Objective-C开发指南--3
- 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 |
| -(BOOL) isMemberOfClass: classObj |
is object a member of classObj |
| -(BOOL) respondsToSelector: selector |
does the object have a method named specifiec by the selector |
| +(BOOL) instancesRespondToSelector: selector |
does an object created by this class have the ability to respond to the specified selector |
| -(id) performSelector: selector |
invoke the specified selector on the object |
- 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，其它的 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 )]。