Objective-C中单例模式（Singletons）的实现

单例的介绍

单例模式是在iOS开发中最常用的一种设计模式，单例模式可以让程序的各个模块共享数据，而不用去手动的去传递。所以单例类是一个我们要去理解的非常重要的模式，这种模式贯穿于iPhoneSDK中，例如UIApplication有一个方法叫做shareApplication用来共享当前程序的UIApplication实例。

单例的实现

添加一个ShareData类，在ShareData.h中声明如下：

#import @interface ShareData : NSObject{    NSString *string;}@property (nonatomic, retain) NSString *string;+ (id)sharedData;@end

#import "ShareData.h"@implementation ShareData@synthesize someProperty;#pragma mark Singleton Methods+ (id)sharedData{    static ShareData *sharedData = nil;    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        sharedData = [[self alloc] init];    });    return sharedData;}- (id)init{    if (self = [super init])    {        self.string = [[[NSString alloc] initWithString:@"123"] autorelease];    }    return self;}- (void)dealloc{    self.string = nil;    [super dealloc];}

定义了一个在translation unit内全局唯一的静态变量shareData，通过调用静态方法shareData来初始化和获取变量。通过调用Grand Central Dispatch(GCD)的dispatch_once方法来保证静态变量只初始化一次，并且dispatch_once是系统保证的线程安全的。

在不用GCD的情况shareData方法的写法如下：

+ (id)sharedData{    static ShareData *sharedData = nil;    @synchronized(self)    {        if (sharedData == nil)        {            sharedMyManager = [[self alloc] init];        }    }    return sharedData;}

ShareData *shareData = [ShareData sharedData];

#import "ShareData.h"static ShareData *shareData = nil;@implementation ShareData@synthesize string;#pragma mark Singleton Methods+ (id)shareData{    @synchronized(self)    {        if(shareData == nil)        {            shareData = [[super allocWithZone:NULL] init];        }    }    return shareData;}+ (id)allocWithZone:(NSZone *)zone{    return [[self shareData] retain];}- (id)copyWithZone:(NSZone *)zone{    return self;}- (id)retain{    return self;}- (unsigned)retainCount{    return UINT_MAX; //denotes an object that cannot be released}- (oneway void)release{    // never release}- (id)autorelease{    return self;}- (id)init{    if (self = [super init])    {        string = [[NSString alloc] initWithString:@"123"];    }    return self;}- (void)dealloc{    [string release];    [super dealloc];}@end