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Objective-C 入门教程  

程序员文章站 2022-07-15 11:46:08
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大纲


  • 开始吧
    • 下载这篇教学
      • 所有这篇初学者指南的原始码都可以由 objc.tar.gz 下载。这篇教学中的许多范例都是由 Steve Kochan Programming in Objective-C http://www.assoc-amazon.com/e/ir?t=tristanshomep-20&l=ur2&o=1 . 一书中撰写。如果你想得到更多详细信息及范例,请直接参考该书。这个网站上登载的所有范例皆经过他的允许,所以请勿复制转载。
    • 设定环境
      • Linux/FreeBSD: 安装 GNUStep
        • 为了编译 GNUstep 应用程序,必须先执行位于 /usr/GNUstep/System/Makefiles/GNUstep.sh GNUstep.sh 这个档案。这个路径取决于你的系统环境,有些是在 /usr, some /usr/lib ,有些是 /usr/local 。如果你的 shell 是以 csh/tcsh 为基础的 shell ,则应该改用 GNUStep.csh 。建议把这个指令放在 .bashrc .cshrc 中。
      • Mac OS X: 安装 XCode
      • Windows NT 5.X: 安装 cygwin mingw ,然后安装 GNUStep
    • 前言
      • 这篇教学假设你已经有一些基本的 C 语言知识,包括 C 数据型别、什么是函式、什么是回传值、关于指针的知识以及基本的 C 语言内存管理。如果您没有这些背景知识,我非常建议你读一读 K&R 的书:The C Programming Language http://www.assoc-amazon.com/e/ir?t=tristanshomep-20&l=ur2&o=1 (译注:*出版书名为 C 程序语言第二版)这是 C 语言的设计者所写的书。
      • Objective-C ,是 C 的衍生语言,继承了所有 C 语言的特性。是有一些例外,但是它们不是继承于 C 的语言特性本身。
      • nil :在 C/C++ 你或许曾使用过 NULL ,而在 Objective-C 中则是 nil 。不同之处是你可以传递讯息给 nil (例如 [nil message]; ),这是完全合法的,然而你却不能对 NULL 如法炮制。
      • BOOL C 没有正式的布尔型别,而在 Objective-C 中也不是「真的」有。它是包含在 Foundation classes (基本类别库)中(即 import NSObject.hnil 也是包括在这个头文件内)。BOOL Objective-C 中有两种型态:YES NO ,而不是 TRUE FALSE
      • #import vs #include :就如同你在 hello world 范例中看到的,我们使用了 #import#import gcc 编译程序支援。我并不建议使用 #include#import 基本上跟 .h 档头尾的 #ifndef #define #endif 相同。许多程序员们都同意,使用这些东西这是十分愚蠢的。无论如何,使用 #import 就对了。这样不但可以避免麻烦,而且万一有一天 gcc 把它拿掉了,将会有足够的 Objective-C 程序员可以坚持保留它或是将它放回来。偷偷告诉你,Apple 在它们官方的程序代码中也使用了 #import 。所以万一有一天这种事真的发生,不难预料 Apple 将会提供一个支持 #import gcc 分支版本。
      • Objective-C 中, method message 这两个字是可以互换的。不过 messages 拥有特别的特性,一个 message 可以动态的转送给另一个对象。在 Objective-C 中,呼叫对象上的一个讯息并不一定表示对象真的会实作这个讯息,而是对象知道如何以某种方式去实作它,或是转送给知道如何实作的对象。
    • 编译 hello world
      • hello.m

§ #import <stdio.h>

§

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

§ printf( "hello world/n" );

§ return 0;

}

      • 输出

hello world

      • Objective-C 中使用 #import 代替 #include
      • Objective-C 的预设扩展名是 .m
  • 创建 classes
    • @interface
      • 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;

§ }

§

§ -(void) print;

§ -(void) setNumerator: (int) d;

§ -(void) setDenominator: (int) d;

§ -(int) numerator;

§ -(int) denominator;

§ @end

      • NSObject NeXTStep Object 的缩写。因为它已经改名为 OpenStep ,所以这在今天已经不是那么有意义了。
      • 继承(inheritance )以 Class: Parent 表示,就像上面的 Fraction: NSObject
      • 夹在 @interface Class: Parent { .... } 中的称为 instance variables
      • 没有设定访问权限(protected, public, private )时,预设的访问权限为 protected 。设定权限的方式将在稍后说明。
      • Instance methods 跟在成员变数(即 instance variables )后。格式为:scope (returnType) methodName: (parameter1Type) parameter1Name;
        • scope class instance 两种。instance methods - 开头,class level methods + 开头。
      • Interface 以一个 @end 作为结束。
    • @implementation
      • 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.m

