Objective-C中單例模式（Singletons）的實現

單例的介紹

單例模式是在iOS開發中最常用的一種設計模式，單例模式可以讓程序的各個模塊共享數據，而不用去手動的去傳遞。所以單例類是一個我們要去理解的非常重要的模式，這種模式貫穿於iPhoneSDK中，例如UIApplication有一個方法叫做shareApplication用來共享當前程序的UIApplication實例。

單例的實現

添加一個ShareData類，在ShareData.h中聲明如下：

#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];
}

定義了一個在translation unit內全局唯一的靜態變量shareData，通過調用靜態方法shareData來初始化和獲取變量。通過調用Grand Central Dispatch(GCD)的dispatch_once方法來保證靜態變量只初始化一次，並且dispatch_once是系統保證的線程安全的。

在不用GCD的情況shareData方法的寫法如下：

+ (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