - protocol 的宣告十分简单，基本上就是 @protocol ProtocolName (methods you must implement) @end。
  - 要遵从（conform）某个 protocol，将要遵从的 protocols 放在 <> 里面，并以逗点分隔。如：@interface SomeClass <Protocol1, Protocol2, Protocol3>
  - protocol 要求实作的 methods 不需要放在 header 档里面的 methods 列表中。如你所见，Complex.h 档案里没有 -(void) print 的宣告，却还是要实作它，因为它（Complex class）遵从了这个 protocol。
  - Objective-C 的接口系统有一个独一无二的观念是如何指定一个型别。比起 C++ 或 Java 的指定方式，如：Printing *someVar = ( Printing * ) frac; 你可以使用 id 型别加上 protocol：id <Printing> var = frac;。这让你可以动态地指定一个要求多个 protocol 的型别，却从头到尾只用了一个变数。如：<Printing, NSCopying> var = frac;
  - 就像使用@selector 来测试对象的继承关系，你可以使用 @protocol 来测试对象是否遵从接口。如果对象遵从这个接口，[object conformsToProtocol: @protocol( SomeProtocol )] 会回传一个 YES 型态的 BOOL 对象。同样地，对 class 而言也能如法炮制 [SomeClass conformsToProtocol: @protocol( SomeProtocol )]。

标签： Object 入门教程

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