§ #import "Fraction.h"

§ #import <stdio.h>

§

§ @implementation Fraction

§ -(void) print {

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

§ }

§

§ -(void) setNumerator: (int) n {

§ numerator = n;

§ }

§

§ -(void) setDenominator: (int) d {

§ denominator = d;

§ }

§

§ -(int) denominator {

§ return denominator;

§ }

§

§ -(int) numerator {

§ return numerator;

§ }

@end

      • Implementation @implementation ClassName 开始,以 @end 结束。
      • Implement 定义好的 methods 的方式,跟在 interface 中宣告时很近似。
    • 把它们凑在一起
      • 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 <stdio.h>

§ #import "Fraction.h"

§

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

§ // create a new instance

§ Fraction *frac = [[Fraction alloc] init];

§

§ // set the values

§ [frac setNumerator: 1];

§ [frac setDenominator: 3];

§

§ // print it

§ printf( "The fraction is: " );

§ [frac print];

§ printf( "/n" );

§

§ // free memory

§ [frac release];

§

§ return 0;

}

      • output

The fraction is: 1/3

      • Fraction *frac = [[Fraction alloc] init];
        • 这行程序代码中有很多重要的东西。
        • Objective-C 中呼叫 methods 的方法是 [object method] ,就像 C++ object->method()
        • Objective-C 没有 value 型别。所以没有像 C++ Fraction frac; frac.print(); 这类的东西。在 Objective-C 中完全使用指针来处理对象。
        • 这行程序代码实际上做了两件事: [Fraction alloc] 呼叫了 Fraction class alloc method 。这就像 malloc 内存,这个动作也做了一样的事情。
        • [object init] 是一个建构子(constructor )呼叫,负责初始化对象中的所有变量。它呼叫了 [Fraction alloc] 传回的 instance 上的 init method 。这个动作非常普遍,所以通常以一行程序完成:Object *var = [[Object alloc] init];
      • [frac setNumerator: 1] 非常简单。它呼叫了 frac 上的 setNumerator method 并传入 1 为参数。
      • 如同每个 C 的变体,Objective-C 也有一个用以释放内存的方式: release 。它继承自 NSObject ,这个 method 在之后会有详尽的解说。
  • 详细说明...
    • 多重参数
      • 目前为止我还没展示如何传递多个参数。这个语法乍看之下不是很直觉,不过它却是来自一个十分受欢迎的 Smalltalk 版本。
      • 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

§ ...

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

...

      • Fraction.m

§ ...

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

§ numerator = n;

§ denominator = d;

§ }

...

      • main.m

§ #import <stdio.h>

§ #import "Fraction.h"

§

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

§ // create a new instance

§ Fraction *frac = [[Fraction alloc] init];

§ Fraction *frac2 = [[Fraction alloc] init];

§

§ // set the values

§ [frac setNumerator: 1];

§ [frac setDenominator: 3];

§

§ // combined set

§ [frac2 setNumerator: 1 andDenominator: 5];

§

§ // print it

§ printf( "The fraction is: " );

§ [frac print];

§ printf( "/n" );

§

§ // print it

§ printf( "Fraction 2 is: " );

§ [frac2 print];

§ printf( "/n" );

§

§ // free memory

§ [frac release];

§ [frac2 release];

§

§ return 0;

}

      • output

§ The fraction is: 1/3

Fraction 2 is: 1/5

      • 这个 method 实际上叫做 setNumerator:andDenominator:
      • 加入其他参数的方法就跟加入第二个时一样,即 method:label1:label2:label3: ,而呼叫的方法是 [obj method: param1 label1: param2 label2: param3 label3: param4]
      • Labels 是非必要的,所以可以有一个像这样的 methodmethod::: ,简单的省略 label 名称,但以 : 区隔参数。并不建议这样使用。
    • 建构子(Constructors
      • 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

§ ...

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

...

      • Fraction.m

§ ...

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

§ self = [super init];

§

§ if ( self ) {

§ [self setNumerator: n andDenominator: d];

§ }

§

§ return self;

§ }

...

      • main.m

§ #import <stdio.h>

§ #import "Fraction.h"

§

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

§ // create a new instance

§ Fraction *frac = [[Fraction alloc] init];

§ Fraction *frac2 = [[Fraction alloc] init];

§ Fraction *frac3 = [[Fraction alloc] initWithNumerator: 3 denominator: 10];

§

§ // set the values

§ [frac setNumerator: 1];

§ [frac setDenominator: 3];

§

§ // combined set

§ [frac2 setNumerator: 1 andDenominator: 5];

§

§ // print it

§ printf( "The fraction is: " );

§ [frac print];

§ printf( "/n" );

§

§ printf( "Fraction 2 is: " );

§ [frac2 print];

§ printf( "/n" );

§

§ printf( "Fraction 3 is: " );

§ [frac3 print];

§ printf( "/n" );

§

§ // free memory

§ [frac release];

§ [frac2 release];

§ [frac3 release];

§

§ return 0;

}

      • output

§ The fraction is: 1/3

§ Fraction 2 is: 1/5

Fraction 3 is: 3/10

      • @interface 里的宣告就如同正常的函式。
      • @implementation 使用了一个新的关键词:super
        • 如同 JavaObjective-C 只有一个 parent class (父类别)。
        • 使用 [super init] 来存取 Super constructor ,这个动作需要适当的继承设计。
        • 你将这个动作回传的 instance 指派给另一新个关键词:selfSelf 很像 C++ Java this 指标。
      • if ( self ) ( self != nil ) 一样,是为了确定 super constructor 成功传回了一个新对象。nil Objective-C 用来表达 C/C++ NULL 的方式,可以引入 NSObject 来取得。
      • 当你初始化变量以后,你用传回 self 的方式来传回自己的地址。
      • 预设的建构子是 -(id) init
      • 技术上来说,Objective-C 中的建构子就是一个 "init" method ,而不像 C++ Java 有特殊的结构。
    • 访问权限
      • 预设的权限是 @protected
      • Java 实作的方式是在 methods 与变量前面加上 public/private/protected 修饰语,而 Objective-C 的作法则更像 C++ 对于 instance variable (译注:C++ 术语一般称为 data members )的方式。
      • Access.h

§ #import <Foundation/NSObject.h>

§

§ @interface Access: NSObject {

§ @public

§ int publicVar;

§ @private

§ int privateVar;

§ int privateVar2;

§ @protected

§ int protectedVar;

§ }

@end

      • Access.m

§ #import "Access.h"

§

§ @implementation Access

@end

      • main.m

§ #import "Access.h"

§ #import <stdio.h>

§

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

§ Access *a = [[Access alloc] init];

§

§ // works

§ a->publicVar = 5;

§ printf( "public var: %i/n", a->publicVar );

§

§ // doesn't compile

§ //a->privateVar = 10;

§ //printf( "private var: %i/n", a->privateVar );

§

§ [a release];

§ return 0;

}

      • output

public var: 5

      • 如同你所看到的,就像 C++ private: [list of vars] public: [list of vars] 的格式,它只是改成了@private, @protected, 等等。
    • Class level access
      • 当你想计算一个对象被 instance 几次时,通常有 class level variables 以及 class level functions 是件方便的事。
      • ClassA.h

§ #import <Foundation/NSObject.h>

§

§ static int count;

§

§ @interface ClassA: NSObject

§ +(int) initCount;

§ +(void) initialize;

@end

      • ClassA.m

§ #import "ClassA.h"

§

§ @implementation ClassA

§ -(id) init {

§ self = [super init];

§ count++;

§ return self;

§ }

§

§ +(int) initCount {

§ return count;

§ }

§

§ +(void) initialize {

§ count = 0;

§ }

@end

      • main.m

§ #import "ClassA.h"

§ #import <stdio.h>

§

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

§ ClassA *c1 = [[ClassA alloc] init];

§ ClassA *c2 = [[ClassA alloc] init];

§

§ // print count

§ printf( "ClassA count: %i/n", [ClassA initCount] );

§

§ ClassA *c3 = [[ClassA alloc] init];

§

§ // print count again

§ printf( "ClassA count: %i/n", [ClassA initCount] );

§

§ [c1 release];

§ [c2 release];

§ [c3 release];

§

§ return 0;

}

      • output

§ ClassA count: 2

ClassA count: 3

      • static int count = 0; 这是 class variable 宣告的方式。其实这种变量摆在这里并不理想,比较好的解法是像 Java 实作 static class variables 的方法。然而,它确实能用。
      • +(int) initCount; 这是回传 count 值的实际 method 。请注意这细微的差别!这里在 type 前面不用减号 - 而改用加号 + 。加号 + 表示这是一个 class level function 。(译注:许多文件中,class level functions 被称为 class functions class method
      • 存取这个变数跟存取一般成员变数没有两样,就像 ClassA 中的 count++ 用法。
      • +(void) initialize method is Objective-C 开始执行你的程序时被呼叫,而且它也被每个 class 呼叫。这是初始化像我们的 count 这类 class level variables 的好地方。
    • 异常情况(Exceptions
      • 注意:异常处理只有 Mac OS X 10.3 以上才支持。
      • 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。
      • CupWarningException.h

§ #import <Foundation/NSException.h>

§

§ @interface CupWarningException: NSException

@end

      • CupWarningException.m

§ #import "CupWarningException.h"

§

§ @implementation CupWarningException

@end

      • CupOverflowException.h

§ #import <Foundation/NSException.h>

§

§ @interface CupOverflowException: NSException

@end

      • CupOverflowException.m

§ #import "CupOverflowException.h"

§

§ @implementation CupOverflowException

@end

      • Cup.h

§ #import <Foundation/NSObject.h>

§

§ @interface Cup: NSObject {

§ int level;

§ }

§

§ -(int) level;

§ -(void) setLevel: (int) l;

§ -(void) fill;

§ -(void) empty;

§ -(void) print;

@end

      • Cup.m

§ #import "Cup.h"

§ #import "CupOverflowException.h"

§ #import "CupWarningException.h"

§ #import <Foundation/NSException.h>

§ #import <Foundation/NSString.h>

§

§ @implementation Cup

§ -(id) init {

§ self = [super init];

§

§ if ( self ) {

§ [self setLevel: 0];

§ }

§

§ return self;

§ }

§

§ -(int) level {

§ return level;

§ }

§

§ -(void) setLevel: (int) l {

§ level = l;

§

§ if ( level > 100 ) {

§ // throw overflow

§ NSException *e = [CupOverflowException

§ exceptionWithName: @"CupOverflowException"

§ reason: @"The level is above 100"

§ userInfo: nil];

§ @throw e;

§ } else if ( level >= 50 ) {

§ // throw warning

§ NSException *e = [CupWarningException

§ exceptionWithName: @"CupWarningException"

§ reason: @"The level is above or at 50"

§ userInfo: nil];

§ @throw e;

§ } else if ( level < 0 ) {

§ // throw exception

§ NSException *e = [NSException

§ exceptionWithName: @"CupUnderflowException"

§ reason: @"The level is below 0"

§ userInfo: nil];

§ @throw e;

§ }

§ }

§

§ -(void) fill {

§ [self setLevel: level + 10];

§ }

§

§ -(void) empty {

§ [self setLevel: level - 10];

§ }

§

§ -(void) print {

§ printf( "Cup level is: %i/n", level );

§ }

@end

      • main.m

§ #import "Cup.h"

§ #import "CupOverflowException.h"

§ #import "CupWarningException.h"

§ #import <Foundation/NSString.h>

§ #import <Foundation/NSException.h>

§ #import <Foundation/NSAutoreleasePool.h>

§ #import <stdio.h>

§

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

§ NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

§ Cup *cup = [[Cup alloc] init];

§ int i;

§

§ // this will work

§ for ( i = 0; i < 4; i++ ) {

§ [cup fill];

§ [cup print];

§ }

§

§ // this will throw exceptions

§ for ( i = 0; i < 7; i++ ) {

§ @try {

§ [cup fill];

§ } @catch ( CupWarningException *e ) {

§ printf( "%s: ", [[e name] cString] );

§ } @catch ( CupOverflowException *e ) {

§ printf( "%s: ", [[e name] cString] );

§ } @finally {

§ [cup print];

§ }

§ }

§

§ // throw a generic exception

§ @try {

§ [cup setLevel: -1];

§ } @catch ( NSException *e ) {

§ printf( "%s: %s/n", [[e name] cString], [[e reason] cString] );

§ }

§

§ // free memory

§ [cup release];

§ [pool release];

}

      • output

§ Cup level is: 10

§ Cup level is: 20

§ Cup level is: 30

§ Cup level is: 40

§ CupWarningException: Cup level is: 50

§ CupWarningException: Cup level is: 60

§ CupWarningException: Cup level is: 70

§ CupWarningException: Cup level is: 80

§ CupWarningException: Cup level is: 90

§ CupWarningException: Cup level is: 100

§ CupOverflowException: Cup level is: 110

CupUnderflowException: The level is below 0

      • NSAutoreleasePool 是一个内存管理类别。现在先别管它是干嘛的。
      • Exceptions (异常情况)的丢出不需要扩充(extendNSException 对象,你可简单的用 id 来代表它: @catch ( id e ) { ... }
      • 还有一个 finally 区块,它的行为就像 Java 的异常处理方式,finally 区块的内容保证会被呼叫。
      • Cup.m 里的 @"CupOverflowException" 是一个 NSString 常数物件。在 Objective-C 中,@ 符号通常用来代表这是语言的衍生部分。C 语言形式的字符串(C string )就像 C/C++ 一样是 "String constant" 的形式,型别为 char *
  • 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 Java 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 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 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 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 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 getClass() method 。这个 class 对象被使用于前述的 methods 中。
      • Selectors 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 @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 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 NSObject 的一部份,它允许 subclass 扮演 superclass
    • Protocols
      • Objective-C 里的 Protocol Java interface 或是 C++ 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++ Java 的指定方式,如:Printing *someVar = ( Printing * ) frac; 你可以使用 id 型别加上 protocolid <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 )]
  • 内存管理
    • 到目前为止我都刻意避开 Objective-C 的内存管理议题。你可以呼叫对象上的 dealloc ,但是若对象里包含其他对象的指针的话,要怎么办呢?要释放那些对象所占据的内存也是一个必须关注的问题。当你使用 Foundation framework 建立 classes 时,它如何管理内存?这些稍后我们都会解释。
      • 注意:之前所有的范例都有正确的内存管理,以免你混淆。
    • Retain and Release (保留与释放)
      • Retain 以及 release 是两个继承自 NSObject 的对象都会有的 methods 。每个对象都有一个内部计数器,可以用来追踪对象的 reference 个数。如果对象有 3 reference 时,不需要 dealloc 自己。但是如果计数器值到达 0 时,对象就得 dealloc 自己。[object retain] 会将计数器值加 1 (值从 1 开始),[object release] 则将计数器值减 1 。如果呼叫 [object release] 导致计数器到达 0 ,就会自动 dealloc
      • Fraction.m

§ ...

§ -(void) dealloc {

§ printf( "Deallocing fraction/n" );

§ [super dealloc];

§ }

...

      • 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 "Fraction.h"

§ #import <stdio.h>

§

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

§ Fraction *frac1 = [[Fraction alloc] init];

§ Fraction *frac2 = [[Fraction alloc] init];

§

§ // print current counts

§ printf( "Fraction 1 retain count: %i/n", [frac1 retainCount] );

§ printf( "Fraction 2 retain count: %i/n", [frac2 retainCount] );

§

§ // increment them

§ [frac1 retain]; // 2

§ [frac1 retain]; // 3

§ [frac2 retain]; // 2

§

§ // print current counts

§ printf( "Fraction 1 retain count: %i/n", [frac1 retainCount] );

§ printf( "Fraction 2 retain count: %i/n", [frac2 retainCount] );

§

§ // decrement

§ [frac1 release]; // 2

§ [frac2 release]; // 1

§

§ // print current counts

§ printf( "Fraction 1 retain count: %i/n", [frac1 retainCount] );

§ printf( "Fraction 2 retain count: %i/n", [frac2 retainCount] );

§

§ // release them until they dealloc themselves

§ [frac1 release]; // 1

§ [frac1 release]; // 0

§ [frac2 release]; // 0

}

      • output

§ Fraction 1 retain count: 1

§ Fraction 2 retain count: 1

§ Fraction 1 retain count: 3

§ Fraction 2 retain count: 2

§ Fraction 1 retain count: 2

§ Fraction 2 retain count: 1

§ Deallocing fraction

Deallocing fraction

      • Retain call 增加计数器值,而 release call 减少它。你可以呼叫 [obj retainCount] 来取得计数器的 int 值。 当当 retainCount 到达 0 ,两个对象都会 dealloc 自己,所以可以看到印出了两个 "Deallocing fraction"
    • Dealloc
      • 当你的对象包含其他对象时,就得在 dealloc 自己时释放它们。Objective-C 的一个优点是你可以传递讯息给 nil ,所以不需要经过一堆防错测试来释放一个对象。
      • 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。
      • AddressCard.h

§ #import <Foundation/NSObject.h>

§ #import <Foundation/NSString.h>

§

§ @interface AddressCard: NSObject {

§ NSString *first;

§ NSString *last;

§ NSString *email;

§ }

§

§ -(AddressCard*) initWithFirst: (NSString*) f

§ last: (NSString*) l

§ email: (NSString*) e;

§ -(NSString*) first;

§ -(NSString*) last;

§ -(NSString*) email;

§ -(void) setFirst: (NSString*) f;

§ -(void) setLast: (NSString*) l;

§ -(void) setEmail: (NSString*) e;

§ -(void) setFirst: (NSString*) f

§ last: (NSString*) l

§ email: (NSString*) e;

§ -(void) setFirst: (NSString*) f last: (NSString*) l;

§ -(void) print;

@end

      • AddressCard.m

§ #import "AddressCard.h"

§ #import <stdio.h>

§

§ @implementation AddressCard

§ -(AddressCard*) initWithFirst: (NSString*) f

§ last: (NSString*) l

§ email: (NSString*) e {

§ self = [super init];

§

§ if ( self ) {

§ [self setFirst: f last: l email: e];

§ }

§

§ return self;

§ }

§

§ -(NSString*) first {

§ return first;

§ }

§

§ -(NSString*) last {

§ return last;

§ }

§

§ -(NSString*) email {

§ return email;

§ }

§

§ -(void) setFirst: (NSString*) f {

§ [f retain];

§ [first release];

§ first = f;

§ }

§

§ -(void) setLast: (NSString*) l {

§ [l retain];

§ [last release];

§ last = l;

§ }

§

§ -(void) setEmail: (NSString*) e {

§ [e retain];

§ [email release];

§ email = e;

§ }

§

§ -(void) setFirst: (NSString*) f

§ last: (NSString*) l

§ email: (NSString*) e {

§ [self setFirst: f];

§ [self setLast: l];

§ [self setEmail: e];

§ }

§

§ -(void) setFirst: (NSString*) f last: (NSString*) l {

§ [self setFirst: f];

§ [self setLast: l];

§ }

§

§ -(void) print {

§ printf( "%s %s <%s>", [first cString],

§ [last cString],

§ [email cString] );

§ }

§

§ -(void) dealloc {

§ [first release];

§ [last release];

§ [email release];

§

§ [super dealloc];

§ }

@end

      • main.m

§ #import "AddressCard.h"

§ #import <Foundation/NSString.h>

§ #import <stdio.h>

§

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

§ NSString *first =[[NSString alloc] initWithCString: "Tom"];

§ NSString *last = [[NSString alloc] initWithCString: "Jones"];

§ NSString *email = [[NSString alloc] initWithCString: "tom@jones.com"];

§ AddressCard *tom = [[AddressCard alloc] initWithFirst: first

§ last: last

§ email: email];

§

§ // we're done with the strings, so we must dealloc them

§ [first release];

§ [last release];

§ [email release];

§

§ // print to show the retain count

§ printf( "Retain count: %i/n", [[tom first] retainCount] );

§ [tom print];

§ printf( "/n" );

§

§ // free memory

§ [tom release];

§

§ return 0;

}

      • output

§ Retain count: 1

Tom Jones <tom@jones.com>

      • AddressCard.m ,这个范例不仅展示如何撰写一个 dealloc method ,也展示了如何 dealloc 成员变量。
      • 每个 set method 里的三个动作的顺序非常重要。假设你把自己当参数传给一个自己的 method (有点怪,不过确实可能发生)。若你先 release ,「然后」才 retain ,你会把自己给解构(destruct ,相对于建构)!这就是为什么应该要 1) retain 2) release 3) 设值 的原因。
      • 通常我们不会用 C 形式字符串来初始化一个变量,因为它不支持 unicode 。下一个 NSAutoreleasePool 的例子会用展示正确使用并初始化字符串的方式。
      • 这只是处理成员变量内存管理的一种方式,另一种方式是在你的 set methods 里面建立一份拷贝。
    • Autorelease Pool
      • 当你想用 NSString 或其他 Foundation framework classes 来做更多程序设计工作时,你需要一个更有弹性的系统,也就是使用 Autorelease pools
      • 当开发 Mac Cocoa 应用程序时,autorelease pool 会自动地帮你设定好。
      • 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 <Foundation/NSString.h>

§ #import <Foundation/NSAutoreleasePool.h>

§ #import <stdio.h>

§

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

§ NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

§ NSString *str1 = @"constant string";

§ NSString *str2 = [NSString stringWithString: @"string managed by the pool"];

§ NSString *str3 = [[NSString alloc] initWithString: @"self managed string"];

§

§ // print the strings

§ printf( "%s retain count: %x/n", [str1 cString], [str1 retainCount] );

§ printf( "%s retain count: %x/n", [str2 cString], [str2 retainCount] );

§ printf( "%s retain count: %x/n", [str3 cString], [str3 retainCount] );

§

§ // free memory

§ [str3 release];

§

§ // free pool

§ [pool release];

§ return 0;

}

      • output

§ constant string retain count: ffffffff

§ string managed by the pool retain count: 1

self managed string retain count: 1

      • 如果你执行这个程序,你会发现几件事:第一件事,str1 retainCount ffffffff
      • 另一件事,虽然我只有 release str3 ,整个程序却还是处于完美的内存管理下,原因是第一个常数字符串已经自动被加到 autorelease pool 里了。还有一件事,字符串是由 stringWithString 产生的。这个 method 会产生一个 NSString class 型别的字符串,并自动加进 autorelease pool
      • 千万记得,要有良好的内存管理,像 [NSString stringWithString: @"String"] 这种 method 使用了 autorelease pool ,而 alloc method [[NSString alloc] initWithString: @"String"] 则没有使用 auto release pool
      • Objective-C 有两种管理内存的方法, 1) retain and release or 2) retain and release/autorelease
      • 对于每个 retain ,一定要对应一个 release 「或」一个 autorelease
      • 下一个范例会展示我说的这点。
      • 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

§ ...

§ +(Fraction*) fractionWithNumerator: (int) n denominator: (int) d;

§ ...

      • Fraction.m

§ ...

§ +(Fraction*) fractionWithNumerator: (int) n denominator: (int) d {

§ Fraction *ret = [[Fraction alloc] initWithNumerator: n denominator: d];

§ [ret autorelease];

§

§ return ret;

§ }

...

      • main.m

§ #import <Foundation/NSAutoreleasePool.h>

§ #import "Fraction.h"

§ #import <stdio.h>

§

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

§ NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

§ Fraction *frac1 = [Fraction fractionWithNumerator: 2 denominator: 5];

§ Fraction *frac2 = [Fraction fractionWithNumerator: 1 denominator: 3];

§

§ // print frac 1

§ printf( "Fraction 1: " );

§ [frac1 print];

§ printf( "/n" );

§

§ // print frac 2

§ printf( "Fraction 2: " );

§ [frac2 print];

§ printf( "/n" );

§

§ // this causes a segmentation fault

§ //[frac1 release];

§

§ // release the pool and all objects in it

§ [pool release];

§ return 0;

}

      • output

§ Fraction 1: 2/5

Fraction 2: 1/3

      • 在这个例子里,此 method 是一个 class level method 。在对象建立后,在它上面呼叫 了 autorelease 。在 main method 里面,我从未在此对象上呼叫 release
      • 这样行得通的原因是:对任何 retain 而言,一定要呼叫一个 release autorelease 。对象的 retainCount 1 起跳 ,然后我在上面呼叫 1 autorelease ,表示 1 - 1 = 0 。当 autorelease pool 被释放时,它会计算所有对象上的 autorelease 呼叫次数,并且呼叫相同次数的 [obj release]
      • 如同批注所说,不把那一行批注掉会造成分段错误(segment fault )。因为对象上已经呼叫过 autorelease ,若再呼叫 release ,在释放 autorelease pool 时会试图呼叫一个 nil 对象上的 dealloc ,但这是不允许的。最后的算式会变为:1 (creation) - 1 (release) - 1 (autorelease) = -1
      • 管理大量暂时对象时,autorelease pool 可以被动态地产生。你需要做的只是建立一个 pool ,执行一堆会建立大量动态对象的程序代码,然后释放这个 pool 。你可能会感到好奇,这表示可能同时有超过一个 autorelease pool 存在。
  • Foundation framework classes
    • Foundation framework 地位如同 C++ Standard Template Library 。不过 Objective-C 是真正的动态识别语言(dynamic types ),所以不需要像 C++ 那样肥得可怕的样版(templates )。这个 framework 包含了对象组、网络、线程,还有更多好东西。
    • NSArray
      • 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 <Foundation/NSArray.h>

§ #import <Foundation/NSString.h>

§ #import <Foundation/NSAutoreleasePool.h>

§ #import <Foundation/NSEnumerator.h>

§ #import <stdio.h>

§

§ void print( NSArray *array ) {

§ NSEnumerator *enumerator = [array objectEnumerator];

§ id obj;

§

§ while ( obj = [enumerator nextObject] ) {

§ printf( "%s/n", [[obj description] cString] );

§ }

§ }

§

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

§ NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

§ NSArray *arr = [[NSArray alloc] initWithObjects:

§ @"Me", @"Myself", @"I", nil];

§ NSMutableArray *mutable = [[NSMutableArray alloc] init];

§

§ // enumerate over items

§ printf( "----static array/n" );

§ print( arr );

§

§ // add stuff

§ [mutable addObject: @"One"];

§ [mutable addObject: @"Two"];

§ [mutable addObjectsFromArray: arr];

§ [mutable addObject: @"Three"];

§

§ // print em

§ printf( "----mutable array/n" );

§ print( mutable );

§

§ // sort then print

§ printf( "----sorted mutable array/n" );

§ [mutable sortUsingSelector: @selector( caseInsensitiveCompare: )];

§ print( mutable );

§

§ // free memory

§ [arr release];

§ [mutable release];

§ [pool release];

§

§ return 0;

}

      • output

§ ----static array

§ Me

§ Myself

§ I

§ ----mutable array

§ One

§ Two

§ Me

§ Myself

§ I

§ Three

§ ----sorted mutable array

§ I

§ Me

§ Myself

§ One

§ Three

Two

      • 数组有两种(通常是 Foundation classes 中最数据导向的部分),NSArray NSMutableArray ,顾名思义,mutable (善变的)表示可以被改变,而 NSArray 则不行。这表示你可以制造一个 NSArray 但却不能改变它的长度。
      • 你可以用 Obj, Obj, Obj, ..., nil 为参数呼叫建构子来初始化一个数组,其中 nil 表示结尾符号。
      • 排序(sorting )展示如何用 selector 来排序一个对象,这个 selector 告诉数组用 NSString 的忽略大小写顺序来排序。如果你的对象有好几个排序方法,你可以使用这个 selector 来选择你想用的方法。
      • print method 里,我使用了 description method 。它就像 Java toString ,会回传对象的 NSString 表示法。
      • NSEnumerator 很像 Java 的列举系统。while ( obj = [array objectEnumerator] ) 行得通的理由是 objectEnumerator 会回传最后一个对象的 nil 。在 C nil 通常代表 0 ,也就是 false 。改用 ( ( obj = [array objectEnumerator] ) != nil ) 也许更好。
    • NSDictionary
      • 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 <Foundation/NSString.h>

§ #import <Foundation/NSAutoreleasePool.h>

§ #import <Foundation/NSDictionary.h>

§ #import <Foundation/NSEnumerator.h>

§ #import <Foundation/Foundation.h<

§ #import <stdio.h>

§

§ void print( NSDictionary *map ) {

§ NSEnumerator *enumerator = [map keyEnumerator];

§ id key;

§

§ while ( key = [enumerator nextObject] ) {

§ printf( "%s => %s/n",

§ [[key description] cString],

§ [[[map objectForKey: key] description] cString] );

§ }

§ }

§

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

§ NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

§ NSDictionary *dictionary = [[NSDictionary alloc] initWithObjectsAndKeys:

§ @"one", [NSNumber numberWithInt: 1],

§ @"two", [NSNumber numberWithInt: 2],

§ @"three", [NSNumber numberWithInt: 3],

§ nil];

§ NSMutableDictionary *mutable = [[NSMutableDictionary alloc] init];

§

§ // print dictionary

§ printf( "----static dictionary/n" );

§ print( dictionary );

§

§ // add objects

§ [mutable setObject: @"Tom" forKey: @"tom@jones.com"];

§ [mutable setObject: @"Bob" forKey: @"bob@dole.com" ];

§

§ // print mutable dictionary

§ printf( "----mutable dictionary/n" );

§ print( mutable );

§

§ // free memory

§ [dictionary release];

§ [mutable release];

§ [pool release];

§

§ return 0;

}

      • output

§ ----static dictionary

§ 1 => one

§ 2 => two

§ 3 => three

§ ----mutable dictionary

§ bob@dole.com => Bob

tom@jones.com => Tom


Last modified: April 13, 2004.
中文翻译:William Shih (xamous)January 7, 2005